Frightening stories about sea monsters often focus on the giant squid. This sea creature can be more than 60 feet (18 meters) long and weigh 1,000 pounds (454 kilograms). Its ten arms are lined with suckers that form a powerful vacuum when they attach to prey. The giant squid, however, is only one of several hundred different kinds of squid. Although most are similar to their huge relative, they are far less scary looking. In fact, some squid are less than an inch (2.5 centimeters) in length.

Physical features

Squid are a type of mollusk. Mollusks are invertebrates (animals without a backbone) with soft bodies. Unlike most mollusks, however, squid, octopuses, and other members of the scientific group Cephalopoda do not have a hard outer shell. Instead they are strengthened by an internal shell.

A squid's body is long and shaped like a tube. The vital organs, such as the heart and gills, are enclosed by a thick, muscular fold of flesh called the mantle. The ten arms extend from the head area. Two of them—the tentacles—are much longer than the others. The tentacles have flattened ends with four rows of suckers, making them especially useful in catching prey. A squid's eyes are very large and usually set on the sides of the head. They resemble those of humans but do not have lids.

Behavior

Squid live in coastal and deep-sea waters throughout the world. They travel together in groups, or schools, often chasing after schools of fish to eat. Squid are generally very fast swimmers. They push themselves forward by taking water into their bodies and then forcing it out, which creates a jet of water.

Squid themselves are food for many animals. Sperm whales eat giant squid, and fish and sea birds feed on smaller types. Squid often use their speed to escape danger, but they also have other methods for protecting themselves. They can hide from enemies by becoming transparent (see-through) or by changing color to blend in with rocks or seaweed. Squid can also spray a cloud of dark ink to confuse an enemy.

After mating, a female squid lays hundreds or even thousands of eggs in the form of jellylike strands. The strands are often attached to the ocean floor or to floating weeds. The female leaves the eggs alone to develop. Within several weeks they hatch into a small form of the adult.

Squid and humans

Squid are valuable to humans both for commercial purposes and for research. In many countries people eat squid, and sometimes they are used as bait for fishing. In the laboratory, scientists have studied the squid's unique nervous system, which has very large nerve cells. These studies may provide information that will someday help to cure human diseases of the nervous system, such as Alzheimer's disease.