It is also important to note that aside from their lack of tube feet, most apodid cucumbers lack the respiratory tree and most of the associated structures characteristic of the other sub-classes of sea cucumbers. Instead, these animals tend to gain oxygen and expel carbon dioxide primarily across the body surfaces. Because these animals lack a respiratory tree and the associated tubules of Cuvier (defensive structures with which the most potent toxins of sea cucumbers are associated), they are relatively nontoxic in comparison to some of their more potent relatives such as the sea apples ( Pseudocolochirus spp.) and spotted sea cucumbers (in particular, Bohadaschia argus and Actinopyga agassizii ). The Cuvierian tubules commonly found in species of Holothuria, Bohadaschia, Stichopus and Actinopyga liberate a highly toxic saponin compound known as holothurin. This compound acts to quickly stun and even kill potential predators, which are effectively asphyxiated (suffocated) by the toxin. The toxins of some of these species are so effective that South Pacific island cultures use the macerated bodies of these sea cucumbers to stun and capture fishes, crabs and lobsters as a method of traditional fishing (Frey 1951; Ruppert and Barnes 1994)! Despite the fact that the medusa worms lack these structures and are considered ‘relatively non-toxic’ by comparison to many of these species, this does not mean that they are by any means non-toxic. For example, the beautiful sea apples (e.g., Pseudocolochirus violaceus and P. tricolor ) also lack these Cuverian tubules, but along with Bohadachia argus and Actinopyga agassizii, are considered among the most toxic sea cucumbers in the world. In fact, according to Wilkens (1998), it takes only about 1g of tissue from any of these particularly toxic species to poison the fish in a 25g tank. These toxins also affect humans, and many people suffer moderate to severe skin and eye irritation if they come into contact with these toxins (Cunningham and Goetz 1996). Some cases in which toxins came into direct contact with the eyes have resulted in blindness, and deaths have even been reported in cases where people have eaten these animals without the proper preparation.
In the most general terms, virtually any soft-bodied animal that would make easy prey on a coral reef, such as these sea cucumbers, will typically be defended in some way by distasteful chemicals or physical armament, and medusa worms are no exception. Like virtually all sea cucumbers, they are soft- bodied, lacking any real physical armament, and so they all tend to have a variety of nasty chemicals associated with their bodies to deter predators from feeding on them. Even without the Cuvierian tubules and their potent toxins, medusa worms have a variety of distasteful chemicals associated with the skin and body wall to protect them from being eaten by fishes, crabs and lobsters on the coral reef. Although the specific toxins associated with synaptid cucumbers are somewhat different from those of most other cucumbers studied to date (Kuznetsova et al. 1989; Ponomarenko et al. 2001), these animals are still reported to be highly toxic to fishes in marine aquaria if they are seriously injured (e.g., Delbeek and Sprung 1994; Fenner 2000; Michael undated online; Sprung 2001). In general, toxins are only released when the cucumbers are under severe stress (such as being chewed up after being sucked into a powerhead or overflow grate), and a diligent aquarist will usually prevent this from ever happening, and therefore never experience any problems with one of these animals. If however, and accident happens, and the cucumber is stressed severely enough to release its chemical defenses, then a good water change, together with an efficient skimmer and some activated carbon are usually sufficient to prevent any fish from asphyxiating from the soap-like holothurin. I will not detour further into the chemical defenses of sea cucumbers in this article, however I may come back to the subject another time.
