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For fish, does a bony skeleton have any advantages over a cartilage skeleton?

For fish, does a bony skeleton have any advantages over a cartilage skeleton?


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I have learned that two important lineages of fish are the bony fishes (Osteichthyes) and the cartilaginous fishes (Chondrichthyes). Many websites mention an advantage of the cartilaginous skeletons of sharks and other chondrichthyans: they're lighter than bony skeletons.

But it has been harder for me to find out if we know of any things bony skeletons do for fish that cartilage skeletons can't do, or don't do as well.

My research and hypotheses

I found an old Manual of Geology by Samuel Haughton (1865) that says

[a bony] skeleton only affords an advantage over a cartilaginous skeleton by allowing a greater variety of points of attachment for the muscles of the Fish, and so admits of more powerful motions.

But maybe other people have discovered more things since this was written. Also, I'm curious if this is just a theoretical argument, or if we have actual evidence that bony fish are capable of using their muscles more effectively than cartilaginous fish.

I have three main hypotheses at present:

  1. Haughton was right, and a bony skeleton is just more effective at serving as an attachment point for muscles.

  2. Bony skeletons are more effective at protecting the fish.

  3. There are no advantages to bony skeletons; bony fish just happened to evolve this way. (As far as I can tell, the early evolutionary history of fish has been a bit unclear and I haven't been able to determine if there is current consensus about whether bone or cartilage skeletons came first, and whether chondrichthyan cartilage skeletons are an inherited primitive feature or an innovation.)


As a fish morphologist, here's my best attempt. I really like the first reference I listed at the bottom, too.

1) Ancestral Condition:: The cartilaginous skeleton of chondrichthyes (sharks, skates, rays, chimaeras) is almost certainly a derived feature--a synapomorphy-- defining the clade. There are a few lines of evidence for this. First, the cartilage in a shark skeleton is a very specialized form of calcified cartilage, unique to the group. Second, in other gnathostome and pre-gnathostome ancestors (Osteostracans, other armored jawless fishes) had big bony head shields. The groups most closely associated with sharks (Placoderms, paraphyletic acanthodians) has both bone and cartilage in their skeletons. Basically I guess my point is that, while cartilaginous skeletons were ancestral for gnathostomes, that cartilage was very different from chondrichthyan cartilage, and bone turned up in many stem fishes before chondrichthyan cartilage. So the ancestor for sharks and bony fishes probably had both cartilage and bone but not the specialized calcified cartilage of sharks

2) Bone as a Plesiomorphic (ancestral feature), but not the entire skeleton: If we take what I've said above, it certainly suggests that your point 3 is in the right direction w.r.t. split between chondrichthyes and osteichthyes. The chondrichthyan skeleton is light… But the osteichthyes definitely took endochondral bone and ran with it--which brings us back to your original question: why did they replace their entire skeleton with bone? Why is always a tricky question in evolution, but we can speculate:

3) Potential adaptive value of a bony skeleton:

  • bone is better at transmitting muscle force than cartilage, so in that sense, you are correct
  • when fish were coming into prominence and diversifying, there were a lot of big, terrifying, predatory arthropods. Many early fishes have big bony armor plates-- which may have served to project them from their chitonous predators
  • More evidence for that -- when big scary predators went extinct, fishes began to pare down their bony armor (most teleosts have highly reduced skeletons).

Without a time machine we can't be certain of these things, but I'm pretty sure what I've laid out is fairly close to the common consensus.

Relevant lit:


Differences between bony fish and cartilaginous fish

The sharks , rays and chimeras (deep-sea fish, also called rat fish) of this class (from the Greek chondros = cartilage + ichthys = fish) are the most primitive living vertebrates with complete and separate vertebrae, movable jaws and even fins.

This group is ancient and represented by numerous fossil remains. They belong to some of the largest and most efficient marine predators. All have a cartilaginous skeleton, specialized teeth that are renewed throughout life and a skin thickly covered by tooth-shaped scales.

Almost all are marine, although there are species of sharks and rays that regularly penetrate estuaries and rivers, and, in tropical regions, freshwater species.

All cartilaginous fish are predators, although the phytoplankton also ingest phytoplankton. In this case there are rigid projections of the gill arcs, which function as filters. Much of their diet is made up of live prey, although they eat corpses when available.

The Bone Fish

Bony fish are the largest group (corresponding to 9 out of 10 species) and diverse fishes present. These animals inhabit all types of water, sweet, brackish, salty, hot or cold (although most are limited to temperatures between 9 and 11ºC). This is the most recent class from a phylogenetic point of view as well as considered more evolved. The taxonomy within this class has often been altered, due to the discovery of new species, as well as of new relations between the already known ones.

Typically the bony fish are not larger than 1 m in length but there are reduced forms (certain gobies are only 10 mm long) and gigantic (swordfish with 3.70 m, sturgeon with 3.80 m and 590 kg of weight or fish -Water with 900 kg weight).

They have adapted to live in sometimes difficult conditions, such as lakes at high altitude, polar zones, hydrothermal vents, puddles with high salinity or poor in oxygen, etc.

Many fish periodically migrate from site to site or from deep water to the surface, both for spawning and feeding.

Its main features include a body, taller than wide and oval in silhouette, which facilitates movement through the water.

The head extends from the tip of the muzzle to the opening of the operculum, the trunk from there to the anus, behind which is the tail. The body has a strong segmental musculature – myomeros -, separated by delicate connective septa.

The skeleton is formed by true bones, although some species may have cartilaginous bones (sturgeon, for example), with numerous distinct vertebrae, although notochord is persistent in the intervertebral spaces.

The skeleton has 3 main parts: spine , skull and rays of the fins . The ribs and the pectoral girdle (there is no pelvic girdle, connecting these fins by means of tendons, without attachment to the spinal column). Numerous other small bones support the rays of the fins.

The main differences between bony fish and cartilaginous fish

We can classify the fish into two large groups that are quite different from one another: condrictes (cartilaginous fish) and osteitis (bony fish). Despite some glaring differences, it is common to make mistakes by differentiating the two groups. The following are the main differences between a cartilaginous fish and a bony fish. It is worth noting that we can find some representatives that do not follow the rules below.

First we can differentiate the two groups by the skeleton. Cartilaginous fish have a skeleton made up entirely of cartilage, while bone fish have a skeleton made up of bones.

Another striking difference is the gills. The bony fish have a membrane that covers the gill slits, while the cartilaginous fish have their gills exposed, without any protection.

Scales can also be used to differentiate these two groups. While cartilaginous fish have placoid scales and dermal and epidermal origin, bone fish have scales of exclusively dermal origin.

By observing the mouth, you can also see a difference. While the cartilaginous fish have a ventral mouth, the bony fish present their mouth in the anterior region of the body.

The bony fish present, among other characteristics, the presence of operculum

Reproduction is also an important factor. While bony fish have external fertilization, the cartilaginous ones have a structure called the clasper, which acts to aid in internal reproduction. The clasper is a modified pelvic fin which helps in the introduction of spermatozoa. Besides this difference, we can highlight the fact that in the cartilaginous fish there is no larvae appearing, whereas in the bony fish there is a larva that later develops and forms the fry.

We can also notice that cartilaginous fish have cloaca, differently from bony fish.

Another difference concerns the swim bladder, a structure that assists in the flotation of the fish. This structure is found only in bony fish.

We can mention as examples of cartilaginous fish the shark, ray and the cation. Among the bony fish, we can mention the catfish, painted and carp.


For fish, does a bony skeleton have any advantages over a cartilage skeleton? - Biology

A Quick Course in Ichthyology

by Jason Buchheim
Director, Odyssey Expeditions

  • FISH Definition
  • FISHES- class agnatha
  • FISHES- Class Chondrichthyes
    • Shark Attack
    • REDUCING THE RISK
    1. ________________
    2. ________________
    3. ________________
    4. ________________
    5. ________________
    6. ________________
    7. Fish are fun!

    FISH : Any of a large group of cold-blooded, finned aquatic vertebrates. Fish are generally scaled and respire by passing water over gills.Modern fish are divided into three classes.

    I. AGNATHA, primitive jawless fish.Lampreys and Hagfish

    II. CHONDRICHTHYES, the jawed fish with cartilaginous skeletons. Sharks, Rays, Rat-Fishes

    III. OSTEICHTHYES, fish with bony skeletons.Lungfish, Trout, Bass, Salmon, Perch, Parrot Fish


    Fish come in all shapes and sizes, some are free swimming, while others rest on the bottom of the sea, some are herbivores and others are carnivores, and some lay eggs while others give live birth and parental care to their young.
    FISH: the members of a single species
    FISHES: more than one species of fish
    FISHES- class Agnatha

    • Primitive
    • No jaws
    • Cartilaginous skeleton
    • Scaleless skin
    • Oral sucker in place of jaws
    • Predators and filter feeders
    • anticoagulating saliva
    • fresh and salt water
    • some anadromous
    • Cartilaginous skeleton
    • Skin covered with denticles, not scales
    • Five to seven gill slits per side
    • No swim bladder
    • Internal fertilization
    • Spiral valve intestines
    • Five to seven gill arches
    • Cartilaginous jaws, loosely attached lower jaws

    In fact, most sharks are entirely incapable of this feat. The largest fish of all, the Whale Shark, which can reach sizes of up to 59 feet and weigh 88,000 lb., is a very calm and approachable plankton feeder. There are many species of sharks which can inflict severe bodily injury and require the utmost of respect. The most feared of all, the Great White Shark, has been responsible for most of the fatal shark attacks off the California and Australian coastlines. While the Great White gets all the notoriety, pound for pound, the Bull Shark is probably the most ferocious. The Great White generally attacks a person because it has confused it with its favorite food, the seals and sea lions, but the Bull Shark will attack a person just because they are there. Even with these dangerous animals roaming the ocean, your chances of getting attacked by a shark are very remote.

