TPWD photo.
“With its untold depths, couldn't the sea keep alive such huge specimens of life from another age, this sea that never changes while the land masses undergo almost continuous alteration? Couldn't the heart of the ocean hide the last-remaining varieties of these titanic species, for whom years are centuries and centuries millennia?”
~Jules Verne, Twenty Thousand Leagues Under the Sea

For thousands of years, stories have existed of giant, many armed sea creatures. In Homer's Odyssey, Odysseus had to navigate his boat around the many-headed sea monster, Scylla. Jules Verne later wrote in his classic novel, 20,000 Leagues Under the Sea, of a monstrous creature whose massive arms "could entangle a ship of five-hundred tons and hurry it into the abyss of the ocean." In the first century B.C., Pliny the Elder – Roman author, naturalist, and natural philosopher, as well as naval and army commander of the early Roman Empire – first wrote of an enormous squid in his Natural History. The animal he described had 30-foot-long arms, weighed 700 pounds, and had a head “as big as a cask.” In 1978, sharp, curved hooks cut up the rubber coating on the hull of the USS Stein. The size suggested the largest squid known at the time. These stories are all likely based on sightings of the giant squid, Architeuthis dux, a real but elusive creature and one of the world's largest known invertebrates.

The giant squid's smaller cousins, though less famous, are better known and no less fascinating. Squid are actually mollusks, though they don't really resemble their relatives the gastropods (snails and slugs), bivalves (clams, oysters, and scallops), scaphopoda (tusk shells), and polyplacophorans (chitons). They are further delineated into the class Cephalopoda, meaning "head foot," the super order Decapodiformes, meaning "ten-footed," and the order Teuthida.

Squid emerged on the ecological timeline about 500 million years ago, during the Cambrian, a particularly prolific period. So many different animal groups appeared at this stage that scientists have dubbed it the "Cambrian explosion." The earliest squid were most likely slow-moving creatures that lived in shallow waters. They shared a common ancestor with the primitive externally-shelled nautilus, but their paths diverged some 438 million years ago, before the first primitive fish began swimming the oceans and before plants grew upright on land. In those days, thousands of species of cephalopods existed. Today, only four remain: squid, cuttlefish, octopuses, and nautiluses.

Squid are swift, agile, and fairly intelligent creatures with brains closer in proportion to those of mammals than those of fish or reptiles. They have excellent vision. Though they are mostly colorblind, they see a much higher level of ultraviolet light than many other animals. This helps the squid to see reflected light and movement at depth. The eyes, on either side of the head, each contain a hard lens. Squid eyes are similar in structure to human eyes, but the image a squid sees is focused by changing the position of the lens, as in a camera or telescope, rather than changing the shape of the lens, as in the human eye. Small pits located beneath the eyes register smell; they have an awesome sense of smell and almost everything they eat leaves a scent trail. Compared to fish, squid do not seem to hear particularly well. Most cephalopods lack a gas-filled chamber, such as the swim bladder, that fish use to hear. That anatomical fact suggests they can't detect the pressure wave component of sound. However, some fish use another organ, the statocyst, to register sound. The statocyst is a balance sensory receptor, a sac-like structure containing a mineralized mass (statolith) and sensitive hairs. The statolith shifts as the animal moves, and any movement large enough to throw the creature off balance causes the statolith to brush against the sensitive hairs, which send a message to the brain to correct the animal's balance. Many invertebrates also have a statocyst and use it for hearing, so squids might also be tapping into this for their own audial benefit.

