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Characteristics of a giant squid's skin

Characteristics of a giant squid's skin


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Is the skin of a giant squid/colossal squid similar to the common squid? Does it possess chromatophores to change color? Or did it lose that ability because of the depth at which it resides or due to the comparative lack of predators?


Architeuthis - the Giant Squid

1:1 Giants of the Sea: There is also a giant squid among the illustrious exhibits in the Stralsund Ozeaneum.


The Alecton crew tries to catch a giant squid. From
"20000 leagues under the seas" by Jules Verne.
Reference: Andreas Fehrmann.

Up to the middle of the 19th century, the giant squid was a sea monster from ancient legends, about which science only knew what tales sailors and fishermen told. Olaus Magnus in the 16th century, Egede and Pontoppidan in the 18th century, had described the kraken. The mostly exaggerated tales from the middle ages more look like a cock-and-bull story than scientific reports.

Whale hunters reported, that sperm whales in their death throes often disgorged something that resembled a squid's arms, only it was much larger. When the whale was cut open, often they found horn beaks the size of their hands, resembling a parrot's beak. The whalers concluded, that sperm whales fought with giant squids in the deep. Herman Melville concurs, 1851 in "Moby Dick" he describes the encounter of his whale ship Pequod with a giant squid.

It was not until three years after the Pequod 's literary encounter with the giant squid, that the subject was first dealt with scientifically.

The Danish scientist Japetus Steenstrup received the beak, the shell and some suckers of a giant squid, that had been washed ashore a year previously on the Danish coast. He compared the material with corresponding organs of smaller squid species and concluded that it had to have belonged to a giant squid species, which he named Architeuthis, the first and largest of the squids. The genus Architeuthis S teenstrup 1856 until today means the giant squids from the Atlantic.


A giant squid's shell. Because of their sword-like form, those
are also called a gladius. Reference: Aldebaran Expedition.

A large part of the scientific research on giant squids at that time is closely connected with Newfoundland Island in the Northwest of America.

Twenty years after Steenstrup had described the giant squid Architeuthis, in 1873 a giant squid attacked a small fisher boat near Portugal Cove, but withdrew, after the fishermen had hacked off two of its arms. Those arms, parts of the larger tentacles, were taken ashore and so got into the hands of Reverend Harvey in nearby St. John's. Harvey examined the arm parts, described the fishermen's fight in several newspaper articles and then sent the arm parts to the biologist Verrill in New Haven, Connecticut. Two months later the fishermen in the Logy Bay, also Newfoundland, managed, so to speak to catch the Big One.


Reverend Harvey in a BBC movie.

They managed to catch a giant squid nine metres (27 feet) long and to take it ashore. Reverend Harvey bought the giant squid and had it photographed in his living room. Afterwards he sent it to Professor Verril who conformed what had been assumed for some time so far: Architheuthis exists and it is really a giant squid.

Since then giant several hundred specimens of giant squids have been caught and washed ashore in different species (three distinct species are assured, there are numerous more insecure species) all over the world. The last specimen was washed ashore on the 22nd of June 2002 in Tasmania on the South coast of Australia.

Of the giant squid's biology, only the morphology has so far been researched. Until today nobody has managed to observe giant squids in their natural environment. Giant squids have been caught with dragnets from the ocean floor, as well as with floating nets in the open sea. So today it is assumed that giant squids live in depths of 300 to 1000 metres (1000 to 3000 feet) on the continental shelf' slope. As beaks of juvenile giants squids have been found in the stomachs of albatrosses and certain fish, it is also assumed that giant squids dive into ever deeper water during their live.

Giant squids are cephalopods ( Cephalopoda ). They have ten arms altogether, of which two long tentacles with club-like broadened ends armed with suckers to catch the prey.

Eight short arm around the mouth then guide the prey to the mouth. There a horn beak cuts the prey in small pieces, that are then further ground by the radula. While freshly caught squids and those washed ashore usually appear white, their natural colour is supposed to be red. Those squids appear to be white, because the outer layers of the skin have been grazed off by the surf.


A giant squid's beak.
Reference: American Museum of Natural History.

Enlarged picture of a giant squid's suckers.
Reference: Aldebaran Expedition.

A giant squid fighting against a sperm whale.
Diorama in the Natural History Museum in New York.
Picture: Mike Goren, Wikipedia.

Giant squids are often said to effuse a disgusting smell of ammonia. That is because giant squids are among those squid species that keep ammonia in the muscular tissue to enhance buoyancy. It is assumed that giant squids can float in the water that way, without having to invest muscle power. So for humans giant squids are indigestible. The contrary is fact, however, with sperm whales (Physeter macrocephalus) . A large part if a sperm whale's diet consists of squids, among those also giant ones. As described before, in the stomachs of sperm whales, collections of squid beaks can be found. Sperm whales' skin often carry the scars of giant squids' suckers armed with teeth. The idea to calculate the squid's size from those scars is made more difficult by the fact that sperm whales, of course, grow and so the result is falsified.