    Worldwide, there are only about three hundred documented shark attacks a year. The chances are much higher that you will be hit by a drunk driver while driving to the beach then they are that you will even encounter a dangerous shark when you get there. There are some activities that will greatly increase your chance of a shark attack, such as carrying speared fish with you while diving or collecting abalone in turbid waters. Statistics of 1,652 shark attacks show that males are much more likely to be attacked than females (10 to 1 ratio), this is probably because males are much more active in the water, surfing and going to deeper depths where sharks are more common.

    The presence of large numbers of fish, or fish behaving in an unusual manner, has been reported preceding many attacks. In 40 percent of the reported shark attacks, people were pole-fishing or spear-fishing in the area of an attack. A comparison of the number of people swimming to those fishing and spear-fishing seems to show that these two pastimes have by far the highest risk of inducing an attack. While swimming, the chance of drowning is more than 1,000 times greater than that of dying from a shark attack.

    Most shark attacks occur in shallow water, where most bathers are, and in 94 percent of the cases the attack was by an individual shark acting alone. About 10 percent of reported shark attacks are on divers since the number of divers in the water at one time must be much smaller than 10 percent of beach bathers, the odds of being attacked must be significantly greater for divers.

    Close passes were seldom made before the attack, and in the majority of the cases there was only one strike. Few attacks involved more than one bite. This indicates that in many cases the attacking shark mistook the victim for a more usual kind of food and did not attack any further when the error was discovered. It is fortunate that sharks, in most cases, do not consider humans to be suitable food. This information also refutes the long-standing notion that fresh human blood is a powerful attractant that excites sharks into a feeding frenzy. If this were so, the presence of blood would certainly have induced that attacking shark to strike the victim repeatedly. Most wounds occur on the appendages- the hands, arms, legs, and feet. Lacerations of varying severity are the most common types of injury. About 25 percent of attacks kill the victim. The most usual cause of death is shock, combined with a severe loss of blood.

    REDUCING THE RISK

    Swimmers and divers can reduce the chance of being attacked by following a few simple rules: Never swim in areas where sharks are known to be common. Never enter the water where people are fishing, either from the beach or from inshore boats. If there are a number of people in the water, do not separate yourself from them. There is safety in numbers. Avoid swimming near deep channels, or where shallow water suddenly becomes deeper. Do not swim alone, or at dusk or after dark, when sharks are feeding actively and are likely to be closer to the shore. Do not enter the water, or if in the water leave immediately, if large numbers of fish are seen, or if fish seem to be acting strangely. Be alert for unusual movements in the water. Do not wear a watch or other jewelry that shines and reflects light. Do not enter the water with an open wound, and women should not swim during their menstrual periods.

    FISHES- Chondrichthyes, Sharks

    Sharks are animals that are superbly adapted to their environment. Almost all are carnivores or scavengers, although the species that live close to the sea floor feed mostly on invertebrates. Most possess a keen sense of smell, a large brain, good eyesight, and highly specialized mouth and teeth. Their bodies are usually heavier than water, and they do not have an air filled swim bladder for buoyancy like most bony fishes. All sharks have an asymmetric tail fin, with the upper lobe being larger than the lower one. This feature, together with flattened pectoral fins, and an oil-filled liver compensates for the lack of a swim bladder. There are 344 known species of sharks living in all parts of the oceans, from shallow to deep water and from the tropics to the polar regions. A few even venture into fresh water and have been found in rivers and lakes. Contrary to popular belief, most sharks are harmless to humans. Sharks are classified into eight orders:

    1. Sawsharks (Pristophoriformes), one family, five sp.Live on the bottom in warm temperate or tropical seas. Easily recognized because of tube, blade like snouts. Bear live young.

    2. Dogfish Sharks (Squaliformes), three families, 73 sp. Bottom dwelling deep water sharks, distributed worldwide. Bear live young and eat bony fishes, crustaceans, squid and other sharks. Harmless to humans.

    3. Angel Sharks (Squatiniformes), one family, 13 sp. Flattened, bottom dwelling sharks. Found on continental shelves and upper slopes of cold temperate and tropical seas. Have very sharp, awl-like teeth that are used to impale small fish and crustaceans.

    4. Bullhead Sharks (Heterodontiformes), one family, 8 sp. Live on rocky reefs where there are plenty of cracks and crevices. Found in Pacific and Indian Ocean. Eat invertebrates.
    5. Gilled Sharks (Hexanchiformes), two families, five sp. Deep-water, bottom-dwelling sharks. Worldwide distribution. Only shark with six or seven gill slits. Bear live young and eat bony fish, crustaceans, and other sharks.

    6. Mackerel Sharks (Lamniformes), seven families, 16 sp. Small, highly diverse order. Found in tropical to cold temperate or even Arctic waters. Oceanic and coastal. Most very large, eat bony fish, other sharks, squid, and marine mammals. Includes the Mako and Great White and the plankton eating Megamouth and Basking Sharks.

    7. Carpet Sharks (Otectolobiformes) seven families, 31 sp. Warm tropical to temperate waters. All members except whale shark live on bottom. Flattened. Most eat small fishes and invertebrates. Whale shark is plankton feeder. Some bear live young and others lay eggs.

    8. Ground Sharks (Carcharhiniformes) 8 families, 193 sp. Largest order of sharks. Worldwide distribution, temperate and tropical waters. Most live near coast, although some found in deeper waters. Eat bony fishes, other sharks, squid, and small invertebrates. Includes the dangerous Tiger shark.

    Sharks have numerous structural and physiological features that make them unique among the fishes. They have a simple cartilaginous skeleton with no ribs, and a cartilaginous jaw, backbone, and cranium.

    Thick skin supports the flimsy skeleton. The skin is elastic and aids in movement when the tail is arched, it pulls on the skin, which pulls back like a rubber band. The jaws are not connected to the skull and become unhinged, protruding forward from the skull allowing for a wider gape when feeding. The teeth are ossified with minerals known as 'apatite'. They form a conveyer belt with as many as eight teeth in a row. When a shark looses a tooth, another one just pops up. Sharks go through up to 2,400 teeth a year.

    Sharks have placoid scales which are fixed, slightly ossified and layered. They are smooth to the touch in one direction and extremely course in another. Just rubbing a shark the wrong way can inflict serious wounds.

    All sharks, rays, and skates are carnivores. They have normal sensory modalities, a small brain (most of which is dedicated to the olfactory lobes giving them an acute sense of smell) and well developed eyes with color vision and adaptation to low light levels.

    Some sharks lay eggs (all skates and ratfish do), but most are ovoviviparous (all rays are). The young develop with their yolk sacks within the mother, but without a placenta or umbilical cord. Some sharks (the Great White) are oviphagous the young eat the other developing young and embryos inside their mother and only the fiercest is born! A few sharks (hammerheads and reef sharks) are viviparous like mammals, the young are nourished with a placenta within the mother. The gestation period is around 22 months and 2-80 pups are born per litter. Because most sharks are ovoviviparous or viviparous, they do not produce mass numbers of young like other fish do. They are slow to develop and for this reason shark population numbers have been decreasing rapidly due to the recent popularity of shark fin soup. Fishermen are taking many more sharks than the maximum sustainable yield will allow. Some sharks will soon be endangered species. Rays

    Rays in general are physiologically exactly like sharks except the rays pectoral fins are fussed to their heads. Their gills are ventrally located. They swim with their ventral fins, like wings. Their eyes are dorsally [top] located and have spericules behind them. The spericules are used to breathe in with.

    Rays are modified as bottom feeders, feeding on invertebrates found in the sand. Sometimes you can watch a ray making quite a ruckus on the sand bottom in search of the invertebrates.

    Manta rays are planktivores and cruise the open water filter feeding out small animals. Mantas are the largest of the rays.

    Electric rays swim with their caudal fin and use their modified pectoral fins to electrically shock and stun their prey.

    Sawfish look like sharks but have true fused pectoral fins and gills on the ventral surface.

    Stingrays have a toxin filled spine at the base of their tail. Stingrays are not the mean creatures roaming the waters to hurt swimmers, as many people believe them to be. Stingrays are actually very approachable and can be hand fed and petted, just don't step on them!

