Squid and Octopus are mollusks that differ from their snail and bivalve cousins by having little or no shell. They are two of four creatures classified as cephalopods. Squid have reduced their shell to a feather shaped structure that looks and feels like clear plastic, called a pen. It allows them some rigidity in the water column, so they can grow to greater size. Actually, the giant squid is the largest invertebrate on the planet, reaching a length of 50 feet or more and weighing nearly a ton. The octopus lacks a shell all together. Without this rigid structure, they cannot reach the great size of the largest squid. The largest octopus (found in the Pacific) is about half the size of the giant squid – big enough.

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Octopus at Pier © Harry Purcell

Another very prominent difference between these cephalopods and other mollusks is the large head with a brain and eyes with excellent vision. Squid and octopus have 8 arms that evolved from the primitive mollusks foot and are at the head end of the body. Each arm has a series of sucker cups along the length of the limb. Squid have 2 additional tentacles that differ from arms only in length and the suction cups are only near the end of the limb. Both keep their appendages close to their bodies. When in range of prey, they quickly extend an arm grabbing the prey and bringing it back to the mouth where a sharp parrot-like beak is found. Squid prefer fish and pelagic shrimp. Octopus are inclined to hunt crustaceans and other mollusks, but will grab a fish when the opportunity presents itself.



While a shell is not foolproof when it comes to being prey, a mollusk without a shell needs other methods of defense. One is speed. With no heavy shell, squid are some of the fastest invertebrates in the sea – being clocked at 16 mph. While octopus are not as fast, they are more flexible and live in environments with more hiding places. Both animals possess color-changing cells called chromatophores that they control with their muscles and brains. This ability to change color and hide is unmatched in the animal kingdom. Squid and octopus are quite common in our waters but not easy to see as both are more nocturnal in nature and have this unique ability to camouflage.


They have a more developed brain and nervous system than other invertebrates. The large brain encircles their esophagus and is not only used to ascertain the colors of the environment, but also has the capability of learning and memory.



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