In contrary to sperm whales, giant squids orient optically. As among smaller squid species, a giant squid's eyes are large in relation to its body. Those larges optical organs in the animal kingdom are as big as a soup plate.

There are also interesting things to be said about giant squids' methods of reproduction. Female giant squids have been caught with spermatophores (sperm packets, see also snails' reproduction) in their arm tissue, supposedly placed there by males. It remains unclear, how the female gets hold of the sperm cells to fertilise any eggs. It is assumed either the female opens the spermatophores on its own, or that the spermatophores are removed hormonally from the tissue. Even young females have been caught with very many implanted spermatophores, which leads to the conclusion, that giant squids are solitary and rarely meet other giant squids.

Very little is known about giant squids' behaviour. The assumption is that they are not highly active hunters like their smaller relatives, but carrion eaters and lurking hunters. In giant squids' stomachs, fish and other squids have been found. Because of he ammonia in giant squids' musculature, for a long time the assumption has been that giant squids are slow lurking animals. This may be doubted after the first movie pictures of a live giant squid, which instead point towards giant squids being highly aggressive hunters.


Futurama (2.12): The Deep South.

Large sea animals, like sperm whales, are however not among their usual prey. So it may be called sure that sperm whales hunt the giant squid and not the other way round.

Much of giant squids' biology is either unknown, are must be concluded, together with the anatomical material present, from the behaviour of smaller, better known squid species.

For a long time, the experiment to supply sperm whales with cameras, to film giant squids, have been without success. 2004 Japanese scientists ( Tsunemi Kubodera of the National Science Museum and Kyoichi Mori of the Ogasawara Whale Watching Association ) managed to film a live giant squid with a network a cameras. Because the giant squid lost parts of an arm in the process, even DNA research was possible.

In the meantime there have been impressive observations of the multiple possibilities to use tentacles, as well as giant squids' bioluminescence, used for hunting as well as for communication.

Certainly, today we know much more about giant squids, as did Japetus Steenstrup. But we hardly can say to know everything about those creatures living in the deepest parts of the sea. And so even the giant squid, at least partly, will always remain a mystery, until the progress of technology allows man to finally completely reveal this secret of nature.


Because so many different kinds of squids exist, there are hundreds of different scientific names for them. All are cephalopods, which means they are members of the scientific class Cephalopoda, along with octopuses and cuttlefish. The class name comes from the Greek words for head and foot. They are members of the superorder Decapodiformes, which is derived from the Greek words for 10 feet. Squids belong to the order Teuthida, a term that comes from the Greek word for fierce.

Squids can look different from one another, depending on the species, but in general all squid have an elongated, tubular body called the mantle which ends at a somewhat flattened head. On either side of the mantle are fins that aid the squid in moving through the water. Depending on the species these fins can be quite large, running the full length of the mantle, or very small, located just at one end. A squid also has relatively large eyes, one on either side of its head, that allow it to see 360 degrees around it.

At the lower end of the squid’s body are the arms and tentacles, attached to the head. Each of the arms has suckers on it, as do the tentacles. The suckers of some squids are also armed with sharp hooks that allow them to grip their prey tightly. They don’t have a skeleton as we do, but squid do have a small, internal skeleton made of chitin, which is the same thing you’ll find on the outside of an insect.

The shape of the squid allows it to slip quickly through the water. When swimming slowly it uses its fins for propulsion, but if the squid is in a hurry it moves by taking in water through its mantle and then squirting it out through its siphon, jet-propelling it through the water. The siphon can be moved to point in any direction, allowing the squid to quickly move whichever way it chooses.

Squid are usually black, white, brown, or gray, but many of them can change their appearance at will. The Humboldt squid, for example, can flash red and white, and other squids can match their color to their surroundings or display a colorful pattern on their bodies. They can use color to signal to other squid or to help camouflage themselves to avoid predators.

Deep-sea squid often have bioluminescent organs, and these lighted body parts can be seen from outside the animal. Typically, squid can also squirt out a cloud of ink in the event they feel threatened. The ink hides them and gives them time to escape to safety. A notable exception to this is the vampire squid, which squirts out a sticky bioluminescent cloud into the water which glows for about 10 minutes, giving the vampire squid time to get away.