    FISHES- the BONY FISH, OSTIEICTHYES

    The bony fish comprise the largest section of the vertebrates, with over 20,000 species worldwide. They are called bony fish because their skeletons are calcified, making them much harder than the cartilage bones of the chondrichthyes. The bony fishes have great maneuverability and speed, highly specialized mouths equipped with protrusible jaws, and a swim bladder to control buoyancy.

    The bony fish have evolved to be of almost every imaginable shape and size, and exploit most marine and freshwater habitats on earth. Many of them have complex, recently evolved physiologies, organs, and behaviors for dealing with their environment in a sophisticated manner.

    Eels -Anguilliformes 597 spp

    Salmon -salmoniformes 350 spp

    Flyingfishes -Cyprinodontiformes 845 spp

    Silversides -Atheriniformes 235 spp

    Squirrelfishes -Beryciformes 164 spp

    Scorpionfishes -Scopaeniformes 1160 spp

    Flatfish -Pleuronectiformes 538 spp

    Triggerfish -Tetraodontiformes 329 spp

    Perch Like -Perciformes 7791 spp, largest order

    Deep Sea Fish -Stomiiformes 250 spp Gobies -Gobiesociformes 114 spp Trumpetfish -Syngnathiformes 257 spp

    FISH SEX- how fish reproduce

    Fish have come up with three modes of reproduction depending on the method they care for their eggs.

    • Ovopartity -- Lay undeveloped eggs, External fertilization (90% of bony fish), Internal fertilization (some sharks and rays)
    • Ovoviviparity - Internal development- without direct maternal nourishment-Advanced at birth (most sharks + rays)-Larval birth (some scorpeaniforms-rockfish)
    • Viviparity - Internal development- direct nourishment from mother-Fully advanced at birth (some sharks, surf perches)

    Parental care: In fishes, parental care is very rare as most fish are broadcast spawners, but there are a few instances of parental care. Male gobies guard the eggs in a nest until they are born. The male yellowhead jawfish actually guards the eggs by holding them in his mouth! Weird Fish Sex!

    Some fish are very kinky creatures by human standards, displaying behavior that would probably get a human incarcerated for a long time.

    • Hermaphroditism : Some fish individuals are both males and females, either simultaneously or sequentially. There is no genetic or physical reason why hermaphroditism should not be present. About 21 families of fish are hermaphrodites.
    • Simultaneous hermaphrodite : There are some instances where being a member of both sexes could have its advantages. Imagine all the dates that you could have! In the deep sea, the low light levels and limited food supply make for a very low population density meaning that potential mates are few and far between. Members of the fish family Salmoniformes (eg salmon) and Serranidae (hamlets) are simultaneous hermaphrodites they can spawn with any individual encountered.
    • Sequential hermaphrodite: Very strange life histories develop in species whose individuals may change sex at some time in their life. They may change from being males to females (protandry) or females to males (protogyny).

    A classic example of protogyny is found in the wrasses and parrotfishes. The males in these species form harems, with one large male sequestering and defending a group of smaller females. The male enjoys spectacular reproductive success, as it has many females to mate with. The females also enjoy a limited reproductive success, producing as many eggs as they can, all fertilized by the one male. The male has the advantage over the females it has many females producing eggs for him to fertilize, whereas the females only have themselves. It is great to be the king!

    The weird sex stuff comes in when we analyze what the reproductive success of a smaller male may be. As only the largest male, the 'SuperMale' gets to mate with the females, a smaller male would enjoy zero reproductive success. There is no advantage to being a small male, and this is where the hermaphrodism comes in. If all the smaller fish were females, they could all enjoy a limited reproductive success while they are growing. If the male dies, the one that has grown to be the largest female will change sexes and become the male, in turn enjoying a much greater reproductive success than if she did not switch. So there are no small males and everything is all said and done, but wait! Evolution has a keen ability in finding weaknesses in any system, and it has done so with the parrotfish. In nature, we do find smaller male parrotfish, why should this be so? It has to do with the kind of thing that if a parrotfish was a human, could get the parrotfish into a great deal of trouble. The 'supermale' has to run around all of the time keeping track of and protecting all of his females as well capturing and eating food himself, so he does not necessarily have time to pay close attention to the details. When parrotfish mate, they form a spawning aggregation where the supermale will release his sperm into the water and the many females release their eggs. The sperm and egg find each other in the water column and fertilization takes place, and this is where the weakness of the system lays. Along comes the smaller male, who has evolved to look just like a female. Most of the time the smaller male will make itself completely inconspicuous by behaving just like the females, but during the spawning aggregations, he will be releasing sperm instead of eggs. The supermale will probably not even know that he has been conned. Everything gets really mixed up as males are changing into females changing into males. FISH- Schooling Behavior

    Everyone has heard of a school of fish, an aggregation of fish hanging out together but why, they are obviously not learning reading, writing, and arithmetic. Schools of fish may be either polarized (with all the fish facing the same direction) or non polarized (all going every which way)

    There are some factors that can make it advantageous to hang out with other fish.

      A. Confusion effect. A large school of fish may be able to confuse a potential predator into thinking that the school is actually a much larger organism.

    B. Dilution affect. If a fish hangs out with a lot of other fish and a predator does come around, the predator must usually select one prey item. With so many choices, the chances are that it will not be you. This is known as the 'selfish herd'.

    Enhanced Foraging: A school of fish may have better abilities to acquire food. With many more eyes to detect food, many more meals may be found but there would also be many more mouths to feed. By working as a team, the school may be able to take larger food items than any one individual could manage to capture.

    Migration: The migration abilities of fish in schools may possibly be enhanced due to better navigation, etc. Hydrodynamic efficiency: Due to the complex hydrodynamic properties of water (properties the fish probably discovered only by accident), a fish may gain a swimming advantage by being in a school. The slipstream from the fish ahead of it may make it easier to pass through the water. Good for all the fish except for the ones in front.

    The density of water makes it very difficult to move in, but fish can move very smoothly and quickly.

    A swimming fish is relying on its skeleton for framework, its muscles for power, and its fins for thrust and direction.

    The skeleton of a fish is the most complex in all vertebrates. The skull acts as a fulcrum, the relatively stable part of the fish. The vertebral column acts as levers that operate for the movement of the fish.

    The muscles provide the power for swimming and constitute up to 80% of the fish itself. The muscles are arranged in multiple directions (myomeres) that allow the fish to move in any direction. A sinusoidal wave passes down from the head to the tail. The fins provide a platform to exert the thrust from the muscles onto the water.

    Diagram of forces when a fish swims.

    Thrust- force in animal's direction

    Lift- force opposite in right angles to the thrust

    Drag- force opposite the direction of movement

    • Cruisers: These are the fish that swim almost continuously in search for food, such as the tuna. Red Muscle- richly vascularized (blood-carrying capacity), rich in myoglobin (oxygen holder and transferor into the muscles active sites) * able to sustain continuous aerobic movement.
    • Burst Swimmers: These fish usually stay relatively in the same place such as most reef fish.
    • Caudal fin-- provides thrust, and control the fishes direction
    • Pectorals-- act mostly as rudders and hydroplanes to control yaw and pitch. Also act as very important brakes by causing drag.
    • Pelvic fins-- mostly controls pitch
    • Dorsal/anal-- control roll
    • A tuna fish which has a fusiform similar to a torpedo can cruise through the water at very high speeds.
    • The attenuated shape of the eel allows it to wiggle into small crevices where it hunts prey.
    • The depressed shape of the angler fish is advantageous for its "sit and wait" strategy of hunting.
    • The compressed shape found on many reef fishes such as the butter fish gives the fish great agility for movement around the reef and can support sudden bursts of acceleration.
    • Ectothermic: fish derive their heat from the environment
    • Poikilothermic : fish conform to the heat in the environment

    They maintain a higher body temperature through the use of a specialized counter-current heat exchanger called a reta mirabile. These are dense capillary beds within the swimming muscle that run next to the veins leaving the muscles. Blood passes through the veins and arteries in a counter current (opposite) direction. The heat produced from the muscle contraction flows from the exiting veins into the incoming arteries and is recycled.

    Why should they bother having an elevated body temperature? To increase the speed of the fish. The higher the body temperature, the greater the muscular power. Thirty degrees Celsius is the optimum temperature for muscular speed. With increased speed, the tuna can capture the slower, cold blooded fish it prey upon. Tuna have been clocked at record speed of 50-70 mph!

    Bony fish have swim bladders to help them maintain buoyancy in the water. The swim bladder is a sac inside the abdomen that contains gas. This sac may be open or closed to the gut. If you have ever caught a fish and wondered why its eyes are bulging out of its head, it is because the air in the swim bladder has expanded and is pushing against the back of the eye. Oxygen is the largest percentage of gas in the bladder nitrogen and carbon dioxide also fill in passively.

