Loliginidae and Spirulidae - Squids

Squids are marine cephalopods which include octopuses, cuttlefishes and the nautilus. They belong to the phylum Mollusca and are related to gastropods and bivalves, but unlike their hard-shelled cousins, squids do not have an external shell. They have a remnant of a shell, internally, called a pen or gladius. The gladius is long, slim and very thin and flexible. (The ram's horn squid internal shell is a chambered, coiled structure.) Squids have an outer body called the mantle with two lateral fins. The fins are used for balance and steering more than for locomotion. A siphon behind the eyes provides propulsion and expels waste. Attached to the mantle are the eyes, the beak-like mouth, eight arms and two longer retractable tentacles. The underside of the arms and the ends of the tentacles are covered with suckers.

Squid have the ability to rapidly change their colors and color patterns to camouflage themselves with the surrounding background. They also protect themselves by squirting black mucous ink into the water to confuse their predators. Some squid produce bioluminescence which is another method of protection because the light they produce disguises their contours in surrounding dark waters.

Squids travel in large schools. Most undergo diel (24-hour) vertical migration, where they may occur at depths of 200 to 400 m during the day and then migrate to surface waters at night. There are extensive commercial fisheries for squid, worldwide. In the U.S. they are commonly marketed as "calamari". Squids are also important in many biomedical researches. They have highly developed brain and sensory organs and have the capacity to learn and remember. This makes them valuable for behavioral and neurological studies.

The family Spirulidae contains only one species, Spirula spirula, the deepwater ram's horn squid.