Squid is the common name for a group of marine mollusks (order Mollusca) with highly developed eyes and brain, and complex swimming behavior. About one-half the length (24-36 in; 60-90 cm) of the common North Atlantic species Loligo pealei consists of its streamlined cylindrical body, and the other half is its set of eight arms and two arm-like tentacles. These appendages are equipped with small suction-cups, and surround the mouth opening. Squids have no external shell, but have an internal stiffening structure known as the rod or pen. The squids of the Mediterranean are called calamares, from the Greek kalamos and Latin calamus, meaning writing-reed or pen. The body of squids has a pair of flexible, roughly triangular fins. The skin contains many pigment cells or chromatophores, capable of changing the color of the animal through expansion and contraction.
Squids are classified in the class Cephalopoda and subclass Coleoidea (Dibranchiata), which places them with octopuses and cuttlefishes. Further classification places squids in order Decapoda and octopuses in the order Octopoda. Mollusks in the subclass coleoidea are separate from the chambered nautilus (subclass Nautiloidea) and the ammonoids (subclass Ammonoidea). Ammonoids became extinct at the end of the Cretaceous period. They were apparently active predators, with a coiled shell similar to the nautilus. Some species were up to 3 ft (1 m) in diameter.
The first fossil mollusks originated during the primary radiation of the Precambrian era. These were gastropods (snails), followed by bivalves. Nautiloids appear later, and the Coleoidea (squids and related forms) later still. In terms of lineage, therefore, squids and octopuses are the most recently evolved of the major groups of mollusks.
There are about 650 species of living Coleoidea. Stasek (1972) regarded the Coleoidea as the result of evolution of a nautiloid lineage, modified in the direction of loss of shell "and increased streamlining, speed, and muscularization of the mantle." A group of squid-like animals called belemnoids appeared in the Permian period and flourished in the Jurassic and Cretaceous, leaving behind the fossil counterpart of the internal rod or pen of modern squids.
Reproduction in squids consists of the male inserting a packet of sperm into the mantle cavity of the female, using an arm especially adapted for this purpose, followed by egg laying on the ocean floor. The fertilized eggs, each about 0.01 in (2.5 mm) in diameter, develop in a mass of jelly, and hatch out as tiny squids, rather than as trochophore or veliger larvae as in other mollusks. Species of squids are found in all oceans, at all depths. The deep-sea species often have luminescent organs, which probably aid individuals to contact each other for breeding in absolute darkness.
The range of size of squids is extremely wide. The smallest squid, with one of the longest names, is Pickfordiateuthis pulchella. It is a shallow-water species less than 1 in (2.2 cm) long. The largest is a giant squid, Architeuthis dux, a deep-sea species growing as long as 59 ft (18 m). This giant squid is therefore about 800 times longer than the smallest one. Dwarf and giant species occur in other Mollusca; species of snails vary in diameter by about 300 times, and the largest and smallest bivalves differ in length by about 200 times.
Giant squids have been found dead on beaches in both the Northern and Southern hemispheres, and some have been captured off Australia and New Zealand. When the head and tentacles are fully extended, their total length may be up to 3.5 times the mantle length, great enough to inspire many sea-stories about deep-sea monsters. Scientific studies based on specimens of giant squid captured in the nets of trawlers have just begun. One study of tiny growth rings in statoliths of Architeuthis kirki suggested a life span of only about 2.5 years. This species was taken at an average depth of 1,750 ft (530 m) off the New Zealand coast. The largest animal had a mantle length of 7 ft (2.14 m). (A statolith is a small, stone-like part within the fluid-filled statocyst, an organ that enables squid to sense position and acceleration.)
Squid are active swimmers, moving swiftly to capture prey and to avoid being captured themselves. Swimming speeds of up to 11 yards per second (10 m/s) have been recorded, about the same as a world-class sprinter running a 109-yd (100-m) dash. The mantle cavity is normally filled with sea water, and when the muscles of the mantle contract at the same time, a jet of water is forced out through a "funnel." A principal anatomical feature of this system of propulsion is a nerve center (ganglion) with a giant axon (extensions of a neuron) emerging from it and carrying a signals to all of the muscles via branched endings. This giant axon, 0.13-0.25 in (0.5-1.0 mm) in diameter, is "giant" only in the sense that the axons of all other species are much smaller. Its size has been exploited in laboratory research, and has permitted crucial experiments that have advanced knowledge of how neurons work far beyond what was known before the giant axon was discovered. On the assumption that neurons throughout the animal kingdom function similarly, our understanding of the human nervous system has benefited greatly from study of the giant axon of squids.
Nesis, K.N. Cephalopods of the World. TFH Publ. Neptune City, NJ: 1987.
Gauldie, R.W., I.F. West, and E.C. Forch. "Statocyst statolith, and Age Estimation of the Giant Squid, Architeuthis kirki." Veliger. 37 (1994): 93-109.