    Physoclistous- swim bladder is closed to the gut. The gas gets in through a special gas gland in the front of the swim bladder. Gas leaves the bladder through an oval body in the back of the swim bladder. The system works in a pretty miraculous way. Oval body, filled by venous blood -gasses leave here

    Gas gland, fed by arterial blood -gasses enter here

    inside the spots= giant secretory cells- secrete lactate -in capillary clusters rete mirabile

    Increased lactate levels from the giant secretory cells lower the surrounding pH, causing the blood hemoglobin to dump off its oxygen. The oxygen diffuses back into the incoming capillary, increasing the partial pressure of oxygen in the incoming capillary. This continues until the partial pressure of the oxygen in the capillary is higher than that of the swim bladder (which has a high concentration of oxygen). This complex system is necessary because the concentration of oxygen is higher in the swim bladder than it is in the blood, so simple diffusion would tend to pull the oxygen out of the bladder instead of pushing it in. If the fish wants more buoyancy, it must tell its secretory cells to release more lactate. Since oxygen diffuses easily with oxygen-poor venous blood, the gas can be forced out.

    *Fish that migrate vertically tend to have high oxygen levels in their bladders because it fills in faster and leaves faster.

    *Fish that maintain a stable depth tend to have more nitrogen because it is inert, enters slowly, and exits slowly.

    How in the heck can a fish, which is underwater, breath if there is no air? When we go under water, we have to bring air with us to survive. Whales and dolphins have lungs that store air from the surface. Fish don't have lungs, and they rarely ever venture into the air, so how do they survive. We all know it has something to do with gills, but what exactly.

    The water surrounding a fish contains a small percentage of dissolved oxygen. In the surface waters there can be about 5 ml. of oxygen per liter of water. This is much less than the 210 ml. of oxygen per liter of air that we breath, so the fish must use a special system for concentrating the oxygen in the water to meet their physiological needs. Here it comes again, a counter current exchange system, similar to the one we found in the fish's swim bladder and in the tuna's muscles.

    The circulation of blood in fish is simple. The heart only has two chambers, in contrast to our heart which has four. This is because the fish heart only pumps blood in one direction. The blood enters the heart through a vein and exits through a vein on its way to the gills. In the gills, the blood picks up oxygen from the surrounding water and leaves the gills in arteries, which go to the body. The oxygen is used in the body and goes back to the heart. A very simple closed-circle circulatory system.

    • The blood flows thorough the gill filaments and secondary lamellae in the opposite direction from the water passing the gills. This is very important for getting all of the available oxygen out of the water and into the blood.
    • If the blood flowed in the same direction as the water passing it, then the blood would only be able to get half of the available oxygen from the water. The blood and water would reach an equilibrium in oxygen content and diffusion would no longer take place.
    • By having the blood flow in the opposite direction, the gradient is always such that the water has more available oxygen than the blood, and oxygen diffusion continues to take place after the blood has acquired more than 50% of the water's oxygen content. The countercurrent exchange system gives fish an 80-90% efficiency in acquiring oxygen.
    • When fish are taken out of the water, they suffocate. This is not because they cannot breathe the oxygen available in the air, but because their gill arches collapse and there is not enough surface area for diffusion to take place. There are actually some fish that can survive out of the water, such as the walking catfish (which have modified lamellae allowing them to breathe air.
    • It is possible for a fish to suffocate in the water. This could happen when the oxygen in the water has been used up by another biotic source such as bacteria decomposing a red tide.

    --Ram Ventilation: Swim through the water and open your mouth. Very simple, but the fish must swim continuously in order to breathe, not so simple.

    Successful survival in any environment depends upon an organism's ability to acquire information from its environment through its senses. Fish have many of the same senses that we have, they can see, smell, touch, feel, and taste, and they have developed some senses that we don't have, such as electroreception. Fish can sense light, chemicals, vibrations and electricity.

    Light: photoreception [Vision]. Fish have a very keen sense of vision, which helps them to find food, shelter, mates, and avoid predators. Fish vision is on par with our own vision many can see in color, and some can see in extremely dim light.

    Fish eyes are different from our own. Their lenses are perfectly spherical, which enables them to see underwater because it has a higher refractive index to help them focus. They focus by moving the lens in and out instead of stretching it like we do. They cannot dilate or contract their pupils because the lens bulges through the iris. As the depth at which fish are found increases, the resident fish's eye sizes increase in order to gather the dimmer light. This process continues until the end of the photic zone, where eye size drops off as their is no light to see with. Nocturnal fish tend to have larger eyes then diurnal fish. Just look at a squirrelfish, and you will see this to be so. Some fish have a special eye structure known as the Tapetum lucidum, which amplifies the incoming light. It is a layer of guanine crystals which glow at night. Photons which pass the retina get bounced back to be detected again. If the photons are still not absorbed, they are reflected back out of the eye. On a night dive, you may see these reflections as you shine your light around!

    Chemicals: chemoreception [Smell and Taste]. Chemoreception is very well developed in the fishes, especially the sharks and eels which rely upon this to detect their prey. Fish have two nostrils on each side of their head, and there is no connection between the nostrils and the throat. The olfactory rosette is the organ that detects the chemicals. The size of the rosette is proportional to the fish's ability to smell. Some fish (such as sharks, rays, eels, and salmon) can detect chemical levels as low as 1 part per billion.

    Fish also have the ability to taste. They have taste buds on their lips, tongue, and all over their mouths. Some fish, such as the goatfish or catfish, have barbels, which are whiskers that have taste structures. Goatfish can be seen digging through the sand with their barbels looking for invertebrate worms to eat and can taste them before they even reach their mouths.

    Vibrations: mechanoreception [Hearing and touch]. Have you ever seen a fish's ear. Probably not, but they do have them, located within their bodies as well as a lateral line system that actually lets them feel their surroundings.

    Fish do not have external ears, but sound vibrations readily transmit from the water through the fish's body to its internal ears. The ears are divided into two sections, an upper section (pars superior) and a lower section (utriculus) The pars superior is divided into three semicircular canals and give the fish its sense of balance. It is fluid-filled with sensory hairs. The sensory hairs detect the rotational acceleration of the fluid. The canals are arranged so that one gives yaw, another pitch, and the last- roll. The utriculus gives the fish its ability to hear. It has two large otoliths which vibrate with the sound and stimulate surrounding hair cells.

    Fish posses another sense of mechanoreception that is kind of like a cross between hearing and touch. The organ responsible for this is the neuromast, a cluster of hair cells which have their hairs linked in a glob of jelly known as 'cupala'. All fish posses free neuromasts, which come in contact directly with the water. Most fish have a series of neuromasts not in direct contact with the water. These are arranged linearly and form the fishes lateral lines. A free neuromast gives the fish directional input.

    A lateral line receives signals stimulated in a sequence, and gives the fish much more information (feeling the other fish around it for polarized schooling, and short-range prey detection 'the sense of distant touch').

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    Main Differences between Bony fish and Cartilaginous fish

    The main difference between the bony fish and the cartilaginous fish is obviously the structure of the skeleton. As mentioned earlier, bony fish have a bony skeleton while cartilaginous fish have a skeleton made of cartilage.However, there are many other differences between these two kinds of fish. These differences are listed below.

    Habitat

    The vast majority of cartilaginous fish live in marine or salt water habitats.

    These fish can be found in all the seas and oceans of the world. Bone fish, on the other hand, are found in both saltwater and freshwater habitats.

    Gills

    Fish gills are tissues located on both sides of the throat. These tissues contain ions and water in the fish system, where the water’s oxygen and the fish’s carbon dioxide are exchanged. In other words, the gills of the fish act like lungs.

    In the bony fish, the gills are covered by an external skin flap, known as the operculum.

    In cartilaginous fish, the gills are exposed and not protected by any external skin. Most fish, whether bony or cartilaginous, have five pairs of gills.

    Heart and Blood

    In both kinds of fish, the heart is divided into chambers. In the hearts of cartilaginous fish, one of these chambers is known as the arterial cone, a special heart muscle that contracts. Instead of this chamber, bony fish have a bulbous artery, a muscle that doesn’t contract.

    Another difference between bony and cartilaginous fish lies in how each class produces red blood cells. In bony fish, red blood cells are produced in the bone marrow, (the central part of the bone). This process is known as hemopoiesis.

    Cartilaginous fish lack bone marrow for hemopoiesis. Instead, these fish produce red blood cells in the spleen and thymus organs.

    Mandibular structure

    The jaw is part of the mouth, allowing it to open and close to grab and digest food. Its structure is different in cartilaginous and bony fish.

    For example, bony fish have two sets of jaws: the oral jaw and the pharyngeal jaw. The oral jaw allows bony fish to trap food, bite and chew it.

    The teeth usually only grow along one side of the jaw. The pharyngeal jaw, (located in the throat), digests the food further by processing it before it moves from mouth to stomach.

    In contrast, cartilaginous fish lack the pharyngeal jaw. The oral jaw of these fish is made-up of cartilage and is divided into an upper and lower section.

    Each section can contain several teeth, which grow in multiple sets. Cartilaginous fish can even grow their teeth again as they wear out over time.

    Digestive system

    The digestive system between bony and cartilaginous fish is also different. The intestine of cartilaginous fish is typically shorter than that of bony fish.

