What external signals do tell catadromous fish to go from the sea to the rivers?

What external signals do tell catadromous fish to go from the sea to the rivers?

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What signals do tell catadromous fish to go from the sea to the rivers? Is it photoperiodicity? Are there any other signals?

Overfishing: The most serious threat to our oceans

Of all the threats facing the oceans today, overfishing takes the greatest toll on sea life — and people.

What is overfishing?
Overfishing is catching too many fish at once, so the breeding population becomes too depleted to recover. Overfishing often goes hand in hand with wasteful types of commercial fishing that haul in massive amounts of unwanted fish or other animals, which are then discarded.

As a result of prolonged and widespread overfishing, nearly a third of the world's assessed fisheries are now in deep trouble — and that's likely an underestimate, since many fisheries remain unstudied.

Why does overfishing matter?
Overfishing endangers ocean ecosystems and the billions of people who rely on seafood as a key source of protein. Without sustainable management, our fisheries face collapse — and we face a food crisis.

What leads to overfishing?
Poor fishing management is the primary cause. Around the world, many fisheries are governed by rules that make the problem worse, or have no rules at all.

What's the alternative?
With smarter management systems, known as fishing rights, we can reverse the incentives that lead to overfishing. Under fishing rights, fishermen's interests are tied to the long-term health of a fishery. Their income improves along with the fish population.

Does it work?
Yes. In Belize, Denmark, Namibia, the United States and elsewhere, fishing rights have helped transform struggling fisheries. In the Gulf of Mexico, red snapper populations are three times what they were in 2007 when we helped reform that fishery. Over the next five years, we are working to ensure that sustainable fishing is firmly established in the U.S. and other countries.


Food and Agriculture Organization. 2010. The State of World Fisheries and Aquaculture 2010. [page 2 overview]

Heal, G. and W. Schlenker. 2008. Sustainable fisheries. Nature 455: 1044-1045

What external signals do tell catadromous fish to go from the sea to the rivers? - Biology

A Quick Course in Ichthyology

by Jason Buchheim
Director, Odyssey Expeditions

  • FISH Definition
  • FISHES- class agnatha
  • FISHES- Class Chondrichthyes
    • Shark Attack
    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.


    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!


    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').

    Marine Biology resources by Odyssey Expeditions Tropical Marine Biology Voyages

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    Observations of initial water ingestion and ion absorption in the digestive tract of Japanese eel larvae

    The onset of osmoregulation for seawater adaptation was examined during the early life stages of the Japanese eel. Ingested seawater was detected in the digestive tract by using fluorescent dextran as an inert marker. Ingested seawater remained in the forepart of the digestive tract at 0 and 1 days post hatching (dph), but reached the anus at 2 dph. Scanning electron microscope observations showed that the mouth appeared as a slit at 0 dph and developed into a hole-like shape at 2 dph. Expressions of Na + , K + , and 2Cl − cotransporter 2β (NKCC2β), and Na + and Cl − cotransporter β (NCCβ) mRNAs were detected mainly in the intestine and rectum, respectively. These results are consistent with those of the adult eel, suggesting that the intestine and rectum are the sites of active ion absorption in larvae also. Expression levels of NKCC2β steeply increased from 4 to 6 dph, while NCCβ levels were highest on the day of hatching, presumably due to a maternal factor. The expression levels of NCCβ decreased by 3–4 dph and then increased to a constant status at 7 dph. Our findings confirmed that osmoregulation started as early as the day of hatching and stabilized within a week.

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    Animal Navigation

    Students discuss the navigation methods of migratory animals. Then they watch videos, draw mental maps, and make connections between their maps and how migratory animals use mental maps and other cues.

    Biology, Ecology, Experiential Learning, Geography, Physical Geography

    This lists the logos of programs or partners of NG Education which have provided or contributed the content on this page. Program

    1. Discuss how animals navigate.
    Ask: If you want to go somewhere that you have never visited before, how do you find your way? Write students’ responses on the board. Explain to students that animals navigate in a similar way. But animals do not have a compass, GPS, street signs, or maps. Ask: How do animals navigate then? Tell students that animals use environmental cues, instincts, and internal cues to help them navigate. Provide students with an example of animal navigation: the monarch butterfly flies thousands of kilometers or miles over lands it has never seen. Monarchs and other migratory species use a complex combination of navigational aids that scientists do not yet fully understand. Ask: Why is it important to understand how animals navigate?

