Sea Apple

With its egg-shaped purple-red body, the yellow or orange rows of feet running along it and the red-and-white tentacles branching like trees around its mouth, the sea apple is one of the showiest sea cucumbers. It grows to about 15 cm. It opens its tentacles into the current to catch plankton from the water, then puts each tentacle into its mouth in turn to clean it off.

Its risks are as great as its looks. Toxin warning: The tissue of sea cucumbers contains toxic saponin-type substances. When the sea apple is stressed, sucked into a pump, picked at by fish or dies, these substances are released into the water and can cause mass deaths of fish and other organisms. In addition, since it does not photosynthesize, it needs plankton every day; in very "clean" aquariums it slowly starves to death. For these reasons it is suitable only for experienced keepers and well-prepared aquariums. Those on the market are wild-collected. The Australian sea apple (Pseudocolochirus axiologus) is also sold under a similar name and carries the same risks.

It lives around Indonesia, the Philippines and northern Australia, on reefs and lagoon edges swept by nutrient-rich currents. It clings to rocks with its feet and opens its tentacles at the point where the current reaches it best.

• Volume: At least 200 liters; a mature, very well established aquarium with plenty of live rock.
• Water: Salinity specific gravity 1.024–1.026, pH 8.1–8.4, temperature 24–26°C; sudden changes can cause it to shed its tentacles.
• Flow: Moderate to strong flow; but all pump and overflow intakes must be covered with sponges or grates.
• Safety: Activated carbon should always be in use in the aquarium, and extra carbon and premixed saltwater should be kept on hand for emergencies.
• Reef compatibility: It does not harm corals; the main risk is that its toxin can affect the entire aquarium.
• Acclimation and medication: It is acclimated by the drip method for at least 2 hours; copper-based medications are lethal.

A filter feeder. Live or concentrated phytoplankton and fine plankton foods such as rotifers and copepods should be given every day. When feeding, it helps to turn the pumps down briefly and release the food slowly near the tentacles. A shrinking body and tentacles that rarely open are signs of hunger.

Peaceful. It slowly roams the glass and rocks until it finds a suitable spot; after that it usually stays in the same place. From time to time it fully retracts its tentacles and inflates and deflates its body; this alone is not a sign of disease.

Good companions: Clownfish, cardinalfish, gobies and peaceful small reef fish; cleaner shrimp, snails.

Avoid: Fish that may pick at its tentacles, such as butterflyfish, angelfish, triggerfish and pufferfish; large hermit crabs; unprotected pump intakes.

Kept alone. The amount of plankton in an aquarium can feed only one individual; a second one increases both the risk of starvation and the risk of poisoning.

The sexes are separate; it releases its eggs and sperm into the water. Spawning in the aquarium can foul the water; it is not bred in home aquariums.

• Starvation: The most common cause of death; the body slowly shrinks.
• Shedding tentacles: Sudden water changes and stress can cause its tentacles to break off.
• Death: A dead or disintegrating individual should be removed immediately, a large water change done and fresh activated carbon added.

Males and females cannot be distinguished externally.