    However, it spirals internally to create a larger surface area that optimizes nutrient absorption. In the bony fish, the intestine is longer and has no spiral shape.

    J-shaped stomachs can be found in cartilaginous fish, while bony fish have a wide variety of stomach shapes and in some cases, they have no stomach at all.

    The sewer, the opening through which urine and feces are excreted is also different. It can only be found in cartilaginous fish and with lobular fins. In other bony fish, the urinary tract, genitals and anus each have a separate opening.

    Neutral buoyancy

    Fish must have an internal flotation system to prevent them from floating on top of the water or sinking at the bottom, which is known as neutral floating. Bony fish are able to maintain neutral buoyancy with the help of the swim bladder.

    The swim bladder is typically a two-bag organ that controls the volume of internal gases to help fish maintain a certain position in the water.

    This allows them to conserve energy that they could otherwise use swimming to maintain neutral buoyancy.

    Some bony fish have lost the swim bladder through evolution most of these are species that inhabit the bottom.

    Cartilaginous fish can achieve neutral buoyancy due to the lighter weight of their cartilaginous skeleton and its more hydrodynamic exteriors.

    Some cartilaginous fish, such as sharks, even swim to the surface of the water to absorb the air that helps them maintain their position in the water.


    Bony Fish

    This category of fish is also referred to as Teleostomi. It is also considered the largest class in Phylum Chordata. These fish are widely recognized because of the following characteristics:

    • Their endoskeleton is entirely made of bone
    • They have anterior tip mouth opening
    • They can either be fresh water or marine water fishes
    • Their exoskeleton is made up of cycloids (thin bony plates), aligned based on whether the outer edges are spiny or smooth
    • They have an operculum on either side of their gills
    • They possess an air bladder that also performs hydrostatic functions
    • Their tail fin is homocercal
    • They fertilize their eggs externally

    Some of the fish in this category include flying fish, globe fish, sea horses and eels.


    Difference Between Bony Fish and Cartilaginous Fish

    Definition

    Bony Fish: Bony fish refers to a large class of fish distinguished by a skeleton made up of bone.

    Cartilaginous Fish: Cartilaginous fish refers to a class of fish with a skeleton made up of cartilages.

    Alternative Names

    Bony Fish: Bony fish is also known as teleostomi.

    Cartilaginous Fish: Cartilaginous fish is also known as elasmobranchii.

    Class

    Bony Fish: Bony fish belongs to the class Osteichthyes.

    Cartilaginous Fish: Cartilaginous fish belongs to the class Chondrichthyes.

    Number of Species

    Bony Fish: More than 27,000 bony fish species have been identified worldwide.

    Cartilaginous Fish: More than 970 cartilage fish species have been identified worldwide.

    Habitat

    Bony Fish: Bony fish can be found in both fresh and marine water.

    Cartilaginous Fish: Cartilaginous fish can be exclusively found in marine water.

    Endoskeleton

    Bony Fish: Bony fish has an endoskeleton made up of bones.

    Cartilaginous Fish: Cartilaginous fish has an endoskeleton made up of cartilages.

    Exoskeleton

    Bony Fish: The exoskeleton of bony fish is made up of thin bony plates known as cycloids.

    Cartilaginous Fish: The exoskeleton of cartilaginous fish is made up of very small denticles coated with sharp enamel known as placoid.

    Position of the Mouth

    Bony Fish: Bony fish has a mouth at the anterior tip of the mouth.

    Cartilaginous Fish: Cartilaginous fish has a ventrally-positioned mouth.

    Oral Jaw Sets

    Bony Fish: Bony fish has two sets of oral jaws.

    Cartilaginous Fish: Cartilaginous fish has a single set of oral jaws.

    Gill Pairs

    Bony Fish: Bony fish has four pairs of gills.

    Cartilaginous Fish: Cartilaginous fish has five to seven gills.

    Operculum

    Bony Fish: The gills of the bony fish are covered with an operculum.

    Cartilaginous Fish: The gills of the cartilaginous fish are not covered with an operculum.

    Air Bladder

    Bony Fish: Bony fish has an air bladder known as swimbladder for buoyancy.

    Cartilaginous Fish: Cartilaginous fish uses oil-filled liver for buoyancy.

    Tail Fin

    Bony Fish: The tail fin of bony fish is homocercal.

    Cartilaginous Fish: The tail of cartilaginous fish is heterocercal.

    Fertilization

    Bony Fish: Bony fish exhibits external fertilization.

    Cartilaginous Fish: Cartilaginous fish exhibits internal fertilization.

    Excretion

    Bony Fish: Bony fish excretes ammonia.

    Cartilaginous Fish: Cartilaginous fish excretes urea.

    Examples

    Bony Fish: Salmon fish, rohu, trout, flying fish, and seahorse are examples of bony fish.

    Cartilaginous Fish: Shark, skates, and rays are examples of cartilaginous fish.

    Conclusion

    Bony fish and cartilaginous fish are two classes of fish classified under the superclass Pisces. The main difference between bony fish and cartilaginous fish is the composition of the endoskeleton in each class of fish. The endoskeleton of bony fish is completely made up of bones whereas the endoskeleton of cartilaginous fish is made up of cartilages.

    Reference:

    1. Harwood, Jessica, et al. “Bony Fish.” CK-12 Foundation, CK-12 Foundation, 24 Dec. 2016, Available here.
    2. “Osteichthyes – Bony Fish” Wildlife Journal Junior, Available here.
    3. Kennedy, Jennifer. “What Is a Cartilaginous Fish?” ThoughtCo, Available here.


    COMPARISM BETWEEN CARTILAGINOUS FISH (CHONDRICTHYES) AND BONY FISH (OSTEICHTHYES).

    The cartilaginous fish, or Chondricthyes, include the sharks, rays, skates, and chimaeras. There are over eight hundred living species of sharks and rays, and about thirty species of chimaeras. Cartilaginous fish are true fish. They have fins and breathe with gills. Unlike the more familiar bony fish, the Osteichythes, the skeletons of the cartilaginous fish are made of cartilage. Other features that distinguish the cartilaginous fish from the bony fish are multiple gill slits, tiny tooth like scales, nostrils on the side of the head, teeth that are not fused to the jaw, and internal fertilization. Internal fertilization also occurs in some bony fish such as sea horses, guppies, and mollies. As implied in the name, the skeleton in cartilaginous fish does not include bone but consists of cartilage, and all the fins are supported by horny structures rather than fin rays. None of the species possess a swimbladder,The organ most bony fish use to prevent them from sinking to the bottom. Many cartilaginous fish species are therefore either bottom dwellers or accomplish neutral buoyancy by maintaining a high fat or oil content in their tissues. The gill openings in cartilaginous fish are not covered with operculae, and are seen as a series of slits on the side of the fish just behind the head, or on the underside of the fish. Unlike bony fish cartilaginous fish do not have scales, but their body is sometimes covered with small tooth like structures (denticles), which make their skin feel like sandpaper. Their jaws are short and the mouth is protrusible. The teeth, which are highly specialized, are positioned in rows and are continuously shed and replaced. Cartilaginous fish have very keen senses, and in addition to the senses used by other fish they are able to sense electric impulses from prey-fish that are burrowed in the bottom. In some species the eggs hatch while still in the abdomen of the female, other species lay very big hard-shelled eggs from which fully developed juveniles hatch. This kind of reproduction ensures a high survival rate of the young but also limits the number of offspring per female. Many species of cartilaginous fish are also long-lived (certain marine species may live more than 100 years), and they are slow to reach sexual maturity. Cartilaginous fish are therefore more sensitive to a high fishing pressure than many bony fish.
    The bony fish (Osteon = “bone” “icthys” = “fish”) are the most diverse and numerous of all vertebrates. Bony fish (Class Osteichthyes) are first seen in fossils from the Devonian (about 395 million years before present). They differ from most of the cartilaginous fishes in having a terminal mouth and a flap (operculum) covering the gills. Bony fish (Osteichthyes) are distinguished from other fish species that have a cartilaginous skeleton (Chondrichthyes—sharks, rays and chimaeras, for example) by the presence of true bone—a mixture of calcium phosphates and carbonates—in their skeletons. Other differences between the two groups are modifications in the structure and arrangement of the scales and fins and the presence of more specialized teeth in bony fish. When feeding, bony fish display a far wider range of adaptations than cartilaginous species: the former may be either carnivorous (like most cartilaginous species), plant-eating, or both. Combined, these features have helped them to exploit a much wider range of feeding and living habitats.
    In addition, most have a swim bladder, which is ordinarily used to adjust their buoyancy, although among the air-breathing fishes it is attached to the pharynx and serves as a simple lung. The skin has many mucus glands and is usually adorned with dermal scales. Their jaws are well developed, articulated with the skull, and armed with teeth. Although the skeleton of most is bone, that of sturgeons and a few others is largely made of cartilage. They have a two-chambered heart built on the same plan as the Chondrichthyes (two-chambered with a conus arteriosus and a sinus venosus). The sexes are separate, most are oviparous, and fertilization is usually external. There are two subclasses: subclass Actinopterygii (ray-finned fishes) and subclass Sarcopterygii (lobe-finned fishes). All bony fish possess gills. For the majority this is their sole or main means of respiration. Lungfish and other osteichthyan species, are capable of respiration through lungs or vascularized swim bladders. Other species can respire through their skin, intestines, and/or stomach,[ Osteichthyes are primatively ectothermic (cold blooded), meaning that their body temperature is dependent on that of the water. But some members of the family scombridae such as the swordfish and tuna have achieved various levels of endothermy. They can be any type of heterotroph: omnivore, carnivore, herbivore, or detrivore. Some bony fish are hermaphrodites, and a number of species exhibit parthenogenesis. Fertilization is usually external, but can be internal. Development is usually oviparous (egg-laying) but can be ovoviviparous, or viviparous. Although there is usually no parental care after birth, before birth parents may scatter, hide, guard or brood eggs, with sea horses being notable in that the males undergo a form of ‘pregnancy’, brooding eggs deposited in a ventral pouch by a female. While most bony fish breed by shedding egg and milt freely into the water, all cartilaginous fish reproduce through internal fertilization.