    2. Give students a "curiosity quiz" about animal navigation methods.
    Write the following list on the board: 1) genetics 2) mental maps 3) instinct 4) sun and moon 5) stars 6) smell 7) magnetic field 8) communication and signaling among individuals 9) ocean currents. Ask students to number a blank piece of paper 1-9. Ask them to write H for human method, W for wild animal method, or B for both. Tell students they will have an opportunity to check and revise their answers later in this activity.

    3. Build background about migratory animal navigation methods.
    Read aloud the information below about navigation methods used by migratory animals. As you read, have students list additional examples or questions they think of on the same piece of paper as their curiosity quiz.

    • Genetics—some scientists believe that migratory animals genetically inherit migratory routes from their parents.
    • Mental maps—rather than a paper map, a mental map is carried in the mind and includes known landmarks, such as rivers, trees, and mountains. Simple migrations, such as altitudinal migrations (up and down a mountain) can be navigated with a mental map.
    • Instinct—instinct also helps animals with simple migrations. For example, gray whales mostly follow the west coast of Canada and the United States as they migrate between Alaska and Baja, Mexico. Dolphins follow the topography of the ocean floor.
    • Sun and Moon—some animals follow the sun as it crosses the sky from east to west. Starlings orient themselves using the path of the sun. Clouds, time of year, and moving at night can make it impractical to use the sun as the only cue for direction.
    • Stars—hundreds of years ago, explorers used the stars to navigate their course as they traveled over land and sea. Animals use stars, such as Betelgeuse and the North Star, most likely because those stars are very bright and often visible. Using the stars, Mallard ducks can find north.
    • Smell—over small distances, or at specific locations on a migratory path, scents can help animals find their way. For example, salmon use scents in rivers to find spawning areas to lay their own eggs—in the same area where they were hatched. Scientists think wildebeest follow the scent of rain on the dry Serengeti soils to reach greener pastures.
    • Magnetic field—the Earth has a magnetic field, and although humans usually cannot detect it without a compass, some animals have the ability to detect and use it for their migrations. It helps them know which way is north. Scientists are not sure exactly how animals use the magnetic field, but it’s similar to humans using a compass to find magnetic north.
    • Communication and signaling among individuals—some animals that migrate in groups communicate as they travel to help with navigation. For example, whales use sound to tell each other where they are and where they are headed.
    • Ocean currents—some animals can use ocean currents to navigate to and from breeding or feeding grounds. Some eggs, larvae, and young fish drift passively with ocean currents. Some adult fish migrate to breeding grounds by deliberately moving against ocean currents.

    4. Watch video clips from Great Migrations.
    Show students the video clips “Red Crab Eggs,” “Wildebeest Migration,” and “Sperm Whale Migration.” Ask:

    • Which navigational method(s) does each species use?
    • Do you think any of the species’ routes are more difficult to navigate than others? Explain.

    5. Have students draw mental maps of a familiar place.
    Tell students that both humans and migratory animals use mental maps. Have students draw mental maps showing areas they are very familiar with, such as the area between students’ homes and your school, or between two well-known, local landmarks. Ask students to include a title, symbols for landmarks, roads, and any other relevant symbols. Then ask them to include a legend that explains those symbols.

    6. Have students share their maps and discuss how migratory animals use mental maps.

    Have students share their maps with each other. Ask:

    • How are your maps similar? How are they different?
    • Are any two maps exactly the same? Why or why not?
    • How are your mental maps different from those of migrating animals?
    • What is the farthest you have ever been from home? How accurate would your mental map be if you needed to use it to get to that faraway place?
    • How do you think migratory animals use their mental maps for faraway places where they travel?

    Informal Assessment

    As a class, discuss students' answers from the curiosity quiz in Step 2. Encourage students to use information from the activity to confirm or revise their answers. As you discuss each item, ask students to give additional examples of how humans, wildlife, or both use a particular navigation method.

    Extending the Learning

    Ask: Do you think humans have an internal compass like migratory animals? Do you think you can accurately find north if you are lost? As a class, go outside or into a large indoor room with open space, such as a gym. Use a compass to determine which direction is north. Then have students work in pairs, taking turns being blindfolded and being the leader. Ask each leader to help the blindfolded student “get lost.” After about one minute, have leaders remove the blindfolds and ask the “lost” student if he or she can correctly identify north. Have each pair take 3-5 turns being blindfolded, and see how many times students can correctly identify north. Have each pair tally their data. As a class, discuss how accurately students could locate north. Put all of the class data into a chart to analyze. Ask: Was your original assumption correct? How do we use our internal compass? Do you think internal navigation cues are still important for humans even though we have technology, such as GPS, street signs, and maps that helps us figure out where we are going? Why or why not?