Squid come in many different sizes. The heaviest squid on record was a colossal squid discovered in New Zealand in 2007. This huge animal weighed more than 1,000 pounds (453.6 kg), almost as heavy as a grizzly bear. The longest squid ever found was a giant squid. While not as heavy as a colossal squid, the biggest giant squid was 49 feet (14.9 meters) long, longer than a semitrailer. Most squid are much smaller, with the average being about 2 feet (60 cm) long, the size of an average man. The smallest squid known is the Southern pygmy squid, which is practically invisible at only ¾ of an inch (1.6 cm) long.

Squid tend to live alone, but they do sometimes gather in groups and some of them have even been known to hunt cooperatively, similar to the way a pack of wolves hunts. When they do gather a group of squid is called either a shoal or a squad, with the exception of the giant squid. A group of giant squid is called a school.


Squids: Sophisticated skin

Squids have long been a source of fascination for humans, providing the stuff of legend, superstition and myth. And it's no wonder -- their odd appearances and strange intelligence, their mastery of the open ocean can inspire awe in those who see them.

Legends aside, squids continue to intrigue people today -- people like UC Santa Barbara professor Daniel Morse -- for much the same, albeit more scientific, reasons. Having evolved for hundreds of millions of years to hunt, communicate, evade predators and mate in the vast, often featureless expanses of open water, squids have developed some of the most sophisticated skin in the animal kingdom.

"For centuries, people have been amazed at the ability of squids to change the color and patterns of their skin -- which they do beautifully -- for camoflage and underwater communication, signaling to one another and to other species to keep away, or as attraction for mating and other kinds of signaling," said Morse, a Distinguished Professor Emeritus of Biochemistry and Molecular Genetics.

Like their cephalopod cousins the octopus and cuttlefish, squids have specialized pigment-filled cells called chromatophores that expand to expose them to light, resulting in various shades of pigmentary color. Of particular interest to Morse, however, is the squids' ability to shimmer and flicker, reflecting different colors and breaking light over their skin. It's an effect that is thought to mimic the dappled light of the upper ocean -- the only feature in an otherwise stark seascape. By understanding how squids manage to fade themselves into even the plainest of backgrounds -- or stand out -- it may be possible to produce materials with the same, light tuning properties for a variety of applications.

Morse has been working to unlock the secret of squid skin for the last decade, and with support from the Army Research Office and research published in the journal Applied Physics Letters, he and co-author Esther Taxon come even closer to unraveling the complex mechanisms that underlie squid skin.

An Elegant Mechanism

"What we've discovered is that not only is the squid able to tune the color of the light that's reflected, but also its brightness," Morse said. Research had thus far has established that certain proteins called reflectins were responsible for iridescence, but the squid's ability to tune the brightness of the reflected light was still something of a mystery, he said.

Previous research by Morse had uncovered structures and mechanisms by which iridocytes -- light-reflecting cells -- in the opalescent inshore squid's (Doryteuthis opalescens) skin can take on virtually every color of the rainbow. It happens with the cell membrane, where it folds into nanoscale accordion-like structures called lamellae, forming tiny, subwavelength-wide exterior grooves.

"Those tiny groove structures are like the ones we see on the engraved side of a compact disc," Morse said. The color reflected depends on the width of the groove, which corresponds to certain light wavelengths (colors). In the squid's iridocytes, these lamellae have the added feature of being able to shapeshift, widening and narrowing those grooves through the actions of a remarkably finely tuned "osmotic motor" driven by reflectin proteins condensing or spreading apart inside the lamellae.

While materials systems containing reflectin proteins were able to approximate the iridescent color changes squid were capable of, attempts to replicate the ability to intensify brightness of these reflections always came up short, according to the researchers, who reasoned that something had to be coupled to the reflectins in squid skin, amplifying their effect.

That something turned out to be the very membrane enclosing the reflectins -- the lamellae, the same structures responsible for the grooves that split light into its constituent colors.

"Evolution has so exquisitely optimized not only the color tuning, but the tuning of the brightness using the same material, the same protein and the same mechanism," Morse said.

Light at the Speed of Thought

It all starts with a signal, a neuronal pulse from the squid's brain.

"Reflectins are normally very strongly positively charged," Morse said of the iridescent proteins, which, when not activated, look like a string of beads. Their same charge means they repel each other.

But that can change when a neural signal causes the reflectins to bind negatively charged phosphate groups that neutralize the positive charge. Without the repulsion keeping the proteins in their disordered state they fold and attract each other, accumulating into fewer, larger aggregations in the lamellae.

These aggregations exert osmotic pressure on the lamellae, a semipermeable membrane built to withstand only so much pressure created by the clumping reflectins before releasing water outside the cell.

"Water gets squished out of the accordion-like structure, and that collapses the accordion so the thickness in spacing between the folds gets reduced, and that's like bringing the grooves of a compact disc closer together," Morse explained. "So the light that's reflected can shift progressively from red to green to blue."