    Bones

    There are two types of bone tissue within the endoskeleton of humans:

    The Cortical Bone

    The cortical bone—also called the ‘compact bone’— is the dense bone tissue that forms the hard exterior and gives long bones their strength.

    Compact bone is formed of a calcified matrix containing very few spaces, although it does contain many small cylindrical columns of only a few millimeters wide called lamellae. These lamellae form the osteon or the haversian system.

    Within the osteon is the haversian canal, the central canal which surrounds blood cells and nerves.

    Surrounding the haversian canal are the osteocytes, which store the mineral tissue of bones such as calcium. These osteocytes are connected to each other in a network of tiny canals called canaliculi, which allows them to transport minerals, fatty acids and waste and between each other.

    The Cancellous Bone

    The cancellous bone, also known as trabecular bone or ‘spongy bone’, makes up the interior of the bone structure. Cancellous bone is typically found at the ends of the long bones as well as the rubs, skull, pelvic bones and the vertebrae of the spinal column.

    It is a lightweight and porous bone with the tissue arranged into a honeycomb-like matrix with large spaces these spaces are often filled with blood vessels and bone marrow. The main structure of the cancellous bone is formed of thin rod-like bones called trabeculae.


    Sharks are cartilaginous fish. On the other hand, bony fish are the largest group of fish which have a skeleton made from bones. They share similarities as well as differences. The below infographic presents the difference between sharks and bony fish in tabular form.


    Contents

    For every type of fin, there are a number of fish species in which this particular fin has been lost during evolution.

    • A peculiar function of pectoral fins, highly developed in some fish, is the creation of the dynamic lifting force that assists some fish, such as sharks, in maintaining depth and also enables the "flight" for flying fish.
    • In many fish, the pectoral fins aid in walking, especially in the lobe-like fins of some anglerfish and in the mudskipper.
    • Certain rays of the pectoral fins may be adapted into finger-like projections, such as in sea robins and flying gurnards.
      • The "horns" of manta rays and their relatives are called cephalic fins this is actually a modification of the anterior portion of the pectoral fin.
      • In gobies, the pelvic fins are often fused into a single sucker disk. This can be used to attach to objects. [1]
      • Pelvic fins can take many positions along the ventral surface of the fish. The ancestral abdominal position is seen in (for example) the minnows the thoracic position in sunfish and the jugular position, when the pelvics are anterior to the pectoral fins, as seen in the burbot. [2]

      Dorsal fins are located on the back. A fish can have up to three dorsal fins. The dorsal fins serve to protect the fish against rolling, and assist it in sudden turns and stops.

      • In anglerfish, the anterior of the dorsal fin is modified into an illicium and esca, a biological equivalent to a fishing rod and lure.
      • The bones that support the dorsal fin are called Pterygiophore. There are two to three of them: "proximal", "middle", and "distal". In rock-hard, spinous fins the distal is often fused to the middle, or not present at all.

      The function of the adipose fin is something of a mystery. It is frequently clipped off to mark hatchery-raised fish, though data from 2005 showed that trout with their adipose fin removed have an 8% higher tailbeat frequency. [4] [5] Additional information released in 2011 has suggested that the fin may be vital for the detection of, and response to, stimuli such as touch, sound and changes in pressure. Canadian researchers identified a neural network in the fin, indicating that it likely has a sensory function, but are still not sure exactly what the consequences of removing it are. [6] [7]

      A comparative study in 2013 indicates the adipose fin can develop in two different ways. One is the salmoniform-type way, where the adipose fin develops from the larval-fin fold at the same time and in the same direct manner as the other median fins. The other is the characiform-type way, where the adipose fin develops late after the larval-fin fold has diminished and the other median fins have developed. They claim the existence of the characiform-type of development suggests the adipose fin is not "just a larval fin fold remainder" and is inconsistent with the view that the adipose fin lacks function. [3]

      Research published in 2014 indicates that the adipose fin has evolved repeatedly in separate lineages. [8]

    The caudal fin is the tail fin (from the Latin cauda meaning tail), located at the end of the caudal peduncle and is used for propulsion. See body-caudal fin locomotion.

    (A) - Heterocercal means the vertebrae extend into the upper lobe of the tail, making it longer (as in sharks). It is the opposite of hypocercal.

    • Hypocercal, also known as reversed heterocercal, means that the vertebrae extend into the lower lobe of the tail, making it longer (as in the Anaspida). It is the opposite of heterocercal. [9]

    (B) - Protocercal means the vertebrae extend to the tip of the tail and the tail is symmetrical but not expanded (as in amphioxus)

    (C) - Homocercal where the fin appears superficially symmetric but in fact the vertebrae extend for a very short distance into the upper lobe of the fin

    (D) - Diphycercal means the vertebrae extend to the tip of the tail and the tail is symmetrical and expanded (as in the bichir, lungfish, lamprey and coelacanth). Most Palaeozoic fishes had a diphycercal heterocercal tail. [10]

    Most modern fishes (teleosts) have a homocercal tail. These appear in a variety of shapes, and can appear:

    • rounded
    • truncated, ending in a more-or-less vertical edge (such as salmon)
    • forked, ending in two prongs
    • emarginate, ending with a slight inward curve.
    • lunate or shaped like a crescent moon

    Finlets

    Some types of fast-swimming fish have a horizontal caudal keel just forward of the tail fin. Much like the keel of a ship, this is a lateral ridge on the caudal peduncle, usually composed of scutes (see below), that provides stability and support to the caudal fin. There may be a single paired keel, one on each side, or two pairs above and below.

    Finlets are small fins, generally behind the dorsal and anal fins (in bichirs, there are only finlets on the dorsal surface and no dorsal fin). In some fish such as tuna or sauries, they are rayless, non-retractable, and found between the last dorsal and/or anal fin and the caudal fin.

    Bony fishes form a taxonomic group called Osteichthyes. They have skeletons made of bone, and can be contrasted with cartilaginous fishes which have skeletons made of cartilage. Bony fishes are divided into ray-finned and lobe-finned fish. Most fish are ray-finned, an extremely diverse and abundant group consisting of over 30,000 species. It is the largest class of vertebrates in existence today. In the distant past, lobe-finned fish were abundant. Nowadays they are mainly extinct, with only eight living species. Bony fish have fin spines and rays called lepidotrichia. They typically have swim bladders, which allows the fish to create a neutral balance between sinking and floating without having to use its fins. However, swim bladders are absent in many fish, most notably in Lungfishes, which are the only fish to have retained the primitive lung present in the common ancestor of bony fish from which swim bladders evolved. Bony fishes also have an operculum, which helps them breathe without having to use fins to swim.

    Lobe-fins Edit

    Lobe-finned fishes form a class of bony fishes called Sarcopterygii. They have fleshy, lobed, paired fins, which are joined to the body by a single bone. [11] The fins of lobe-finned fish differ from those of all other fish in that each is borne on a fleshy, lobelike, scaly stalk extending from the body. Pectoral and pelvic fins have articulations resembling those of tetrapod limbs. These fins evolved into legs of the first tetrapod land vertebrates, amphibians. They also possess two dorsal fins with separate bases, as opposed to the single dorsal fin of ray-finned fish.

    The coelacanth is a lobe-finned fish which is still extant. It is thought to have evolved into roughly its current form about 408 million years ago, during the early Devonian. [12] Locomotion of the coelacanths is unique to their kind. To move around, coelacanths most commonly take advantage of up or downwellings of the current and drift. They use their paired fins to stabilize their movement through the water. While on the ocean floor their paired fins are not used for any kind of movement. Coelacanths can create thrust for quick starts by using their caudal fins. Due to the high number of fins they possess, coelacanths have high maneuverability and can orient their bodies in almost any direction in the water. They have been seen doing headstands and swimming belly up. It is thought that their rostral organ helps give the coelacanth electroperception, which aids in their movement around obstacles. [13]

    Lungfish are also living lobe-finned fish. They occur in Africa (Protopterus), Australia (Neoceratodus), and South America (Lepidosiren).