    How Jellyfish Work

    Jellyfish are probably some of the most unusual and mysterious creatures that you'll ever encounter. With their gelatinous bodies and dangling tentacles, they look more like something from a horror movie than a real animal. But if you can get past the weirdness -- and the fact that getting too close to one results in a nasty sting -- you'll discover that jellyfish are pretty fascinating. They've been around for more than 650 million years, and there are thousands of different species, with more species discovered all of the time.

    In this article, we'll learn all about these mysterious animals and find out what to do if you do happen to get in the way of a stinging jellyfish tentacle.

    Jellyfish live mainly in the ocean, but they aren't actually fish -- they're plankton. These plants and animals either float in the water or possess such limited swimming powers that currents control their horizontal movements. Some plankton are microscopic, single-celled organisms, while others are several feet long. Jellyfish can range in size from less than an inch to nearly 7 feet long, with tentacles up to 100 feet long.

    Jellyfish are also members of the phylum Cnidaria, (from the Greek word for "stinging nettle") and the class Scyphozoa (from the Greek word for "cup," referring to the jellyfish's body shape). All cnidarians have a mouth in the center of their bodies, surrounded by tentacles. The jellyfish's cnidarian relatives include corals, sea anemones and the Portuguese man-o'-war.

    Jellyfish are about 98 percent water. If a jellyfish washes up on the beach, it will mostly disappear as the water evaporates. Most are transparent and bell-shaped. Their bodies have radial symmetry, which means that the body parts extend from a central point like the spokes on a wheel. If you cut a jellyfish in half at any point, you'll always get equal halves. Jellyfish have very simple bodies -- they don't have bones, a brain or a heart. To see light, detect smells and orient themselves, they have rudimentary sensory nerves at the base of their tentacles.

    A jellyfish's body generally comprises six basic parts:

    • The epidermis, which protects the inner organs
    • The gastrodermis, which is the inner layer
    • The mesoglea, or middle jelly, between the epidermis and gastrodermis
    • The gastrovascular cavity, which functions as a gullet, stomach, and intestine all in one
    • An orifice that functions as both the mouth and anus
    • Tentacles that line the edge of the body

    An adult jellyfish is a medusa (plural: medusae), named after Medusa, the mythological creature with snakes for hair who could turn humans to stone with a glance. After the male releases its sperm through its orifice into the water, the sperm swim into the female's orifice and fertilize the eggs.

    Several dozen jellyfish larvae can hatch at once. They eventually float out on the currents and look for a solid surface on which to attach, such as a rock. When they attach they become polyps -- hollow cylinders with a mouth and tentacles at the top. The polyps later bud into young jellyfish called ephyrae. After a few weeks, the jellyfish float away and grow into mature medusae. A medusa can live for about three to six months.

    It sounds like something out of an old "Godzilla" movie: Giant sea monsters have invaded Japanese waters. They're 6 feet long and weigh up to 450 pounds. They have wreaked havoc on the country's fishing industry, and inflicted at least a few deadly stings on humans. They were even responsible for temporarily shutting a nuclear power plant after they lodged in its cooling system. These creatures were jellyfish, which the Japanese call echizen kurage. Some experts blame the influx of jellyfish on heavy rains in China, which they say drove the sea creatures into Japanese waters. Fortunately, the Japanese have found a use for many of the enormous jellyfish they've caught: dried, salted jellyfish, anyone?

    Economic Importance for Humans

    European sea bass is a famous sport fish, rated by British fishermen as their best fighting fish. It’s an equally important species for sport fishermen and commercial fishermen. There is a strong international market for European sea bass and high prices are paid for them.

    Breeding European Sea Bass

    Intensive breeding of sea bass follows a complex process that was the subject of prolonged scientific research programs during the 1960s and 1970s.

    Its improvement allowed the beginning of the sea bass’ aquaculture of sea bass (and also of sea breams) in the Mediterranean in the ‘80. The hatchery has highly technical aspects and requires a staff with a high level of training, as it’s necessary to monitor that larvae grow in good conditions, guarantee the optimal functioning of the recirculation system, produce food, etc. All this led to the specialization of this first stage of the breeding process.

    Although there are cases of vertical integration, European farms are generally independent and sell the fry to fattening farms. Breeding generally develops in three stages:

    Larvae Cultivation

    The larva loses its yolk sac six days after leaving the egg. At that time it receives a very specific diet, first based on algae and rotifers (a microscopic zooplankton) and then, when its size allows, based on brine shrimp

    Broken up

    After around 40 to 50 days, the larvae move to the broken-up unit, where they gradually become accustomed to a diet very rich in protein, especially based on oil and fishmeal. This food, which is administered in the form of tiny granules, is very similar to the one that will receive the sea bass during the rest of its breeding. It is this protein regime, as well as water quality, that maximize larval growth and survival during these crucial first months.

