The sea apple is a colorful, shape-shifting filter feeder that can weaponize its own body.

Nature · Published 3 October 2026 Swipe more like this →

In short: Found in Indo-Pacific waters, the sea apple is a striking sea cucumber that survives by filtering plankton through feathery tentacles. When threatened, this creature can perform a violent internal purge, swell to double its size to ride ocean currents, or release a potent toxin to deter potential predators.

The sea apple, belonging to the genus Pseudocolochirus, is a holothuroid echinoderm known for its vibrant, ovate body. Adults can reach lengths of up to 7.8 inches, or 20 centimeters. These creatures are equipped with rows of tube feet for movement and adhesion, while a central mouth is surrounded by specialized tentacles used for feeding. In the Australian species, these features manifest in a striking palette of purple bodies, red feet, and purple-and-white tentacles.

Survival for the sea apple involves sophisticated, if extreme, defense mechanisms. When stressed or disturbed, they can undergo evisceration, a process where they violently expel their internal organs from their posterior. Furthermore, they can secrete a toxic saponin known as holothurin into the surrounding water. If the environment becomes unsuitable, they can rapidly ingest seawater to inflate their bodies to nearly twice their normal size, allowing them to be transported by currents far more efficiently than they could travel by crawling.

Despite their aesthetic appeal, sea apples are notoriously difficult to maintain in captivity. In marine aquariums, they often struggle to find sufficient plankton, leading to starvation. Their delicate tentacles are also frequently harassed by fish and crustaceans, which may peck at them to steal food. When provoked by such harassment, the sea apple may release its toxin, which can prove fatal to other aquarium inhabitants. Because of these risks and their specific dietary needs, they are often considered unsuitable for most home reef displays.

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