A Phylum in Five Acts
BY ALPER ÖZKAN (MSN/PhDII)
Truth be told, this column was originally intended to cover Welsh mythology, featuring the core cycle of stories from which the Arthurian saga was derived (mostly in the 12th century, when French troubadours took to writing Lancelot/Guinevere fan fiction -- though as terrible a knight as Lancelot was, he still can't hold a candle to Sir Tristram, who not only cuckolded his king but also had the gall to be all self-righteous about it), replete with magical warriors who enter contracts with Fate itself to gain awesome might (and if they ever break the covenant…well, you've seen "Final Destination," right?) and bearing what might be the world's premier source for cruel and unusual punishments (the most prominent example involves two brothers being transformed into mating pairs of various animals and, um, delivering offspring to the king they wronged, for three full years).
However, I recently had to endure a nightmarish week involving a couple of reports to write, some dozen papers to skim through and an experiment protocol that culminated in a full-blown battle with an FT-IR machine (this device is something of an interrogation tool for materials science: you first torture a solid of interest under several tons of pressure, and then shoot lasers at it until it yields its secrets), and thus couldn't do the necessary reading on Arthurian romances and the Mabinogion. As usual, I will cope by substituting a topic I am somewhat familiar with -- and this week, the topic in question happens to be echinoderms.
Echinoderms are built around the number five -- in addition to the pentaradial symmetry found ubiquitously in the taxon, there are five main groups of living echinoderms: starfish, brittle stars, sea cucumbers, sea urchins and crinoids. Starfish and brittle stars are closely related, quite familiar and often regarded as cute, though they are also covered in countless tiny claws (pedicellariae), which some use to tear fish limb from limb; they may even combine their forces to subdue larger prey, or quarrel with each other over a smaller morsel. Other starfish, however, are content with less mobile, more defensive prey, which are digested with the infamous, extensible cardiac stomach (there is an amazing time-lapse video of this event, taken from within a mussel as it is devoured alive). Basket stars (a group within brittle stars) have improvised another feeding technique: their highly branched arms trap small particles and planktonic animals from the water column, allowing these deep-sea echinoderms to feed on the constant influx of organic matter from above. Brisingid starfish use a similar method, though their arms don't feature as many branches.
Sea cucumbers are sometimes called "echinoderm worms," and for good reason: the trademark echinoderm trait of fivefold symmetry is barely noticeable in those elongated animals, and deep-sea forms in particular can diverge considerably from the norm (Psychropotes has a sort of pompadour hairdo that it uses to swim, while Pelagothuria is fully pelagic and looks straight out of R'lyeh). They are also famous for the Cuvierian tubules, sticky threads located at the base of the sea cucumber respiratory system (no, these aren't their intestines, and not every sea cucumber has them) -- when the animal is attacked, they are ejected from the anus to entangle the would-be predator. This, however, isn't the only feat sea cucumbers can achieve with their rears, as they in fact breathe and, in the case of certain species, even feed through their guts. This not particularly surprising, since sea cucumbers are essentially living tubes full of seawater -- pearlfish utilize this fact to live quite comfortably in their body cavities.
Despite looking nothing like them, sea urchins are closely related to sea cucumbers, and they too have their share of quirks: for starters, they are capable of sight, turning their armor into a single, all-seeing optical array by utilizing the space between their spines as tiny eyes (naturally, a sea urchin densely armed with spines gets better resolution than a less spinous one). And finally, crinoids are a relic of a bygone era, currently divided into stalked, flower-like and relatively sessile sea lilies (though they can move, by either relocating their stalks or crawling on their arms -- the latter mode of locomotion is rather unsettling and has become an Internet phenomenon) and stalkless, many-armed feather stars, but also bearing a long and illustrious past that includes such curious designs as floating spheres that serve as "roots" for upside-down animals and other fossil crinoids that pull off the same feat by attaching to floating pieces of wood (many modern barnacles operate on similar terms).
In any case, the aptly-named Echinoblog has an astounding amount of recent research information on echinoderms, and most of this column's contents can be found there in much greater detail. But before I finish, here is one last gem: There are certain starfish that brood their offspring, and Parvulastra does so in its gonads (other locations, such as the stomach, can also be used). However, it's not all roses and sunshine for the young starfish, for the juveniles are cannibalistic. Starfish facing the aboral side (the "back") of their rivals usually get bigger, because they can eat their siblings without fear of a counterattack. To prevent this, baby starfish face each other and engage in "stomach duels" where they wrestle for survival with their stomachs.
Isn't life grand?