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Squid. Squid are free-swimming animals, usually seen in groups in shallow lagoon areas or along the reef edge. They have perfected the mode of jet-propelled movement. While stationary they maintain position with gentle undulations of their lateral fins. Movement, either forward or backward, is achieved by the highly maneuverable water jet. Like the octopus, squids can change their coloration, adopting a sparkling array of brilliant colors and patterns.

Squid have a rudimentary internal "shell," actually a noncalcareous strengthening device called a pen. It is of a clear, flexible substance that looks and feels like a piece of plastic.

Although you will never see one on the reef, the largest cephalopods by far are the giant squids (Architecteuthis), which can reach a length of 18 meters. These animals frequent very deep water, and little is known of their habits. They are the preferred prey of the sperm whale.

Cuttlefish. Cuttlefish (Sepia) superficially resemble squid, but can be distinguished by their generally larger size and more robust shape. Unlike squid, which often travel in large groups, lone cuttlefish can often be seen foraging on the reef slope, and are the most frequently encountered cephalopods.