Squids have three hearts. A large systemic heart pumps blood throughout the body, and on either side of the main heart, two smaller branchial hearts feed the gills. Most squid have a long, tube-shaped body enclosed in a soft and muscular cavity called the mantle, which sits behind the usually small head. The mouth is equipped with a sharp, horny beak composed chiefly of chitin (a tough, protective, semi-transparent substance). The mouth contains the radula (the rough tongue common to all mollusks except bivalves). The head end bears eight arms and two tentacles; sometimes, on some species, these can regenerate if severed. As water flows through the mantle cavity, it passes over the gills which absorb oxygen. Beneath the head is a tube called the funnel. Wastes are excreted through the funnel, as is the squid's defensive ink. Though it could be used as such, squid ink is more sophisticated than a simple smokescreen. It's composed mostly of melanin and mucous (pigment and snot), but includes, among other things, tyrosinase, an enzyme for controlling the production of melanin. It's a useful sort of enzyme, keeps the moray eels at bay. Many of the squid's predators have advanced chemosensory systems, and tyrosinase can irritate, numb, or even deactivate these systems.

Although the nautilus has an external shell, the trend in cephalopods is to internalize and reduce the shell. The shell in cuttlefish is internal and is called the cuttlebone. Squid have a reduced, feather-shaped internal shell called a pen. Octopuses lack a shell altogether. Other than that, the main distinction between a squid and an octopus is that the suckers of squid are armed with hooks or sucker rings (or a combination of the two) while octopuses have simple suckers without secondary armature (though I think no one considers them unarmed).

Squid come in a wide variety of sizes and appearances, and in several ways to be measured. There's total length, relaxed and post mortem; total length, outstretched and alive; standard length, the length of the animal minus its two long tentacles; estimated length, the one that got away; weight; and the length relative to a London double-decker bus. Fun fact (while we're on the topic of size): deep water squid have one of the longest known penis lengths relative to body size of all animals, second in the kingdom only to certain sessile barnacles.

With their soft bodies, squid are appealing prey. They rely on their speed, agility, system of camouflage, and aforementioned ink for defense; some will also fight back with their hooked or razor-edged suckers. Sperm whales will attest to this. A squid's funnel acts like a jet engine, making them powerful swimmers. It draws water into its mantle cavity by expanding its muscles. The mantle stretches like a rubber band, then contracts and forcibly pushes the water out through the funnel. By changing the position of this funnel, a squid can propel itself in almost any direction. When escaping from a predator, a squid can propel itself as quickly as 25 body lengths a second. But sometimes jetting through the currents is not enough to make a successful getaway. Sometimes, a squid needs to get out of the water altogether. So sometimes, they fly. Of course, flying (and swimming) are last resorts. The first choice is camo. To blend in with their surroundings, squid have thousands of pigment cells on their body called chromatophores, which are attached to tiny muscles. Chromatophores expand or contract to change the color or pattern of the squid's skin to match its background (these same cells also help squid attract mates and communicate with other squid). Squid even can change the texture of their skin to simulate their surroundings by raising little flaps and bumps. Some squids produce light. In dark waters, this diverts attention by disguising their contours. But after all that, if they're still spotted by a predator, it's time to fleeeeee!

Several animals like to feast on squid, including sperm whales, tuna, marlin, shark, seals, moray eels, sea stars, other squid, several species of birds, and yours truly. Because several types of fish have such a predilection for squid, they make excellent bait. Squid use the same skills for hunting as for predator evasion. They are carnivorous and their favorite foods include small fish, crabs, shrimp, and other squid. Most squid feed by rapidly lashing and ensnaring prey with suckered feeding tentacles. The squid then pulls the food to its mouth with its arms. It uses its sharp, parrot-like beak to tear off pieces; then the sharp radula on its tongue grinds up the food and pushes it down the squid's throat. Sounds completely feasible, right? Nothing dangerous about some brunch, right? Wrong. Take the giant squid, for example. The giant squid, surpassing lengths of fifty feet, could take down some pretty large prey. However, its superlative size appears at odds with its internal anatomy. All food must reach the gut through the esophagus, which is an admirable three feet in length, or longer, in adult specimens, but its maximum relaxed diameter is less than half an inch, and it passes directly through the brain! We worry about choking if we swallow a big bite. The giant squid has to worry about brain injury.

