Sea Cucumbers for the Marine Tank

As with many aspects of saltwater aquarium-keeping, aquarists tend to be very opinionated about whether or not it is a good idea to keep a sea cucumber in a marine tank. In reality, it is not that complex of an issue, but it is one that requires the aquarists to make some key decisions about the type of tank they plan to keep and their commitment level to maintaining an environment conducive to the health of a sea cucumber. This article attempts to give the aquarist the information they need to make such a decision.

Sea cucumbers are echinoderms (phylum Echinodermata)—related to starfish, sea urchins and brittle stars. Taxonomically-speaking sea cucumbers are part of the subphyla Eleutherozoa or the motile echinoderms (as opposed to the sessile echinoderms like crinoids). The phylum name translates as something like “spiny skin,” but the sea cucumbers so-called spines are really like bumps or warts. Sea cucumbers mostly feed by 1) collecting food with feeder tentacles or 2) “vacuuming” sediment. It is the latter type of sea cucumber the aquarist will probably want to add to his or her system given their capacity to clean and constantly shift the upper layer of the substrate.

First the bad—the number one reason aquarists fear sea cucumbers is because of the animal’s ability to eviscerate itself and release various toxins (e.g. holothurin) that can wipe out all the fish in a reef or fish-only system. Species within the order Aspidochirotida, for example, can force their cuvierian tubules (part of their respiratory system) out their anus as a sort of sticky, stringy web to defend themselves from threatening predators. Some sea cucumbers can also expel parts of their digestive system (evisceration) as a defense mechanism or a response to environmental stressors (although there is some data to suggest they do it seasonally as a means of expelling toxins from their own system). This evisceration is not always accompanied by chemical discharge, but when it is, it can have dire consequences for the fish in the tank.

Now the good—the sand-feeding sea cucumbers are some of the best sandbed cleaners and agitators the aquarist can keep, and many are inexpensive and readily available at the local fish store or online. Many of the species from the genus Holothuria, for example, are prolific detritus consumers able to literally ingest the top layer (1/2” or so) of the sandbed, clean the fine grains, (digesting detritus particles, bacteria and microalgae), and then expel it. This vacuum cleaner-like behavior keeps the sandbed looking great, while requiring very little maintenance on the aquarist’s behalf.

Some of the most commonly available sea cucumbers in the hobby come from the genus Holothuria and include the Black Sea Cucumber (sometimes called the edible cucumber and growing to 12 inches) from the Indo-Pacific, the Pink and Black Sea Cucumber (Holothuria edulis) also from the Indo-Pacific and growing to eight inches, and the Tigertail Sea Cucumber (from the Indo-Pacific and growing to 12 inches)—sometimes called the Sand Sifting Cucumber, the Lollyfish and the Light Spotted Sea Cucumber (Halathuria hilla). All of these species are commonly available for under $20.

In conclusion, many aquarists agree that the benefit of sea cucumbers outweigh the risks, but each hobbyist should make the decision for himself or herself. For the aquarist who purchases a sea cucumber as part of his clean-up crew, keep the following tips in mind:

• Only keep sea cucumbers in a tank with a sandbed.
• Only have one sea cucumber for every 50 to 75 gallons of aquarium water.
• Keep the water quality high.
• Make sure all powerheads and intakes are covered.
• Watch out for species that may nip and harass sea cucumbers (e.g. triggerfish, Wrasses, Butterfly fish, and large hermit crabs).

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