At the same time, the membrane's collapse concentrates the reflectins, causing an increase in their refractive index, amplifying brightness. Osmotic pressure, the motor that drives these tunings of optical properties, couples the lamellae tightly to the reflectins in a highly calibrated relationship that optimizes the output (color and brightness) to the input (neural signal). Wipe away the neural signal and the physics reverses, Morse said.

"It's a very clever, indirect way of changing color and brightness by controlling the physical behavior of what's called a colligative property -- the osmotic pressure, something that's not immediately obvious, but it reveals the intricacy of the evolutionary process, the millennia of mutation and natural selections that have honed and optimized these processes together."

Tunable-Brightness Thin-Films

The presence of a membrane may be the vital link for the development of bioinspired thin films with the optical tuning capacity of the opalescent inshore squid.

"This discovery of the key role the membrane plays in tuning the brightness of reflectance has intriguing implications for the design of future buihybrid materials and coatings with tunable optical properties that could protect soldiers and their equipment," said Stephanie McElhinny, a program manager at the the Army Research Office, an element of the U.S. Army Combat Capabilities Development Command's Army Research Laboratory.

According to the researchers, "This evolutionarily honed, efficient coupling of reflectin of its osmotic amplifier is closely analogous to the impedance matched coupling of activator-transducer-amplifier networks in well-engineered electronic, magnetic, mechanical and acoustic systems." In this case the activator would be the neuronal signal, while the reflectins acts as transducers and the osmotically controlled membranes serve as the amplifiers.

"Without that membrane surrounding the reflectins, there's no change in the brightness for these artificial thin-films," said Morse, who is collaborating with engineering colleagues to investigate the potential for a more squid skin-like thin-film. "If we want to capture the power of the biological, we have to include some kind of membrane-like enclosure to allow reversible tuning of the brightness."


These squids can be found in deep oceans worldwide. Their distribution is incredibly widespread, with specimens found or captured from the north Atlantic Ocean near Norway, and Newfoundland to the south Atlantic Ocean near South Africa. In the Pacific Ocean they can be commonly found from Japan to Australia. It is rare to find them near tropical or polar regions.

These squid feed on deep-sea fish and other species of squid. It is believed they may occasionally cannibalize each other, as fragments of other giant squid beaks have been found in their stomachs.

Prey is captured by quickly grabbing it with the two long tentacles. It is believed that these squid are solitary, because they are only captured in nets individually.


Squids Have Beaks

All squid species have beaks, which are sharp and pointed. These are very hard, and are used to kill and tear their prey apart. Unlike the teeth and jaws of most other animals, these beaks contain no minerals whatsoever, and can even survive digestive juices. This is why squid beaks are often found inside whale stomachs when they’re opened up. Squid beaks are extremely hard to scratch or break, making them tougher and more resilient that virtually all metals and polymers.


The Giant Squid Genome Holds Surprises

I&rsquove been thinking about invertebrates often lately, and so was delighted to learn that the genome of the giant squid has been sequenced. I&rsquoll never tire of reading new genome papers.

One of the largest animals known, the giant squid is also one of the most elusive, appearing to us mainly as body parts sporting telltale suckers that have washed ashore. A full grown giant squid can&rsquot be comfortably stuffed into an aquarium tank. So most of us know about it from fiction.

The giant sea monster of Scandinavian folklore, the Kraken, terrified sailors in vessels along the coastal waters of Norway and Greenland. It was likely a giant squid, as was Homer&rsquos tentacled Scylla in The Odyssey. Jules Verne&rsquos 20,000 Leagues Under the Sea, written in 1869, famously featured the animal.

More recently, the 2005 film The Squid and the Whale evokes the image of a giant squid battling a sperm whale depicted in a mesmerizing diorama at the American Museum of Natural History. Director Noah Baumbach borrowed the image as a metaphor for the battling parents of his young protagonists.

True squid stories are intriguing too.

The first description came from Captain Peter M&rsquoQuhae of the HMS Daedalus. On an August afternoon in 1848, the ship was sailing between the Cape of Good Hope and the island of St. Helena off the coast of Africa, when a huge sea serpent arose from the depths. Many men witnessed the event, and their tales made their way into the newspapers.

The beast, according to captain and crew, was 60 feet long and had gotten quite close to the ship. The captain sketched it, and newspaper readers offered a variety of explanations. Had the sailors seen a dinosaur, an uncommonly long eel, a giant seal, or a gargantuan snake that had lost its way?

In 1857, Japetus Steenstrup, a zoologist at the University of Copenhagen, assembled eclectic clues to the giant&rsquos identity: the smaller squid relatives that people ate, tentacled slimy things washing ashore, a marooned mysterious giant beak, and sailors&rsquo supposedly tall tales.