Many species of cephalopods live fast and die young, their entire life cycle taking just one year. Being so short lived, squid today are versatile creatures. They can make their homes in a variety of marine environments, and are found in all of the world's oceans, from the warm water of the tropics to the near freezing water at the poles. They are found from the wave swept intertidal region to the dark, cold abyss. If you were to clear cut an oak forest, the first plants to grow back would not be more oak trees. First would be the pioneer plants: lichen, moss, algae, some grasses, etc. Then, once the stage is set, in come the weeds. In life history terms, cephalopods are the weeds of the seas. Fun fact: a group of squids is called a shoal. I am "forever disappointed that a group of squids isn't called a squad."

Squids have an uncomplicated life cycle. They reproduce sexually. In most species, the male has a modified arm that he uses to transfer sperm into the female, fertilizing the eggs. The female lays the eggs, usually hiding them under rocks or in holes. After four to eight weeks, baby squid hatch. They look just like miniature squid, they feed on plankton, and they grow into adult squid. No muss; no fuss. About 300 species of squid exist, that we know of. Two other orders of decapodiform cephalopods are also called squid, although they are taxonomically distinct from Teuthida and differ recognizably in their anatomical features. They are the bobtail squid of order Sepiolida and the ram's horn squid of the monotypic order Spirulida. We have the ram's horn in the Gulf, though most people will never see a live one. The internal shell can be distinguished from spiraling gastropod shells by its chambers. Gastropods live in their shells, so the shells won't be chambered. The ram's horn squid spends most of its life in a head down position. The gas-filled internal shell keeps the posterior end up. These squids also have a light-emitting organ between the two posterior fins (hence its other name, tail light squid).

Texas shrimp boats start to haul in the year's first finger squid in March, an occasion that will go unnoticed by most at the dinner table. Atlantic brief squid, dubbed finger squid, are typically sold for bait (when fishermen bother to bring them off their boats). But some seafood dealers say the scrawny cephalopods are worth cookin' up. They reach five inches, but are usually small enough to not require cleaning before being tossed in a stew. I guess the guts add extra flavor?

The two other species we have are the longfin inshore squid and the arrow squid. Ironically, the longfin inshore squid lives in pelagic waters. This squid is more slender and firm than the brief squid and reaches about twenty inches. Neuroscientists in training learn the basics of neurosurgery by practicing on the longfin inshore squid. Their thick axon, thicker than any human nerves, is easier to start with. The arrow squid also lives in pelagic waters and is also more slender and firm than the brief squid, but not as slender as the longfin squid. It reaches about sixteen inches. Both have more chromatophores than the brief squid.

Presently the only commercial fishery for squids in the Gulf of Mexico is a very small-scale fishery in Mexico, in the state of Yucatan. At night, fishermen in small boats use torches and small tethered live fishes to attract arrow squid within range of dip nets. Additionally, three species – the longfin, arrow, and brief squids – are taken in bottom trawls as a bycatch of the Gulf shrimp fishery. While most are discarded along with the rest of the bycatch, a small amount is sold at a low price for bait or human consumption.

“Captain Nemo pointed to this prodigious heap of shellfish, and I saw that these mines were genuinely inexhaustible, since nature's creative powers are greater than man's destructive instincts.”
~Jules Verne, Twenty Thousand Leagues Under the Sea

(Oh, if only Verne could see us now.)

Where I learned about squid, and you can too!

Squid Fishery in Texas: Biological, Economic, and Market Considerations
By Raymond Hixon, Roger Hanlon, Samuel Gillespie, and Wade Griffin

Dallas Observer: It's Finger Squid Season in Texas
By Hanna Raskin

Texas Marine Species: Marine Mollusks of Texas

The Cephalopod Page

The Octopus News Magazine Online: Cephalopod Science Articles

Scientific American: Fact or Fiction: Can a Squid Fly out of Water?

BBC Earth News: Super Squid Sex Organ Discovered

BBC Earth News: The Cephalopods Can Hear You

How Squid Work

inside Discovery: Top 10 Startling Giant Squid Facts

Smithsonian Surprising Science: Fourteen Fun Facts About Squid, Octopuses and Other Cephalopods