The Phylum Echinodermata contains over 6,000 species in 6 classes having 35 orders. The word echinoderm means "spiny skin" and this phylum includes urchins, brittle stars, starfish, cucumbers, and feather stars. These creatures all have an internal calcium skeleton and mostly travel using tube feet. Their vascular system pumps water, not blood. By pumping water into or out of their tube feet, the animal is capable of securing itself to most surfaces. Their mouth area usually contacts the substrate, except for cucumbers. They are scavengers and/or filter/suspension feeders.
Sea Cucumbers 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 relatively 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. For some species, sinking shrimp pellets are a good food source should there not be enough detritus coated material in the aquarium. And, it's not a good practice to place any of the roaming sand and rubble cleaners in newly established aquariums, as they will slowly starve. And be forewarned, cucumbers will grow smaller if they don't find sufficient food. If this happens or they are pestered by tankmates, they should be removed from the aquarium before they release deadly toxins (holothurin and holotoxin). Even though happenings such as this is quite rare, these toxins, chemically called triterpenglycosides, will cause fishes in the aquarium to act skittish, exhibit respiratory distress, and probably die soon after exposure. Other invertebrates are usually not effected. Since these toxins reduce water surface tension, a protein skimmer may overflow. And if a cucumber is sucked into a powerhead and shredded, I recommend a major water change, increased protein skimming and possibly a canister filter with an ample amount of activated carbon be employed.
Even though most are fairly ugly, some are excellent substrate cleaners, such as some members in the genus Holothuria, i.e., H. hilla, H. impatiens and H. thomasi. Sand and organic material that go into one end are separated, with organic matter being digested and sand particles expelled at its opposite end. Always reminds me of something like a car wash!
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