    Diversity of fins in lobe-finned fishes Edit

    Ray-fins Edit

    Ray-finned fishes form a class of bony fishes called Actinopterygii. Their fins contain spines or rays. A fin may contain only spiny rays, only soft rays, or a combination of both. If both are present, the spiny rays are always anterior. Spines are generally stiff and sharp. Rays are generally soft, flexible, segmented, and may be branched. This segmentation of rays is the main difference that separates them from spines spines may be flexible in certain species, but they will never be segmented.

    Spines have a variety of uses. In catfish, they are used as a form of defense many catfish have the ability to lock their spines outwards. Triggerfish also use spines to lock themselves in crevices to prevent them being pulled out.

    Lepidotrichia are usually composed of bone, but in early osteichthyans such as Cheirolepis, there was also dentine and enamel. [14] They are segmented and appear as a series of disks stacked one on top of another. They may have been derived from dermal scales. [14] The genetic basis for the formation of the fin rays is thought to be genes coded for the production of certain proteins. It has been suggested that the evolution of the tetrapod limb from lobe-finned fishes is related to the loss of these proteins. [15]

    Diversity of fins in ray-finned fishes Edit

    Diaphanous hatchetfish Sternoptyx diaphana

    Stellate pufferfish Arothron stellatus

    Cusk-eel Benthocometes robustus

    Shortbill spearfish Tetrapturus angustirostris

    Ghost knifefish Sternarchorhynchus oxyrhynchus

    Cartilaginous fishes form a class of fishes called Chondrichthyes. They have skeletons made of cartilage rather than bone. The class includes sharks, rays and chimaeras. Shark fin skeletons are elongated and supported with soft and unsegmented rays named ceratotrichia, filaments of elastic protein resembling the horny keratin in hair and feathers. [16] Originally the pectoral and pelvic girdles, which do not contain any dermal elements, did not connect. In later forms, each pair of fins became ventrally connected in the middle when scapulocoracoid and puboischiadic bars evolved. In rays, the pectoral fins have connected to the head and are very flexible. One of the primary characteristics present in most sharks is the heterocercal tail, which aids in locomotion. [17] Most sharks have eight fins. Sharks can only drift away from objects directly in front of them because their fins do not allow them to move in the tail-first direction. [18]

    As with most fish, the tails of sharks provide thrust, making speed and acceleration dependent on tail shape. Caudal fin shapes vary considerably between shark species, due to their evolution in separate environments. Sharks possess a heterocercal caudal fin in which the dorsal portion is usually noticeably larger than the ventral portion. This is because the shark's vertebral column extends into that dorsal portion, providing a greater surface area for muscle attachment. This allows more efficient locomotion among these negatively buoyant cartilaginous fish. By contrast, most bony fish possess a homocercal caudal fin. [19]

    Tiger sharks have a large upper lobe, which allows for slow cruising and sudden bursts of speed. The tiger shark must be able to twist and turn in the water easily when hunting to support its varied diet, whereas the porbeagle shark, which hunts schooling fish such as mackerel and herring, has a large lower lobe to help it keep pace with its fast-swimming prey. [20] Other tail adaptations help sharks catch prey more directly, such as the thresher shark's usage of its powerful, elongated upper lobe to stun fish and squid.

    Shark finning Edit

    According to the Humane Society International, approximately 100 million sharks are killed each year for their fins, in an act known as shark finning. [21] After the fins are cut off, the mutilated sharks are thrown back in the water and left to die.

    In some countries of Asia, shark fins are a culinary delicacy, such as shark fin soup. [22] Currently, international concerns over the sustainability and welfare of sharks have impacted consumption and availability of shark fin soup worldwide. [23] Shark finning is prohibited in many countries.

    Generating thrust Edit

    Foil shaped fins generate thrust when moved, the lift of the fin sets water or air in motion and pushes the fin in the opposite direction. Aquatic animals get significant thrust by moving fins back and forth in water. Often the tail fin is used, but some aquatic animals generate thrust from pectoral fins. [24]

    Cavitation occurs when negative pressure causes bubbles (cavities) to form in a liquid, which then promptly and violently collapse. It can cause significant damage and wear. [25] Cavitation damage can occur to the tail fins of powerful swimming marine animals, such as dolphins and tuna. Cavitation is more likely to occur near the surface of the ocean, where the ambient water pressure is relatively low. Even if they have the power to swim faster, dolphins may have to restrict their speed because collapsing cavitation bubbles on their tail are too painful. [26] Cavitation also slows tuna, but for a different reason. Unlike dolphins, these fish do not feel the bubbles, because they have bony fins without nerve endings. Nevertheless, they cannot swim faster because the cavitation bubbles create a vapor film around their fins that limits their speed. Lesions have been found on tuna that are consistent with cavitation damage. [26]

    Scombrid fishes (tuna, mackerel and bonito) are particularly high-performance swimmers. Along the margin at the rear of their bodies is a line of small rayless, non-retractable fins, known as finlets. There has been much speculation about the function of these finlets. Research done in 2000 and 2001 by Nauen and Lauder indicated that "the finlets have a hydrodynamic effect on local flow during steady swimming" and that "the most posterior finlet is oriented to redirect flow into the developing tail vortex, which may increase thrust produced by the tail of swimming mackerel". [27] [28] [29]

    Fish use multiple fins, so it is possible that a given fin can have a hydrodynamic interaction with another fin. In particular, the fins immediately upstream of the caudal (tail) fin may be proximate fins that can directly affect the flow dynamics at the caudal fin. In 2011, researchers using volumetric imaging techniques were able to generate "the first instantaneous three-dimensional views of wake structures as they are produced by freely swimming fishes". They found that "continuous tail beats resulted in the formation of a linked chain of vortex rings" and that "the dorsal and anal fin wakes are rapidly entrained by the caudal fin wake, approximately within the timeframe of a subsequent tail beat". [30]

    Controlling motion Edit

    Once motion has been established, the motion itself can be controlled with the use of other fins. [24] [31]

    The bodies of reef fishes are often shaped differently from open water fishes. Open water fishes are usually built for speed, streamlined like torpedoes to minimise friction as they move through the water. Reef fish operate in the relatively confined spaces and complex underwater landscapes of coral reefs. For this manoeuvrability is more important than straight line speed, so coral reef fish have developed bodies which optimize their ability to dart and change direction. They outwit predators by dodging into fissures in the reef or playing hide and seek around coral heads. [35] The pectoral and pelvic fins of many reef fish, such as butterflyfish, damselfish and angelfish, have evolved so they can act as brakes and allow complex manoeuvres. [37] Many reef fish, such as butterflyfish, damselfish and angelfish, have evolved bodies which are deep and laterally compressed like a pancake, and will fit into fissures in rocks. Their pelvic and pectoral fins have evolved differently, so they act together with the flattened body to optimise manoeuvrability. [35] Some fishes, such as puffer fish, filefish and trunkfish, rely on pectoral fins for swimming and hardly use tail fins at all. [37]

    Reproduction Edit

    Male cartilaginous fishes (sharks and rays), as well as the males of some live-bearing ray finned fishes, have fins that have been modified to function as intromittent organs, reproductive appendages which allow internal fertilization. In ray finned fish they are called gonopodia or andropodia, and in cartilaginous fish they are called claspers.

    Gonopodia are found on the males of some species in the Anablepidae and Poeciliidae families. They are anal fins that have been modified to function as movable intromittent organs and are used to impregnate females with milt during mating. The third, fourth and fifth rays of the male's anal fin are formed into a tube-like structure in which the sperm of the fish is ejected. [40] When ready for mating, the gonopodium becomes erect and points forward towards the female. The male shortly inserts the organ into the sex opening of the female, with hook-like adaptations that allow the fish to grip onto the female to ensure impregnation. If a female remains stationary and her partner contacts her vent with his gonopodium, she is fertilized. The sperm is preserved in the female's oviduct. This allows females to fertilize themselves at any time without further assistance from males. In some species, the gonopodium may be half the total body length. Occasionally the fin is too long to be used, as in the "lyretail" breeds of Xiphophorus helleri. Hormone treated females may develop gonopodia. These are useless for breeding.

    Similar organs with similar characteristics are found in other fishes, for example the andropodium in the Hemirhamphodon or in the Goodeidae [41] or the gonopodium in the Middle Triassic † Saurichthys, the oldest known example of viviparity in a ray-finned fish. [42]

    Claspers are found on the males of cartilaginous fishes. They are the posterior part of the pelvic fins that have also been modified to function as intromittent organs, and are used to channel semen into the female's cloaca during copulation. The act of mating in sharks usually includes raising one of the claspers to allow water into a siphon through a specific orifice. The clasper is then inserted into the cloaca, where it opens like an umbrella to anchor its position. The siphon then begins to contract expelling water and sperm. [43] [44]

    Other functions Edit

    Other uses of fins include walking and perching on the sea floor, gliding over water, cooling of body temperature, stunning of prey, display (scaring of predators, courtship), defence (venomous fin spines, locking between corals), luring of prey, and attachment structures.