In another realm entirely, giant squid axons are famous in the annals of neurology because their gargantuan axons &ndash 0.5 to 1.5 millimeters in diameter and almost a meter long &ndash are big enough to see, and great for experiments on nerve conduction. But &ldquogiant&rdquo in descriptions typically modifies the axon, not the species of squid.

The scientific name of the giant squid is Architeuthis dux. It is a cephalopod mollusk.

&ldquoCephalopod&rdquo means &ldquohead and foot,&rdquo and that&rsquos a pretty good description. The foot of the squid is the counterpart to that of a snail, but elaborated into arms and tentacles. The animal has a prominent head. The roughly 800 species of cephalopods include cuttlefishes, octopuses, and squids, and a few nautiluses. Most are soft and squishy.

Squids have large, complex brains and behaviors, and they can think, but as invertebrates, have no backbones. They live deep in oceans except for the high Arctic and Antarctic waters. Squids grow fast, die soon, and many species eat them. They&rsquore a good source of protein, but I don&rsquot like tentacles in my salads.

Different species range in size. Pygmy squids are a little under an inch long, giant squids average 42 feet long, and the massive 1100-pound colossal squid Mesonychoteuthis hamiltoni is a few feet longer than it&rsquos giant relative.

A squid has eight arms plus two tentacles that it uses to grab prey. The head zooms out of a muscular cone, called the mantle, which contracts to propel the beast. Beneath the mantle lies a hard layer, called a &ldquopen,&rdquo where muscles attach. A beak &ndash the thing that washes ashore or that winds up in the belly of a whale &ndash lies at the center of a ring from which the arms, tipped with suckers, emanate. The animal uses the ring to cut up whatever unfortunate animal winds up in the tentacles, like a salami slicer.

One of the most striking characteristics of the squids is that they are changelings, able to quickly alter the color, texture, pattern, and brightness of their skin. The animals use these disguises to communicate, and in camouflage and mimicry. Hiding in plain sight, they can effectively hunt while avoiding being eaten.

A Little Help from Cephalopod Relatives

Getting enough remains of fresh giant squids to probe their DNA is tough, but researchers have determined from analyzing mitochondrial DNA that giant squid are all the same species. But that&rsquos just a handful of genes. Many copies of a full genome are needed to overlap them sufficiently to derive a complete genome. Otherwise, gaps remain.

Museum samples generally don&rsquot work &ndash between decay and preservatives, the DNA is hard to extract and keep intact. Fortunately, fishermen aboard a ship near New Zealand were able to send a freshly frozen tissue sample from a giant squid to the multinational research team, who went to work on the genome. &ldquoThese new results may unlock several pending evolutionary questions regarding this mantled species,&rdquo said lead investigator Rute da Fonseca from the Center for Macroecology, Evolution and Climate at the Globe Institute of the University of Copenhagen. The report appears in GigaScience.

Alas, many of the squid genes were shattered. So the researchers turned to analyzing messenger RNAs (mRNAs) and proteins from easier-to-handle relatives. A genome is like a hard copy, an instruction manual present in every cell. In contrast, collections of RNAs and proteins differ in different cell types, painting portraits of a living organism&rsquos functions.

But RNA is more delicate than DNA and wouldn&rsquot persist in a lump of rotting squid flesh. So the researchers collected mRNAs from relatives: the common clubhook squid, the Humboldt squid, and the purpleback flying squid, sampling from brains, livers, and sexual organs. They also collected proteins from the muscular mantles of museum specimens of the California two-spot octopus, the Pacific oyster, and the giant owl limpet.

Matching the RNAs and inferring DNA sequences from the proteins&rsquo amino acid sequences, the researchers were able to flesh out the genome of the giant squid.

The giant squid&rsquos genome includes 33,406 protein-encoding genes splayed across 2.7 billion DNA bases, compared to our 20,000 or so genes in a 3.2 billion base genome. About half of its genome is repeated sequences, most of which can jump around, but that&rsquos not surprising. The genomes of corn, insects, and humans are also half or more repeats, with jumping genes too. These sequences, perhaps raw material for evolution, account for much of the variation in genome size among species. Size doesn&rsquot really matter.

The genome of the giant squid also resembles those of other animals in that it includes gene families, groups of genes with related functions. It harbors the dozen WNT genes found in all mollusks. This gene family encodes secreted glycoprotein growth factors that take part in the cell-cell signaling that controls cell proliferation in early development and later in maintaining tissues. Humans have 19 WNT genes.

Genes encoding proteins called protocadherins are &ldquomassively expanded&rdquo among the cephalopods. They oversee cell-cell adhesion, essential to the functioning of a nervous system. Like the WNT growth factors, the protocadherins are in vertebrate genomes too. They&rsquore clustered, which suggests that they evolved from an ancient gene that repeatedly duplicated.

