Quote:
Originally Posted by aquawolf
I understand they are excellent algae eaters and have wanted to try one myself; however, I have been too scared because if I understand correctly when one of them dies it will release toxins that can wipe out your entire tank...
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Well, yes, they do eat algae, but not in the sense that most algae eaters consume algae. They actually consume the biofilm off the surface of sand granules or may eat detritus they can capture with their oral tentacles in the absence of sugar-sized sand. AS far as eating an algal bloom, I think you might be a bit disappointed in their performance...
Holothuroids generally harmless to the tank, they all contain amounts of soap-like substances called saponifiers that they employ as chemical defenses. Some of the more colorful species DO contain higher concentrations of a saponifier substance that
may be lethal in a closed system to vertebrates if the cuke "pukes" i.e.,
eviscerates,
or ejects the sticky Cuvierian tubules. These two defensive mechanisms may appear to the casual observer to be the same mechanism, but in fact are two different variations of similar defensive mechanisms employed by many genera of the
Holothuroids.
In
Holothuria spp. and some
Actinopyga spp. of
Class Holothuroidea, specimens when aggravated or attacked by some potential aggressor/predator will direct the anus towards its attacker and release any number of Tubules of Cuvier; a mass of pink, white or red blind tubules attached to one or both respiratory trees or to the common trunk of both trees in the gut area of these creatures. When threatened, these spp. of sea cucumbers force water into the lumens of these tubules, which then become enlarged and break free of their points of attachments to the respiratory tree. They are then ejected through the posterior opening in the general direction of the potential aggressor. In some spp. of sea cukes, these tubules are very sticky, and may entangle small crabs and shrimps so much that they are immobilized and left to die. In some spp., the tubules contain a substance known as holothurin (as does the body tissue of many spp.), a saponin that changes the ability of semipermeable membranes to function as biological (gradient) barriers. In many vertebrates and in many arthropods, this will cause the attacked specimen to become immobilized and possibly die, allowing the sea cucumber to escape. The cucumbers are able to regenerate the tubules within a few days to full recovery of this mechanism. Although these substances are not true toxins and are more like soaps, they may have a toxic effect in small volumes of water where there is no means of exporting them from the water column. Interestingly en ough, there are some South Pacific islanders that until recently used macerations of certiain Holothurians to cath fish in tidal pools.
This behavior is not to be confused with
evisceration, a much more common activity in holothuroids that may be either seasonal or associated with periods of inactivity due to low food supplies. Depending on the specie, either the anterior or posterioir end of the specimen ruptures as the connective tissue depolymerizes and parts of the body wall and gut rupture and are discharged from the animal. This is thought to be a means in some spp. of ridding the body of wastes stored in internal tissues, and may represent a seasonal change associated with anticipated periods of inactivity or low food aviliability, during which time the specie in question enters a period of inactivity during the regeneration of the lost organs and tissues to reactivate when food supplies are again plentiful. None of the species of holothuroids normally imported or collected for the aquarium hobby display this phenomenon as a normal seasonal activity, but may do so when provoked, accidentally injured, or attacked. The defensive mechanisms employed will vary from specie to specie, and may involve pushing the tubules through the body wall of the specimen in question when under siege...
Another defensive mechanism involving echinoderms as a class (including many spp of sea stars) involves the presence of
mutable (or catch) connective tissue. The stiffness of a specie would seen to normally be based on the presence, concentration and size of ossicles in the skin of the specie in question. One would then surmise that the stiffness of the skin of echinoderms would range from the rigid stiffness of the fused tests of sea urchin ossicles to the fluid worm-like softness and flexibility of the small and widely spaced ossicles of
Ophiuroid serpent stars. This could not be further from the actual truth: Echinoderms as a phylum can voluntarily change the stiffness of their connective tissue quite rapidly from very rigid to very fluid, almost like immediate liquefaction. They utilize this in a number of different mechanisms. Some Sea stars that feed on bivalves will grab a clam's shell halves, then solidify their tissue inside the rays to give the tube feet a scaffold to pull against to open the shell enough to inject their stomach (to digest the mollusk prey). Sea urchins normally wave their spines around in a liquefied base of a moveable socket, however, will move into a crevasse when attacked by fish and set their spines into the walls and present a defensive ball of quills locked rigidly in place.
Brittle stars will soften the tissue in ray to allow the ray to be cast off when attacked by fishes. This same localized depolymerization of the connective tissue is the mechanism that some sea cucumbers use to push their tubules through the walls of their body compartments.
The actual physiological mechanism of this depolymerization and repolymerization is a focus of research, however, there appears to be a neurological control component: although the dermis contains muscle, nerve, ossicle and other related types of tissues, it is the connective tissue itself as an extracellular matrix that is the tissue responsible for this mutability. Apparently there are two types of nerve tissue that terminate in this extracellular matrix and are responsible for the hardening and softening of the matrix. Research into the physiology of this mechanism demonstrates that crosslinking of macromolecules within the connective tissue matrix by calcium ions (a few other indicated in experimental papers as well) is responsible for the degree of stiffness, increasing crosslinking is proportional to the amount of stiffness. This is one of many ionic mechanisms that echinoderms depend on for functionality within marine environments, and may account for the paucity of echinoderms in FW biotopes.
For the most part, these sea cucumbers are reef safe, and unless in tanks where the specimens are provoked by overly curious fishes, or are crushed by falling rocks, etc., generally do not eviscerate nor eject their Cuvierian tubules. Even when they do, skimming, aggressive use of carbon, immediate large percentage water changes, and immediate removal of the offending substances/body tissues should prevent problems in larger systems, although smaller systems may experience problems with vertebrates where the total water column is less than 50 US gal total volume.
I keep
Holothurian spp of sea cucumbers, as they safe for the reef system under most conditions and they keep sand beds clean, a valuable trit of these intereting little echinoderms, btw I have about 10 in a 180 gallon reef tank with a 12cm deep DSB.
Sir Royal Master Cody William Deuter the 3rd
Scott 
My orthopedic thought it wasn't too late to try medical school, but he recommended PA school instead... Not sure that will happen either