    The Indo-Pacific sailfish has a prominent dorsal fin. Like scombroids and other billfish, they streamline themselves by retracting their dorsal fins into a groove in their body when they swim. [45] The huge dorsal fin, or sail, of the sailfish is kept retracted most of the time. Sailfish raise them if they want to herd a school of small fish, and also after periods of high activity, presumably to cool down. [45] [46]

    The oriental flying gurnard has large pectoral fins which it normally holds against its body, and expands when threatened to scare predators. Despite its name, it is a demersal fish, not a flying fish, and uses its pelvic fins to walk along the bottom of the ocean. [48] [49]

    Fins can have an adaptive significance as sexual ornaments. During courtship, the female cichlid, Pelvicachromis taeniatus, displays a large and visually arresting purple pelvic fin. "The researchers found that males clearly preferred females with a larger pelvic fin and that pelvic fins grew in a more disproportionate way than other fins on female fish." [50] [51]

    Evolution of paired fins Edit

    There are two prevailing hypotheses that have been historically debated as models for the evolution of paired fins in fish: the gill arch theory and the lateral fin-fold theory. The former, commonly referred to as the “Gegenbaur hypothesis,” was posited in 1870 and proposes that the “paired fins are derived from gill structures”. [53] This fell out of popularity in favor of the lateral fin-fold theory, first suggested in 1877, which proposes that paired fins budded from longitudinal, lateral folds along the epidermis just behind the gills. [54] There is weak support for both hypotheses in the fossil record and in embryology. [55] However, recent insights from developmental patterning have prompted reconsideration of both theories in order to better elucidate the origins of paired fins.

    Classical theories Edit

    Karl Gegenbaur's concept of the “Archipterygium” was introduced in 1876. [56] It was described as a gill ray, or “joined cartilaginous stem,” that extended from the gill arch. Additional rays arose from along the arch and from the central gill ray. Gegenbaur suggested a model of transformative homology – that all vertebrate paired fins and limbs were transformations of the Archipterygium. Based on this theory, paired appendages such as pectoral and pelvic fins would have differentiated from the branchial arches and migrated posteriorly. However, there has been limited support for this hypothesis in the fossil record both morphologically and phylogenically. [55] In addition, there was little to no evidence of an anterior-posterior migration of pelvic fins. [57] Such shortcomings of the gill-arch theory led to its early demise in favor of the lateral fin-fold theory proposed by St. George Jackson Mivart, Francis Balfour, and James Kingsley Thacher.

    The lateral fin-fold theory hypothesized that paired fins developed from lateral folds along the body wall of the fish. [54] Just as segmentation and budding of the median fin fold gave rise to the median fins, a similar mechanism of fin bud segmentation and elongation from a lateral fin fold was proposed to have given rise to the paired pectoral and pelvic fins. However, there was little evidence of a lateral fold-to-fin transition in the fossil record. [58] In addition, it was later demonstrated phylogenically that pectoral and pelvic fins arise from distinct evolutionary and mechanistic origins. [55]

    Evolutionary developmental biology Edit

    Recent studies in the ontogeny and evolution of paired appendages have compared finless vertebrates – such as lampreys – with chondricthyes, the most basal living vertebrate with paired fins. [59] In 2006, researchers found that the same genetic programming involved in the segmentation and development of median fins was found in the development of paired appendages in catsharks. [60] Although these findings do not directly support the lateral fin-fold hypothesis, the original concept of a shared median-paired fin evolutionary developmental mechanism remains relevant.

    A similar renovation of an old theory may be found in the developmental programming of chondricthyan gill arches and paired appendages. In 2009, researchers at the University of Chicago demonstrated that there are shared molecular patterning mechanisms in the early development of the chondricthyan gill arch and paired fins. [61] Findings such as these have prompted reconsideration of the once-debunked gill-arch theory. [58]

    From fins to limbs Edit

    Fish are the ancestors of all mammals, reptiles, birds and amphibians. [62] In particular, terrestrial tetrapods (four-legged animals) evolved from fish and made their first forays onto land 400 million years ago. [63] They used paired pectoral and pelvic fins for locomotion. The pectoral fins developed into forelegs (arms in the case of humans) and the pelvic fins developed into hind legs. [64] Much of the genetic machinery that builds a walking limb in a tetrapod is already present in the swimming fin of a fish. [65] [66]

    In 2011, researchers at Monash University in Australia used primitive but still living lungfish "to trace the evolution of pelvic fin muscles to find out how the load-bearing hind limbs of the tetrapods evolved." [68] [69] Further research at the University of Chicago found bottom-walking lungfishes had already evolved characteristics of the walking gaits of terrestrial tetrapods. [70] [71]

    In a classic example of convergent evolution, the pectoral limbs of pterosaurs, birds and bats further evolved along independent paths into flying wings. Even with flying wings there are many similarities with walking legs, and core aspects of the genetic blueprint of the pectoral fin have been retained. [72] [73]

    The first mammals appeared during the Permian period (between 298.9 and 252.17 million years ago). Several groups of these mammals started returning to the sea, including the cetaceans (whales, dolphins and porpoises). Recent DNA analysis suggests that cetaceans evolved from within the even-toed ungulates, and that they share a common ancestor with the hippopotamus. [74] [75] About 23 million years ago another group of bearlike land mammals started returning to the sea. These were the seals. [76] What had become walking limbs in cetaceans and seals evolved independently into new forms of swimming fins. The forelimbs became flippers, while the hindlimbs were either lost (cetaceans) or also modified into flipper (pinnipeds). In cetaceans, the tail gained two fins at the end, called a fluke. [77] Fish tails are usually vertical and move from side to side. Cetacean flukes are horizontal and move up and down, because cetacean spines bend the same way as in other mammals. [78] [79]

    Ichthyosaurs are ancient reptiles that resembled dolphins. They first appeared about 245 million years ago and disappeared about 90 million years ago.

    "This sea-going reptile with terrestrial ancestors converged so strongly on fishes that it actually evolved a dorsal fin and tail fin for improved aquatic locomotion. These structures are all the more remarkable because they evolved from nothing — the ancestral terrestrial reptile had no hump on its back or blade on its tail to serve as a precursor." [80]

    The biologist Stephen Jay Gould said the ichthyosaur was his favorite example of convergent evolution. [81]

    Fins or flippers of varying forms and at varying locations (limbs, body, tail) have also evolved in a number of other tetrapod groups, including diving birds such as penguins (modified from wings), sea turtles (forelimbs modified into flippers), mosasaurs (limbs modified into flippers), and sea snakes (vertically expanded, flattened tail fin).

    In the 1990s, the CIA built a robotic catfish called Charlie, designed to collect underwater intelligence undetected [82]
    External video
    Charlie the catfish - CIA video
    AquaPenguin - Festo, YouTube
    AquaRay - Festo, YouTube
    AquaJelly - Festo, YouTube
    AiraCuda - Festo, YouTube

    The use of fins for the propulsion of aquatic animals can be remarkably effective. It has been calculated that some fish can achieve a propulsive efficiency greater than 90%. [24] Fish can accelerate and maneuver much more effectively than boats or submarine, and produce less water disturbance and noise. This has led to biomimetic studies of underwater robots which attempt to emulate the locomotion of aquatic animals. [83] An example is the Robot Tuna built by the Institute of Field Robotics, to analyze and mathematically model thunniform motion. [84] In 2005, the Sea Life London Aquarium displayed three robotic fish created by the computer science department at the University of Essex. The fish were designed to be autonomous, swimming around and avoiding obstacles like real fish. Their creator claimed that he was trying to combine "the speed of tuna, acceleration of a pike, and the navigating skills of an eel." [85] [86] [87]

    The AquaPenguin, developed by Festo of Germany, copies the streamlined shape and propulsion by front flippers of penguins. [88] [89] Festo also developed AquaRay, [90] AquaJelly [91] and AiraCuda, [92] respectively emulating the locomotion of manta rays, jellyfish and barracuda.

    In 2004, Hugh Herr at MIT prototyped a biomechatronic robotic fish with a living actuator by surgically transplanting muscles from frog legs to the robot and then making the robot swim by pulsing the muscle fibers with electricity. [93] [94]

    Robotic fish offer some research advantages, such as the ability to examine an individual part of a fish design in isolation from the rest of the fish. However, this risks oversimplifying the biology so key aspects of the animal design are overlooked. Robotic fish also allow researchers to vary a single parameter, such as flexibility or a specific motion control. Researchers can directly measure forces, which is not easy to do in live fish. "Robotic devices also facilitate three-dimensional kinematic studies and correlated hydrodynamic analyses, as the location of the locomotor surface can be known accurately. And, individual components of a natural motion (such as outstroke vs. instroke of a flapping appendage) can be programmed separately, which is certainly difficult to achieve when working with a live animal." [95]


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