Other invertebrates and vertebrates have arrays of WNT and protocadherin genes. In contract, reflectins are specific to cephalopods, discovered in the Hawaiian bobtail squid. These proteins form flat structures that reflect ambient light, providing the trademark glow that a squid uses to blend in and communicate. In squids as well as octopuses, nine reflectin genes cluster on a chromosome.

RNA Editing and An Exploded Homeotic Gene Cluster

Two characteristics of the giant squid genome have broader implications.


Anders Drud and Frederik Wolff Teglhus, University of Copenhagen, Denmark

The giant beast appears to be expert at editing its RNA. This skill enables the animals&rsquo genomes to knit variations on proteins, particularly those involved in nervous system functioning.

At tens of thousands of places in the genome, the RNA-edited regions lie within &ldquohighly conserved&rdquo DNA sequences, which means that they&rsquore identical, or nearly so, in many species. That is, natural selection has retained them over deep time because they do something essential to successful reproduction.

At the risk of sounding anthropomorphic, this genome organization sets up an intriguing strategy: the conserved sequences keep things stable under the force of positive natural selection, yet at the same time, RNA editing provides a flexibility that benefits establishment of neuron configurations, connections, and excitability. The genome organization is like trying out something new while keeping the old, a common theme in evolution.

The other intriguing characteristic of the giant squid genome is the dispersion of its homeotic (Hox) genes. These genes control where body parts form in relation to each other, from flowers to flies to fungi to the more complex single-celled organisms.

A homeotic mutation mixes up body parts, and they&rsquore behind some diseases of humans. I did my PhD work on the Antennapedia complex of homeotic genes in flies (see next week&rsquos post), specifically mutations that put legs on their heads and antennae on their mouths. The homeobox &ndash the 180-base-pair stretch within homeobox genes that controls the body &ldquoplan&rdquo &ndash was discovered soon after I got my degree, in my lab (that of Thomas Kaufman at Indiana University). I&rsquom a homeogirl.

The most amazing thing about the homeotic genes is that they are arrayed on a chromosome, in the genomes of all these diverse species, in the exact order in which they are deployed in development, like basketball players waiting on a bench at a game.

The squid and the whale diorama at the American Museum of Natural History

But that&rsquos not the case in the giant squid genome. Instead, their homeotic genes are dispersed among chromosomes. Could this be why the body is so huge and blobby, seemingly lacking the nuances of a face, the intricacy of a flower, even the feathery filaments of a mushroom?

&ldquoGain and loss of Hox genes has been attributed to fundamental changes in animal body plans,&ldquo the researchers write. And loss of a major Hox gene in spider mites reduces the number of segments. So did some long-ago mutational event in the giant squid or an extinct ancestor explode the arrayed homeobox genes to new genomic addresses, while retaining enough function to form a body?

The Giant Squid&rsquos Place in the World

It&rsquos hard to tell whether a population that we can&rsquot really observe is threatened, but the researchers point out that oceanic warming and acidification, pollution including mercury and flame retardants, depleted oxygen, and fishing are all threats to the giant squid, as they are to many other species.

&ldquoConsequently, there is an urgent need for greater biological understanding of these important, but rarely encountered animals, in order to aid conservation efforts and ensure their continued existence. With the release of this annotated giant squid genome, we set the stage for future research into the enigmas that enshroud this awe-inspiring creature, the species that has inspired generations to tell tales of the fabled Kraken,&rdquo the researchers conclude.

(Opening image of giant squid sucker rings credit: The Trustees of the Natural History Museum, London)


However, their inhospitable deep-sea habitat has made them uniquely difficult to study, and almost everything scientists know about them is from carcasses that have washed up on beaches or been hauled in by fishermen. Lately, however, the fortunes of scientists studying these elusive creatures have begun to turn. In 2004 researchers in Japan took the first images ever of a live giant squid. And in late 2006, scientists with Japan's National Science Museum caught and brought to the surface a live 24-foot female giant squid.

Giant squid, along with their cousin, the colossal squid, have the largest eyes in the animal kingdom, measuring some 10 inches in diameter. These massive organs allow them to detect objects in the lightless depths where most other animals would see nothing.

Like other squid species, they have eight arms and two longer feeding tentacles that help them bring food to their beak-like mouths. Their diet likely consists of fish, shrimp, and other squid, and some suggest they might even attack and eat small whales.


But that’s not the weird part. What’s truly bizarre (at least from our mammal-centric perspective) is the fact that its esophagus passes through the hole in the middle of its brain. Giant squids have to be really careful while swallowing, because if a given meal isn’t broken down into small pieces first, it can rub against the brain and cause damage.

