For squid residing in the middle levels of the food pyramid, the “audience” includes both predators and prey. In the open ocean, from shallow to deep waters, young fast-growing squid provide a key link in the food pyramid between copepods, krill and other zooplankton and the top-predator fish and marine mammals. Squid come in many sizes, shapes and species. Some travel in groups; others are strictly solo acts. At least 181 squid species have been identified in 25 families that make up the order Teuthoidea, although the systematics is far from complete, and there are almost certainly new species still to be found or sorted out from what amounts to a taxonomic tangle. They range in size from less than an inch (2.5 cm) in length to more than 50 feet (15 m).
The squid body is a marvelous, odd design that somehow works. Squid are mollusks, but they are nothing like clams or mussels. In fact, the shell is located inside the body in a reduced fragmentary form known as the pen. The shell has atrophied in favor of other features of the bizarre squid anatomy.
Just look at a squid next to an octopus. By comparison, an octopus is simple: a head with eight arms attached. On a squid, it’s not so easy to tell which end is which. Swimming through the water, a squid doesn’t give the game away, because it can jet forward or backward. So which way is forward and which backward? The squid appears to have a tail or tail appendages at either end. In fact, the 10 appendages—eight thick octopuslike arms and two thinner, longer sucker-laden tentacles for catching prey—grow out of the head. At the hydrodynamically shaped rear end of the animal, called the mantle, two fins undulate up and down, providing some propulsion in certain squid. The main jet propulsion, however, is produced when the squid draws water into its mantle cavity, then expels it under pressure through the siphon, which it can rotate to change direction. The rapid and constant movement of water through the animal also provides the crucial source of oxygen; gills inside the mantle cavity extract oxygen from the water.
Squid have been called “invertebrate athletes” and “Olympian cephalopods” by squid researchers Ron O’Dor and R. E. Shadwick of Dalhousie University in Halifax, Nova Scotia. Their view is that jet propulsion is an inefficient way of getting around which forced squid to become extremely athletic, employing high-power outputs and rapid oxygen consumption to achieve speed bursts that can exceed those of the fastest fish. But squid are also marathoners, able to complete migrations over many hundreds of miles.
Besides the unusual body plan, a big part of the squid’s success lies in its excellent eyesight, its brain and the high percentage of body weight devoted to its nervous system. First, the eyesight. The rods and cones in the retina suggest the ability to obtain detailed images that may include color. As the squid approaches the near darkness at depth, its pupils expand dramatically to fill its large eyes. But extraocular photoreceptors, roughly comparable to those found in insects and spiders, may allow some squid to sense overall light levels in the water, perhaps partly as a way to fine-tune their own bioluminescence.
Compared with the brains of fish and most other invertebrates, the squid brain is large and complex. It is literally a mass of nerve ganglia situated between the eyes and all around the esophagus. No wonder a squid macerates so thoroughly—its food must pass through its brain!
The squid’s secret source of power, however, is that it possesses the largest nerve axons, or fibers, of any animal—a single axon is about 100 times the diameter of a human nerve fiber. This has made the squid a longtime favorite for scientists studying nervous systems.
Superb eyesight, a complex brain and a supercomputer nervous system with fiber-optic-like axons act as potent tools that, together, give the squid lightning reflexes, enabling it to react to a stimulus and send messages to the muscles faster than in any other known group of animals.