Friday, May 20, 2011

Class Holothuroidea, the Sea Cucumbers

The sea cucumber Bohadschia argus

Echinoderms, in general, are an often overlooked group of creatures.  Other than for being ubiquitous members of tidal pools and the ocean floor, your average person knows little about them.  One class in particular, Holothuroidea, is generally seen as just a curiosity despite having a series of downright bizarre traits and behaviors.  Commonly known as the Sea Cucumbers due to their cylindrical appearance, these scavengers can be found in every ocean.  Not only are they indigenous to all oceans, many species travel in herds that can reach numbers well over a thousand.

Like other living echinoderms such as sea stars, urchins, brittle stars and crinoids, sea cucumbers posses what is known as pentaradial symmetry.  This means that their body plan can be split into five equal sides.  At first sea cucumbers seem to break this rule, but that is only when looked at from above.  When observed on end, the pentaradial symmetry is easily distinguishable.  What appears to be bilateral symmetry at first glance is an intriguing illusion when you take into consideration that all echinoderms evolved from a common ancestor that possessed bilateral symmetry, a feature that is only shown in modern day larval forms. 

Your average sea cucumber feeds by sucking up ocean sediment like a living vacuum cleaner.  Using feeding tentacles, they pick through the sediment and the remaining substrate is passed out the opposite end.  A few species will go so far as to extend their feeding tentacles to pick pieces of food out of the passing current they position themselves near.  There is also one species that gains its nourishment through a commensal relationship with deep-sea angler fish, though little is known about this relationship. They generally propel themselves along on 8-30 small tube feet that make use of echinoderms trademark water vascular system which is essentially a body wide hydraulic system.  One subclass, Apodacea, has gone so far as to completely lose their tube feet and instead burrows through sediment with a series of muscular contractions along their body.

All sea cucumbers have an endoskeleton, but in most, it has become greatly reduced to little more than small ossicles sparsely numbered throughout the mesoderm.  A few species have adopted hardened calciferous plates covering their body for protection, but it is not common.  This leaves most sea cucumbers with a rather soft texture that makes one wonder what sort of cucumbers the person was eating who granted these creatures their common name.  Many sea cucumbers make perfect use of their squishy bodies when it comes to getting into tight spaces.  Their body tissues contain a modified form of collagen known as catch collagen.  In most animals, this provides tissues with a flexible, but strong nature by forming a network of interconnected proteins.  Sea cucumbers have modified this protein so that they can become viscous at will allowing them to literally pour their tissues through a tight crevice.  Once in, they can let their catch collagen reconnect, reforming their tissues into the orderly form it once was.

As if turning themselves to liquid wasn’t odd enough, members of the order Aspidochirotida have a somewhat…unsettling defense mechanism.  They turn their anus at an attacker and through a tear between the coelom (body cavity) and cloaca (the common opening for the intestinal, urinary and reproductive tracts), they expel their respiratory tree which is coated in toxins and a sticky material.  This process, known as evisceration, ensnaring the would be predator in what one can only compare to the most nauseating imitation of Spider-Man ever.

One would think having the ability to spew toxic innards out of one’s ass would keep any sane creature away, but one peculiar fish is not detered.  Known as the pearlfish, many members of the family Carapidae make their home within the confines of the sea cucumber’s cloaca.  These slender fish often cause no harm or benefit to their host and simply use the space to remain hidden within.  A few species have been known to…ahem…eat out the sea cucumber, but this behavior is rare.  A few members of pearlfish have found their home so comfy they invite in their mate.  In a disturbing twist, the pearlfish uses the reproductive space of the sea cumber as its own love abode.

For the most part, sea cucumbers possess few traits useful to humans, though the flesh of some species is eaten in certain oriental cuisine.   Sea cucumbers have also been a staple of traditional Chinese medicine for generations, though the effectiveness is debatable.  A study single has suggested that the fatty acids found in sea cucumber tissue could play a role in tissue repair, but the actual use was never tested.  Another study found an extract could be useful for internal pain when studied on mice, but there was no effect when applied for external pain leaving the validity of the study in doubt.  Finally, a recent study concluded that the Lectin protein (used in the binding of sugar) found in the species Cucumaria echinata actually inhibited the growth of the Malaria parasite.

Most sea cucumbers are dioecious, meaning they have separate male and female sexes.  A few species are protandric, which allows members of one sex transform themselves over to the other sex at one or more points in their life.  The average life cycle involves mass spawning into the current with the ensuing zygote and eventual larvae joining the planktonic drift.  About 30 species, however, fertilize internally.  Once the egg hatches, a feeding tentacle grabs the larva and moves it into a brood pouch for development.  Here they grow to maturity and either crawl out from where they entered or make a rupture out the back end near the anus.  This explosive birth is not fatal to the adult who possesses astonishing regenerative capabilities (after all, if they can re-grow their ‘lungs’ a hernia should pose few problems).

This article was written at the request of one of my readers over at Mad Mikes America.  Anyone who would have a request for an article, feel free to either leave the request in the comments section.  Thank you Morgan Williams for your request!
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