Squid Diversity

Squids come in a wide range of sizes, from smaller-than-your-thumb to the enormous giant squid. (Kim Moeller/Smithsonian Institution)

The giant squid is not just a single species -- or is it? Some researchers think there are as many as 8 species in the genus Architeuthis (Greek for "chief squid"), each a different kind of giant squid. But other researchers think there is just one Architeuthis that swims in the world's ocean. There is no consensus because the squid are so hard to track and there are so few specimens available for study.

However, it is certain that Architeuthis has an abundance of evolutionary relatives. The ocean holds an estimated 500 species of squid—and almost all of those are in the same taxonomic order as the giant squid, called Oegopsina. Some are surprisingly tiny—only about 1 inch (2.5 centimeters) in length. Others are impressively large, including the colossal squid (Mesonychoteuthis hamiltoni), which can grow to be even bigger than the giant squid, reaching 45 feet (14 meters).

These squid species are closely related to snails, clams, and even slugs: they are all mollusks, which are defined by their soft bodies. Some of these soft bodies are encased in hard shells, such as clams and snails, but not the squids.

Squids belong to a particularly successful group of mollusks called the cephalopods, which have been around for about 500 million years. Some ancestors of modern-day squids had shells, such as the ammonites, which ruled the waves 400-65 million years ago. Of those that are still around, one small group—the nautiluses—has an external shell. The other—which includes squids, cuttlefishes, and octopods—does not, although squids and cuttlefishes have an internal, backbone-like support made of chitin called a pen.

Shell or no shell, all cephalopods have well-developed brains and are very active, jet-propelling themselves through the ocean. Most have ink sacs. And many can change skin color and texture in the blink of an eye.