    Breeding of fry:

    Between 3 and 4 weeks later, fry move to the fry breeding unit where they are fed with granules until reaching, (about two months later), a weight of 2 to 5 g, which will allow them to go to the fattening phase.


    The purchase of fry in the hatcheries represents one of the biggest investments of the farms. The fattening takes place in floating cages installed a short distance from the coast, at least in large part of European production (that is, the Mediterranean and the Canary Islands).

    There are also farms that raise the sea bass in tanks located on land, usually

    fed by a recirculation system, which allows controlling the water’s temperature and raising sea bass in more northern latitudes. The seabass are fed with granules composed mainly of fishmeal and fish oil, but also of vegetable extracts.

    In freedom, the sea bass can reach a meter in length and 12 kg in weight, but the seabass of aquaculture is harvested and sacrificed generally when reaching between 300 and 500 g, for which they must pass from one year and a half to two years, in Water temperature function.

    It should also be noted, , that there are still some semi-intensive breeding centers, derived from traditional extensive aquaculture, in which hatchery fry are introduced in coastal lagoons and lagoons and are fattened with an industrial food supplement.

    Let’s watch how they breed them

    Cephalopod camouflage

    Squids, like all cephalopods, are capable of glowing (bioluminescence) as well as changing their skin color. This camouflaging capacity enables them to hide from predators while the bioluminescence allows them to communicate with and/or attract a mate. This complex behavior is produced by a network specialized skin cells and muscles.

    Researchers at the University of Houston have developed a similar device capable of detecting its surrounds and matching this environment in mere seconds. This early prototype uses a flexible, pixelated grid utilizing actuators, light sensors, and reflectors. As the light sensors detect a a change in the surroundings, a signal is sent to the corresponding diode. This creates heat in the area and the thermochromatic grid then changes color. This artificial “skin” could have both military and commercial applications down the road.


    There are many sources of anthropogenic sound in the sea, lakes and rivers, with quite different acoustical characteristics (Hawkins et al., 2015 Popper et al., 2014 ). Many commercial human activities introduce sound, either intentionally for a specific purpose, such as seismic surveys, or unintentionally as a by-product of activities such as shipping and offshore and even onshore construction work. Coastal areas and areas where a high degree of human activity takes place, may be quite noisy including harbours and shipping lanes. However, some high-intensity sources of underwater sound, such as pile drivers and seismic airguns, can be detected over distances of several thousand kilometres. Thus, effects upon fishes may occur well away from the source itself.

    There are two main classes of anthropogenic sound. Some sounds are transient or impulsive, while others are continuous. Impulsive sounds are often of short duration (generally well less than 1 s) and may show large changes in amplitude over their time course. They can either be single or repetitive. Examples of such sounds are those produced by seismic airguns, pile driving and underwater explosions. (Various anthropogenic sounds can be heard at: Most often, such sounds are only present over the course of a particular project and then end.

    Continuous sounds are produced by shipping (both commercial and pleasure boats), operational wind turbines, seabed drilling etc. and may continue for months or even years (e.g., in a harbour or wind farm). A few of these, described below, are perhaps the most ubiquitous sounds potentially affecting fishes over the widest geographic areas. Sonar systems, while used very widely, generally operate within frequency ranges that are not detectable by fishes (Halvorsen et al., 2012d Popper et al., 2007 ).

    7.1 Seismic airguns

    Airguns are impulsive sources used for seismic exploration for sub-sea gas and oil reserves as well as for geological research (Gisiner, 2016 ). These devices use compressed air to produce a gas bubble which expands rapidly when released, creating a high intensity impulsive sound, primarily composed of energy below 200 Hz, but with the bulk of the sound from 20 to 50 Hz (Mattsson et al., 2012 ). The sounds are directed downward into the seabed, though there is also some spreading laterally and they are reflected from various geological formations and then detected by a long array of hydrophones towed by the seismic vessel (see Gisiner, 2016 for a detailed description of seismic surveys).

    7.2 Impact pile driving

    Impact pile driving is widely used for the construction of bridges, harbours, wind farms and other offshore structures (Dahl et al., 2015 Popper & Hastings, 2009 ). Striking by the hammer results in vibration of the pile in water and in the substrate, thereby generating sounds that potentially affect nearby animals (Dahl et al., 2015 Hazelwood & Macey, 2016 ). The sounds produced by pile driving are impulsive, short (of the order of μs) and most of their energy lies below 500 Hz, though some energy may extend up to 1 kHz (Dahl et al., 2015 ). The sound levels (both sound pressure and particle motion) vary substantially, depending on numerous factors such as pile diameter, hammer size, substrate characteristics, etc. The sounds produced by pile drivers are often very intense with SELss often well-exceeding 180 to 200 dB re 1 μPa 2 s −1 and with very sharp rise times.

