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Like the other cephalopods, cuttlefish can squirt out a blob of ink if threatened. The shape of this blob, roughly the size of the animal that ejected it, and its strong smell, distracts the would-be predator while the cuttlefish jets away. In earlier times, this ink was used for writing, as is suggested by the cuttlefish's genus name, Sepia.

Instead of the squid's flexible pen, cuttlefish have a "cuttle-bone," a calcareous structure perhaps most familiar for its use as a dietary supplement for cage birds. Although it provides some stiffness, the most important use of the porous "bone" is for buoyancy control, balancing the animal's vertical movements across the reef face.

Nautilus. These animals, with their distinctive spiralled shell, are the most unusual of the living cephalopods. The chambered shell serves as a form of buoyancy control, like the cuttlefish bone, but much more sophisticated. This control is necessary as the animals undergo a considerable daily vertical migration. During the daylight hours, the nautilus stay at 1,000-1,500 meters, and only rise into relatively shallow water at night. Only very rarely are they found in depths a sport diver could reach. In this way they avoid predators, and perhaps also are able to more easily find their food—carrion and, particularly, the molts of crustaceans.