SQUID

Squid are marine cephalopods in the order Teuthida. They comprise hundreds of species with an elongated mantle, a prominent head, eight grasping arms and two longer feeding tentacles, a beak-like jaw, and a complex nervous system. Most squid can rapidly change color and pattern thanks to chromatophores, aiding camouflage and communication. They range from small, translucent species to large giants such as Architeuthis dux.

Anatomy and locomotion: Squid have a closed circulatory system with three hearts, including two branchial hearts, to drive blood through the gills. Oxygenated blood then circulates to the body through a systemic heart. The mantle muscles along with a siphon enable jet propulsion; fins provide steering and stability. They respire with gills and use ink sacs for defense.

Habitat and diet: Most are pelagic, occupying coastal to deep-sea habitats in oceans worldwide. They are fast and agile predators, feeding on fish, crustaceans, and other cephalopods. Prey is captured with two long tentacles and eight arms that seize with suckers. Many species perform diel vertical migrations, following prey or avoiding predators.

Reproduction and life history: Males transfer spermatophores to females using a specialized arm called a hectocotylus. Females lay egg masses that attach to substrate or vegetation and guard or cloak them until development. Lifespans are short, often one to two years, with larger species living longer in some cases. Notable species include the giant squid (Architeuthis dux) and the colossal squid (Mesonychoteuthis hamiltoni). Humans fish squid for food (calamari) and fishing pressure affects some populations.