Squid: Anatomy, Behavior, and Classification

Squid: Anatomy and Behavior

Squid have two gills, and a circulatory system that consists of a systemic heart and two branchial hearts. Their tentacles are equipped with highly muscular suckers, and if lost, they cannot grow back. Squids can blend in with their surroundings with ease to avoid predators.

They also have cells called chromatophores in their skin that give squids the ability to change color if they feel threatened, a strategy that is combined with the expulsion of the ink they produce. Their shell is internal, unlike other similar animals such as the octopus, which lacks a shell. The squid’s shell is formed by a thin, flat piece connected to its body. They have an organ called a hyponome, allowing them to move by expelling water under pressure.

The mouth of the squid is equipped with a sharp beak, used for killing and butchering prey into manageable pieces. In the stomachs of many captured whales are squid beaks, as they are the only part of this animal that cannot be digested. The mouth of the squid hosts the radula, a kind of tongue common to all mollusks except for bivalves and aplacophores. The squid is exclusively carnivorous, feeding on fish and invertebrates, which they capture with two different, longer tentacles. They are voracious, have fast movements, and grow very rapidly. They can become very abundant in some seas. Most live one year and die after spawning, although some giant species can live for two or more years.

Most squid are no larger than 60 cm, although the giant squid can measure up to 13 meters.

Mollusks

Mollusks are invertebrates with a soft, unsegmented body, usually covered with a shell that can be formed by one or two pieces. This shell is secreted by the mantle, a dorsal fold of the integument. Their body essentially consists of three parts:

  • Head: In the front, where the sensory organs are located.
  • Foot: A muscular locomotor organ on the ventral surface.
  • Visceral Mass: Contains most of the essential organs (heart, intestine). This region is covered by a fold of integument, the mantle, which secretes the calcareous shell.

Mantle Cavity: A gap between the visceral mass and the mantle, which houses the gills and the mouth of the digestive and excretory systems.

Gastropods

Gastropods have an outer shell in one piece, usually coiled and spiral. They have a very distinct head and mouth with a radula (toothed tongue) that the animal uses as a scraper to remove food.

Foot: The organ used in their locomotor movements for swimming. A crushed muscle mass at the bottom adapts to the ground. The foot is very muscular and retracts into the shell. The operculum, located on the back of the foot, completely closes the opening of the shell.

The visceral mass is always included in the shell and has the same spiral coiling. It contains the digestive, circulatory, and excretory systems.

Stomach: Has two parts: a gastric shield that performs a crushing function, and a crystalline style jelly stick that contains enzymes involved in digestion.

Reproduction: Sexes are separate, but hermaphroditism is common. Development is a bitstream.

Bivalves

Bivalves have a body covered by a shell composed of two parts or valves, such as oysters, clams, mussels, and cockles. The shell that covers the body of the bivalves is made up of two valves joined together and coordinated through the hinge. The animal’s body is arranged inside the shell. They do not have a distinct head. The body is reduced to the foot, the visceral mass, and the mantle.

Visceral Mass: Small, like a bag that encloses the internal organs: digestive, circulatory, excretory, and reproductive. There are siphons through which water enters and leaves.

Foot: A burrowing organ, where the species lives buried, sharp as a hatchet to be buried in the ground. In the case of the mussel, filaments are secreted by which it attaches to the substrate, the organ fixing to be fitted with a special gland.

They are filtering species that feed on plankton or small particles in the water.

Reproduction: Sexes are separate or hermaphroditic. Development is a bitstream; before reaching their final form, they pass through larval stages.

Benthic bivalves are in close relation to the bottom. Species that live buried use their foot as an excavating organ and are fitted with traps to catch the water that provides nourishment.

Cephalopods

Cephalopods have a wreath of arms as a prolongation of the head. The head is distinctly developed and has two big eyes like vertebrates.

Mouth: Has a radula and jaws in the shape of a parrot’s beak. The mantle is well-developed in a pouch or bag, within which the viscera are located. Water is pulled out of the mantle cavity, which contains the intestine. The foot is highly modified compared to other mollusks; it forms the arms and the siphon. The mantle develops a kind of fin. The expulsion of water from the siphon causes a reaction.

Reproduction: Sexes are separate. They reproduce by eggs clustered in packages (spermatophores are placed within the mantle cavity of the female).

Crustaceans

Crustaceans are invertebrates belonging to the arthropods. They have a segmented, chitinous body and a variable number of jointed appendages.

Segments: They have a pair of jointed appendages per segment and a chitinous shell.

Decapod Crustaceans

Examples include shrimp, prawns, lobsters, and crabs.

Frills: Calcified shell, body divided into two regions: cephalothorax and abdomen.

Body Parts:

  • Cephalothorax: Fusion of the segments of the head and thorax.
  • Abdomen: Composed of six segments, some of which are developed and muscular.

Macrurids: Have a long and hard carapace (lobster, lobster, crayfish).

Brachyurans: Have a short, wide carapace and a very small abdomen. In macrurids, the segments are articulated to each other by membranes that allow movement. In brachyurans, the abdomen is reduced to a tongue lodged in the ventral part of the carapace.

They detach from the shell to grow their soft tissues. They have to be silent to grow.