    7.3 Other industrial activities

    Many other industrial activities contribute to underwater noise. Such activities generally produce sound that has the most energy at low frequencies (i.e., <1 kHz). Dredging, for example produces high levels of broadband noise (de Jong et al., 2016 Wenger et al., 2017 ) and is used to extract sand and gravel from the seabed and from lakes, maintain shipping lanes and to install pipelines and cables within the seabed. Activities onshore, including the passage of vehicles, may increase noise levels in the sea, lakes and rivers, especially if they generate substrate vibration.

    7.4 Operating wind turbines

    Since c. 2000 there has been an enormous increase in the generation of electricity by wind farms located in coastal waters, especially in European seas. There is some concern that sounds from operating offshore wind turbines might affect fish behaviour, although the sounds generated are very different to those generated during wind-farm construction (Cheesman, 2016 ). Most sound from a wind turbine is concentrated in a narrow band, centred around 180 Hz and the sounds are generally below about 700 Hz (Madsen et al., 2006 Pangerc et al., 2016 ). However, there is also a particle motion component to the sounds generated by wind turbines, accompanying substrate transmission (Sigray & Andersson, 2012 P. Gopu and J. Miller, personal communication, 2018), although this has rarely been monitored and has often been ignored. There is currently limited information available on the acoustic characteristics of offshore turbines, including those utilising tidal and wave energy (Lossent et al., 2018 Schramm et al., 2017 ).

    7.5 Vessel noise

    A significant proportion of anthropogenic noise in the ocean and other water bodies is created by motorised vessels, including large ships, fishing and pleasure boats (Pine et al., 2016 Rossi et al., 2016 ). Most vessels, and especially large ships, produce predominately low frequency sound (i.e., <1 kHz) from onboard machinery and hydrodynamic flow around the hull. Cavitation at propeller blade tips is also a significant source of noise across all frequencies (Ross, 1987 , 1993 ). Low frequency sounds from ships can travel hundreds of kilometres and can increase ambient noise levels over large areas of the ocean (Ellison et al., 2012 Southall, 2005 ).

    Ambient noise levels in busy shipping lanes have recently increased (Hildebrand, 2009 ), across much of the frequency spectrum (Sertlek et al., 2016 ), but especially at lower frequencies (<500 Hz Erbe et al., 2012 Bittencourt et al., 2014 ). Large numbers of smaller pleasure and recreational vessels, including things like jet skis (Erbe, 2013 ), may also result in substantial increases in noise levels in coastal waters, lakes and rivers. Ice-breaking ships can be a significant source of sound in polar regions.

    A review of approaches for classifying benthic habitats and evaluating habitat quality

    We have assessed the current state of knowledge relative to methods used in assessing sub-tidal benthic habitat quality and the classification of benthic habitats. While our main focus is on marine habitat, we extensively draw on knowledge gained in freshwater systems where benthic assessment procedures are at an advanced stage of maturity. We found a broad range of sophistication/complication in terms of the methods applied in assessing and mapping benthic habitats. The simplest index or metric involved some assessment of species richness, while the most complicated required utilizing multi-variate analysis. The simplest mapping attempts equated physical substrate with benthic habitat while the most sophisticated relied on extensive environmental preference and groundtruth data for species of concern. The leading edge of methods for benthic habitat mapping involves combining the advances in optical and acoustic methods that allow for routine classifying and mapping of the seafloor with biological and habitat data for species of concern. The objective of this melding of dispirit methods is to produce benthic habitat maps with broad system wide coverage and sound biological underpinning. It is clear that the disparity in information density between the physical and biological sides of the equation currently hinder applicability and acceptability of benthic habitat mapping efforts. In addition to the lack of basic information on the biological and environmental tolerances of targeted species, the proliferation of metrics for characterizing and assessing biological conditions further clouds the usefulness of any broad scale mapping attempt. The problem of data density mismatch between physical and biological methods will likely not be solved until acoustic methods can routinely resolve the elusive biological components that make a physical substrate a habitat.

    Ever heard the phrase slippery as an eel? Eels can cover their bodies with a mucous layer, making them nearly impossible to capture by hand.

    Biologists study upstream migration of juvenile eels, or elvers, that are using specially designed ramps to migrate around a dam.


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