Likely Reef Tank Suitable
Likely Fish-Only Tank Suitable
These are gentle unassuming creatures that mostly look like a shriveled-up old cucumber, hence the name. Even though they are the most common animals found in deep ocean environments, most well known species and/or aquarium specimens come from shallow reef and coastal areas. They range in length from less than 1 inch (2.5 cm) to more than three feet (1 meter). Some burrow into the substrate and feed by spreading mucus-covered tentacles into the water column to collect suspended matter. Others spend their time gliding over sand and rubble looking for pockets of detritus and/or diatom-coated sand grains, or filter feeding on planktonic material such as the Sea Apple (Pseudocolochirus).
There are also worm-like cucumbers called Medusa worms. These legless sea cucumbers are members of the family Synaptidae (Order: Apodida). They all look quite similar with soft and flaccid bodies with rounded knobs. They project their tentacles directly into the substrate to collect organic coated particles. They are nontoxic when compared to other cukes, especially the Sea Apple. The most common genera are Euapta, Synapta and Synaptula from the Indo Pacific. Those from the Caribbean are usually Euapta lappa or Synaptula hydriformis.
Many of these creatures are nocturnal and all require large amounts of foodstuffs, which they process fully within about one hour.
Basically, sand and/or other organic material go into one end, are separated, with organic matter being digested and unwanted/sand particles expelled at its opposite end. Always reminds me of something like a car wash!
Yet there remains caution if these creatures are of interest for the home aquarium!
If stressed, they can release a toxin (Holothurin) capable of killing fishes, and possibly some small worms/tubeworms and mollusks, yet not corals/other invertebrates.
This toxin can be released in several ways; two are defensive - Evisceration and Cuvierian Tubule Expulsion.
Evisceration - depending upon the species, it can rupture several of the inners and expel them thru the anus, or the frontal part of the body ruptures with the tentacles, pharynx and part of the intestine discharged. Does not kill the cucumber, as these parts are regenerated within a few weeks.
Cuvierian Tubule Expulsion - These long very sticky white or pink internal structures containing Holothurin are expelled thru the anus and help entangle the foe, which dies shortly thereafter in the sticky mass. After the happening is completed, tubule are regenerated and the cucumber returns to its original healthful status.
A third way Holothurin is released is simply by the cucumber dying (usually caused by a lack of proper foodstuffs) or being damaged by a piece of aquarium equipment or being picked on by some fishes.
Cucumbers will also release Holothurin if stressed/experience sudden changes in temperature or salinity.
Releases of Holothurin can, within 5 minutes, kill all the fishes in the aquarium.
Which cucumber are toxic is a very good question, and it is said over 60 species are toxic.
Those such as Actinopyga agassizii, Bohadschia argus, Holothuria edulis, Holothuria impatiens, Holothuria parvula, and Medusa worms in the genus Synapta and Euapta, along with the colorful and commonly seen Sea Apples in the genus Pseudocolochirus are among the worst when it comes to releasing this toxin.
And because of the above described situation, have decided its better to list almost all these cucumbers described here as not safe for reef or fish-only aquariums to simply bring that situation to your attention.
With that said, some are sold for the home aquarium trade and are, if monitored in the aquarium, a useful/interesting creature. Nevertheless, keep in mind they should be closely watched and properly housed with the correct tankmates (not with triggerfishes, pufferfishes, large crabs or lobsters) and have access to the food needed to meet their nutritional needs.
If its health is dwindling, it must be removed.
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