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Most species are detritus feeders, the tentacles being used to pick up sand and pass it into the mouth. Organic matter is digested and the undigested remains are passed out through the anus. Sea cucumbers have to eat a lot of sand in order to obtain enough food, so they often leave a continuous trail of sandy feces behind them. A few species are filter feeders. They hide their bodies in reef crevices and hold their tentacles up in to the water current to feed. The tentacles are rapidly withdrawn if disturbed.

Sea cucumbers appear as elongated and somewhat flaccid forms lying among coral rubble or sea grass, moving slowly in a worm-like way by contractions of their bodies. These are usually black or dull-colored. A few species, such as the sea apple (Pseudocolochirus), are very colorful, however.

Many sea cucumbers are active by day. Since they are not attacked by predatory fish it would seem that they must have some efficient means of defense. Some species can discharge sticky white threads if molested, and most tropical sea cucumbers contain toxins.