Sea apple

Sea apples are the common name for the round, brightly colored sea cucumbers belonging to the genus Pseudocolochirus, which live in the Indo-Pacific. These animals are filter feeders, equipped with feathery tentacles, oval bodies, and tube-like feet. Like many other holothurians, they can expel their internal organs or release a toxin into the water when stressed.

Anatomy and feeding

An adult sea apple’s oval body can reach up to 7.8 inches (20 cm) in length. Around its central mouth are feathery tentacles that add extra length. Rows of tube feet, typical of echinoderms, help the sea apple move and cling to surfaces. Their bodies and tentacles display a variety of colors; the Australian species, for instance, has a mostly purple body, red feet, and purple-and-white tentacles.

Sea apples primarily eat plankton, which they filter from the water using their tentacles. They bring each tentacle to their mouth one at a time, scraping off the captured plankton. They usually feed at night, when their delicate tentacles are less vulnerable to predators.

Defense

When disturbed, sea apples can violently eject their entrails from their rear end—a process called evisceration or autotomy. They can also release a toxic saponin called holothurin into the water as a defense. Additionally, if threatened or placed in an unsuitable environment, they may swallow large amounts of seawater to swell to nearly double their normal size. This allows them to be carried by currents to a new area much faster than they could crawl.

Because of their unusual appearance and behavior, sea apples are often sought after for marine aquariums. They are considered reef-safe in terms of compatibility with other species, but they can be problematic for several reasons. Many sea apples starve in display tanks because plankton levels are usually too low. They may try to feed during the day as well as at night, but with insufficient food they gradually shrink.

Harassment and predation

Hobbyists sometimes try to supplement their diet with plankton and liquid food. Sea apples are also frequently harassed by tank inhabitants like hermit crabs and fish, which peck at their tentacles—either to eat them or to steal trapped food particles. In response to this harassment, sea apples may use their defenses, and the release of their toxin can poison other aquarium animals, which is one reason they are not commonly kept.