Squids usually have a pair of tentacles, huge eyes, eight arms, and a large elongated body. They belong to the Animalia Kingdom and fall under phylum Mollusca, the second-largest type of phylum for invertebrate organisms. Essentially they belong to the class of cephalopods with decapodiformes superorder. Like octopuses, they have soft slippery bodies with a comparatively small skeleton made entirely out of chitin. A squid will have bilateral symmetry, a distinctive head, and a mantle just like other cephalopods. Squids have a special role in the food web of open water. The squid tentacles first catch the prey and then are controlled by the eight arms. The beak then ruptures through the flesh, cuts it into pieces to swallow. They evolved from cephalopods present in the Jurassic period. They use their sight to locate the prey as they move with jet propulsion technique and are fast swimmers. Cooperative hunting is also seen in Humboldt Squid species. These are undoubtedly one of the most intelligent groups of invertebrates.
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What is Squid?
The first question arising in our minds is what is squid and its description. Squids are usually active predators. Teuthida is the squid scientific name. The body of a squid is long with a head and foot attached at each end. The tentacles and arms are present near the mouth region forming a region of disc-like suckers. These are flexible and muscular. Features like suckers, small tooth-like projections, small ganglion for self-control, and strong muscle structure allow them to provide a strong adhesive grip for their prey. The main body has a swimming fin on each side and it is enclosed inside the mantle. The dorsal part of the squid has a longitudinal internal chitin-based portion which is the mollusk shell.
A posterior cone-shaped region is formed by the visceral mass covered by membranous thin epidermis. An opening to the mantle is present on the ventral part of the animal. It contains excretory, reproductive, digestive openings, and gills. Water is drawn into the mantle cavity through an inhalant siphon at the back of the funnel. The funnel is used for jet propulsion. The orientation of the funnel determines the direction of travel. They are extremely fast swimmers as the water is drawn into the mantle cavity and expelled as a strong jet through the funnel. In males, one of the left arms is hectocotylus which helps in transferring the spermatophore inside the female mantle cavity.
Camouflage Techniques of a Squid
The sea squid uses different camouflage techniques for counter illumination and matching of background. It highly helps them to track their prey. Different colour controllable chromatophores are present throughout their skin. Patterns of polarized light can also be created by squids with the help of leucophores and iridophores which are light reflectors. Such techniques help them to communicate with each other, navigate, locate their prey, and hunt.
The effect of countershading by generating light to match the light coming from the ocean surface is done by midwater and firefly squid. Nocturnal predators can also be avoided by using counter illumination techniques. The Hawaiian bobtail squid has a specific symbiotic bacteria for producing light. The light produced in the squid is controlled by changing iris shape or yellow filters present underside. It can easily keep a tract of repeated brightness change. Counter illumination is mostly used during night errands and not during daylight as the Hawaiian squid tends to hide in the sand during daylight.
Generating Ink Cloud for Predator Distraction
After describing general squid meaning we can discuss its ink cloud distraction technique. It simply generates an ink cloud to escape while distracting its predator. The Ink sac as well as the associated ink gland opens in the rectum near the anus. Here the squid animal quickly discharges dark ink into the mantle and surrounding water. The Ink quickly disperses to form a dark suspension as it’s made up of melanin particles. The alkaloid tendency of the suspension may react with the chemoreceptors of the predatory organism which will further weaken it.
Among invertebrates, cephalopods or squid animal usually have the best developed nervous system and are thought to be intelligent. They have a complicated brain structure encircling the esophagus in the form of a nerve ring enclosed in the cranium. The powerful and synchronized muscular movement and jet speed are because nerve messages are transferred with extreme rapidity through huge axons up to 1 mm in length. The eyes are present on either side of the head and have a similar structure to fish eyes. Unlike human eyes who change the shape of their lens, squids change the position of the lens to focus the image as in a telescope or camera. The slit shaped pupil is adjustable to light.
Deep-sea squid often has two types of eyes. The right eye focuses on predators present downward or look forward while the larger left eye searches for animal silhouettes upward in the water column. On either side of the cranium, statocysts are housed in cartilage capsules. These are similar to the inner ear of a fish. It helps a squid to perceive vibrations, maintain its orientation, body position concerning gravity, acceleration, and rotation. Without them, the organism can’t maintain its equilibrium. In addition to that line of hair, cells are present on its arms and head which are extremely sensitive to any pressure change and water movements. It is similar to a fish’s lateral line system.
Locomotive Behavior of Squids
After learning squid meaning, one needs to know several different methods for squid animal movements. Gentle movements are produced by fins present on either side of the body which moves it forward. Jet propulsion provides a much more sustainable movement for squids. Slow jet propulsion allows gill ventilation and normal locomotion at the same time. The inhalant muscle closes while the exhalant valve opens as mantle circular walls contract. The funnel releases water followed by circular muscle relaxation, the recoil of mantle wall tissues, the cavity of mantle expands and opens the inhalant valve, water flows into the cavity and the exhalant valve closes. Continuous locomotion is provided by the repeated cycle of exhalation and inhalation.
One of the escape responses is fast jetting. The mantle cavity is hyper-inflated with the help of both circular and radial muscles to take in a larger volume of water. The water automatically flows out with much greater force on muscular contraction. With this locomotion squid species can even glide like flying fish for about 50m above the surface of the water. Sometimes they even fall on ship decks and eventually die.
Feeding Characteristics of Squids
All types of squid are usually carnivorous. They can swallow even huge animals due to their sturdy suckers and arms. The prey is first perceived by touch or sight following which the rapid tentacles are fired instantly. Then the entire prey is brought down by the arms and held by suckers. The prey is often subdued due to the squid’s saliva as it contains a good amount of toxins. As soon as the squid bites onto its prey, chemicals and toxins are injected into its bloodstream to subdue the heart. Compared to the large body, it has a smaller mouth. Hence, the beak of various types of squid helps to cut the meat into smaller pieces for it to devour. Squids also hunt in packs as they are very intelligent when it comes to their survival.
How Big are Squids?
Giant squid may be up to 13 meters long while the majority of common species are about 60 cm. Benthic pygmy is mostly the smallest squid species whose length is about 10 to 20 mm. The largest squid that was ever noted was discovered back in 2007 on the coast of Antarctica. It weighed about 495 kilograms with almost 10 meters in length. The eyes were even bigger than a football size. These were probably the biggest eyes discovered.
Uses of Squid
In literature and arts, squids have always been described as monsters of depth classically. Greek Mythology’s Gorgon is inspired by squid or octopuses. Even Odyssey or the six-headed monster has a similar birthplace. Aristotle has described giant squid in his History of Animals. Even plenty of science fiction stories have been inspired by giant squid monsters.
Squid is eaten as a nutritious food source all over the world, especially in Japan where they eat it as sashimi or sliced strips of squid or tempura. Across Canada, the main commercial species that are harvested in large quantities is Todarodes Pacificus. Many people call cooked squid meat calamari, especially in English countries. The body can be sliced, cut into pieces, strips, or stuffed as a whole. The tentacles, arms and even the ink is edible. Zinc and manganese are present in high amounts in a squid. It is also high in riboflavin, vitamin B12, copper, and selenium.