History was made by residents of the Ogasawara Islands (located 600 miles south of Japan) on September 30, 2004. Using a line baited with shrimp, zoologist Tsunemi Kubodera and whale-watcher Kyochi Mori attracted an Architeuthis about 2950 feet beneath their vessel. Five hundred still images were then snapped by a submerged camera before the squid took off—leaving behind an 18-foot severed tentacle.


Southern Giant Squid

Click to enlarge image Toggle Caption

Fast Facts

  • Classification Species dux Genus Architeuthis Family Architeuthidae Order Teuthida Subclass Coleoidea Class Cephalopoda Phylum Mollusca
  • Size Range Body to at least 2m, total length to at least 15m. Little is known of size range of this species due to the limited number of observed specimens, although it is reported to weigh up to at least 220kg.

This genus is often referred to as ‘Monsters of the deep’. Giant Squids are the largest of all the living cephalopods and the largest individual invertebrate in the world. There is still little known of the identity, distributions, biology and behaviour of giant squids.

Identification

Two thirds of the length of these squids is made up by a pair of long feeding tentacles each bearing an elongate club on the tip. These metre-long tips bear large suckers armed with toothed horny rings.

Distribution

Southern Ocean, exact distribution unknown but individuals have been captured by deep-sea trawl fishing off New Zealand, Australia, and South Africa.

Habitat

The deep, dark, cold waters of the open ocean - this species has been captured from depths of 400-800m.

Feeding and diet

The stomach contents of some specimens have contained pieces of fin rays from large fish and squid suckers almost as large as their own. It is thought the prey is torn into small pieces by both the large beak and their large toothed tongue, or ‘radula’.

Other behaviours and adaptations

The fins of the Giant Squid are small, and the muscles appear to be poorly developed - so it is unlikely that these squids are fast swimmers. It is believed the animal’s buoyancy is aided by pockets of ammonia solution within the body walls.

Breeding behaviours

Little is known of the breeding behaviours of Architeuthis dux, except for information provided by one female specimen caught off Tasmania in 1996. Numerous ropes of sperm were found radiating from a single point of entry in the skin, which is akin to many other squid species that store sperm in special receptacles or within their skin. It is also known that male Giant Squids have a muscular, well developed penis up to a metre long. It is thought from these two pieces of evidence that males use their penis almost like a hydraulic nail - inserting cords of sperm into the females skin. The female would then store the sperm until her eggs are fully developed and ready to be fertilised- although it is unclear how she might access the stored sperm.

Predators, Parasites and Diseases

Sperm whales are known to feed on the Giant Squid, as the squid beak has been commonly found in the stomachs of beached sperm whales.


Contents

Squid are members of the class Cephalopoda, subclass Coleoidea. The squid orders Myopsida and Oegopsida are in the superorder Decapodiformes (from the Greek for "ten-legged"). Two other orders of decapodiform cephalopods are also called squid, although they are taxonomically distinct from squids and differ recognizably in their gross anatomical features. They are the bobtail squid of order Sepiolida and the ram's horn squid of the monotypic order Spirulida. The vampire squid (Vampyroteuthis infernalis), however, is more closely related to the octopuses than to any squid. [4]

The cladogram, not fully resolved, is based on Sanchez et al., 2018. [4] Their molecular phylogeny used mitochondrial and nuclear DNA marker sequences they comment that a robust phylogeny "has proven very challenging to obtain". If it is accepted that Sepiidae cuttlefish are a kind of squid, then the squids, excluding the vampire squid, form a clade as illustrated. [4] Orders are shown in boldface all the families not included in those orders are in the paraphyletic order "Oegopsida", except Sepiadariidae and Sepiidae that are in the paraphyletic order "Sepiida",

Sepiadariidae (pyjama and bottletail squid)

Evolution

Crown coleoids (the ancestors of octopuses and squid) diverged at the end of the Paleozoic, in the Permian. Squid diverged during the Jurassic, but many squid families appeared in or after the Cretaceous. [5] Both the coleoids and the teleost fish were involved in much adaptive radiation at this time, and the two modern groups resemble each other in size, ecology, habitat, morphology and behaviour, however some fish moved into fresh water while the coleoids remained in marine environments. [6]

The ancestral coleoid was probably nautiloid-like with a strait septate shell that became immersed in the mantle and was used for buoyancy control. Four lines diverged from this, Spirulida (with one living member), the cuttlefishes, the squids and the octopuses. Squid have differentiated from the ancestral mollusc such that the body plan has been condensed antero-posteriorly and extended dorso-ventrally. What may have been the foot of the ancestor is modified into a complex set of appendages around the mouth. The sense organs are highly developed and include advanced eyes similar to those of vertebrates. [6]

