Figure 1: Drawing of a sea cucumber by lupiloops.
The sea cucumber might not be the prettiest creature of the sea, especially compared the pom pom crab. However, the ones found in the Aspidochirotida order are equipped with the most fascinating defense mechanism. A behavioral adaptation that involves literally spewing their guts out, however, through their rectum. When I first read this I questioned why? Where did this adaptation come from?
It seems they actually eject an internal organ, called cuvierian tubules, and spend up to five weeks to regenerate it (Vandenspiegel et al 2000). These tubules expand upon ejection and deter any predators attempting to attack them.
I have been searching for the answer of when this adaptation arose, but there seems to be struggles in the phylogeny of this species as traditional taxonomic identification based on morphology is not as relevant, seeing as they are quite cryptic species, as noted by Byrne et al (2010). The order Aspidochirotida is from Late Ordovician according to Reich (2010), but is that clade in general based on their distinct defense mechanism or on other traits of that order? There seems to be little research into how this defense mechanism came to be, did the first sea cucumber just get so scared it pooped its pants and subsequently scared the predator off? Initially I would not think so, because these are specifically designed organs used to entangle and confuse predators (Flammang et al 2002). But the species could not have specifically designed a new organ to deter predators out of the blue, there must have been a step before. It seems most research is focused on the regeneration of these tubules, and not the origin of them. However, I did find one article that went into some of the evolution of this defense mechanism. Lanterbecq et al (2008) expressed in their abstract that using a reconstruction of a phylogenetic tree, the ancestor of the Aspidochirotida had ramiﬁed, nonadhesive, nonexpellable and nonstretchable tubules.
Overall, however, there does not seem to be a lot of research put into these creatures. Funding to do so might be hard seeing as they are far from any typical flagship species. That does not mean they are not important. They do play an important role in the marine ecosystem as detrivores, which is exactly why these sea cucumbers should receive more attention.
To hopefully increase any further interest in you, I leave you with this dramatic “World’s weirdest” youtube video.
Next week let us diverge from this subject you would not want to be reading about, nor see for that matter, while eating. Trust me, I know.
Let us talk about the incredibly surprising animal that made me interested in marine biology in the first place, Nudibranchs.