The ancestral shell has been lost, with only an internal gladius, or pen, remaining. The pen, made of a chitin-like material, [6] [7] is a feather-shaped internal structure that supports the squid's mantle and serves as a site for muscle attachment. The cuttlebone or sepion of the Sepiidae is calcareous and appears to have evolved afresh in the Tertiary. [8]

Fossil Rhomboteuthis from the Lower Callovian (c. 164 mya, middle Jurassic) of La Voulte-sur-Rhône, France

Fossil Plesioteuthis from the Tithonian (c. 150 mya, upper Jurassic), Solnhofen, Germany

Squid are soft-bodied molluscs whose forms evolved to adopt an active predatory lifestyle. The head and foot of the squid are at one end of a long body, and this end is functionally anterior, leading the animal as it moves through the water. A set of eight arms and two distinctive tentacles surround the mouth each appendage takes the form of a muscular hydrostat and is flexible and prehensile, usually bearing disc-like suckers. [6]

The suckers may lie directly on the arm or be stalked. Their rims are stiffened with chitin and may contain minute toothlike denticles. These features, as well as strong musculature, and a small ganglion beneath each sucker to allow individual control, provide a very powerful adhesion to grip prey. Hooks are present on the arms and tentacles in some species, but their function is unclear. [9] The two tentacles are much longer than the arms and are retractile. Suckers are limited to the spatulate tip of the tentacle, known as the manus. [6]

In the mature male, the outer half of one of the left arms is hectocotylised – and ends in a copulatory pad rather than suckers. This is used for depositing a spermatophore inside the mantle cavity of a female. A ventral part of the foot has been converted into a funnel through which water exits the mantle cavity. [6]

The main body mass is enclosed in the mantle, which has a swimming fin along each side. These fins are not the main source of locomotion in most species. The mantle wall is heavily muscled and internal. The visceral mass, which is covered by a thin, membranous epidermis, forms a cone-shaped posterior region known as the "visceral hump". The mollusc shell is reduced to an internal, longitudinal chitinous "pen" in the functionally dorsal part of the animal the pen acts to stiffen the squid and provides attachments for muscles. [6]

On the functionally ventral part of the body is an opening to the mantle cavity, which contains the gills (ctenidia) and openings from the excretory, digestive and reproductive systems. An inhalant siphon behind the funnel draws water into the mantel cavity via a valve. The squid uses the funnel for locomotion via precise jet propulsion. [10] In this form of locomotion, water is sucked into the mantle cavity and expelled out of the funnel in a fast, strong jet. The direction of travel is varied by the orientation of the funnel. [6] Squid are strong swimmers and certain species can "fly" for short distances out of the water. [11]

Camouflage

Squid make use of different kinds of camouflage, namely active camouflage for background matching (in shallow water) and counter-illumination. This helps to protect them from their predators and allows them to approach their prey. [12] [13]

The skin is covered in controllable chromatophores of different colours, enabling the squid to match its coloration to its surroundings. [12] [14] The play of colours may in addition distract prey from the squid's approaching tentacles. [15] The skin also contains light reflectors called iridophores and leucophores that, when activated, in milliseconds create changeable skin patterns of polarized light. [16] [17] Such skin camouflage may serve various functions, such as communication with nearby squid, prey detection, navigation, and orientation during hunting or seeking shelter. [16] Neural control of the iridophores enabling rapid changes in skin iridescence appears to be regulated by a cholinergic process affecting reflectin proteins. [17]

Some mesopelagic squid such as the firefly squid (Watasenia scintillans) and the midwater squid (Abralia veranyi) use counter-illumination camouflage, generating light to match the downwelling light from the ocean surface. [13] [18] [19] This creates the effect of countershading, making the underside lighter than the upperside. [13]

Counter-illumination is also used by the Hawaiian bobtail squid (Euprymna scolopes), which has symbiotic bacteria (Aliivibrio fischeri) that produce light to help the squid avoid nocturnal predators. [20] This light shines through the squid's skin on its underside and is generated by a large and complex two-lobed light organ inside the squid's mantle cavity. From there, it escapes downwards, some of it travelling directly, some coming off a reflector at the top of the organ (dorsal side). Below there is a kind of iris, which has branches (diverticula) of its ink sac, with a lens below that both the reflector and lens are derived from mesoderm. The squid controls light production by changing the shape of its iris or adjusting the strength of yellow filters on its underside, which presumably change the balance of wavelengths emitted. [18] Light production shows a correlation with intensity of down-welling light, but it is about one third as bright the squid can track repeated changes in brightness. Because the Hawaiian bobtail squid hides in sand during the day to avoid predators, it does not use counter-illumination during daylight hours. [18]

Controllable chromatophores of different colours in the skin of a squid allow it to change its coloration and patterns rapidly, whether for camouflage or signalling.



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