BY ALPER ÖZKAN (MSN/PhD)
d_ozkan@ug.bilkent.edu.tr
Well, so have we come to my final column: I am to graduate this summer, at least by some definition of summer, and I will hopefully leave for parts unknown to undergo the pupal stage called postdoctoral scholarship—graduate students, as we all know, are merely the larval form of college professors, and must feed themselves to surfeit on research before pupating and emerging to infest one university or another. (Truth be told, quite a number of people have recommended that I become an editor instead, but given the years of suffering that my run-on sentences and frequent delays have caused the Bilkent News staff, I don’t think I can head down that path without incurring a hefty load of karmic retribution.) But enough of that for now—I’ve long since lost my original copy of the very first draft I prepared for Bilkent News, but I recall that it was about crabs, so let’s close off this long-running and long-winded column with a brief discussion of the crustaceans.
As I’ve mentioned time and time again, the higher relationships of arthropods are a topic of frequent and heated debate: on one side you have supporters of the uniramia/biramia divide, who align insects with myriapods (centipedes and millipedes) on the basis of their limb morphology, and on the other are these who swear by Pancrustacea, grouping insects and crustaceans together on the basis of molecular evidence. Genetics generally triumphs over morphology in these matters, and such was the case for arthropods: there is now substantial evidence that insects are not just related to, but in fact derived from crustaceans, their closest relatives being unassuming animals called remipedes—similar to centipedes in both shape and feeding habits, these are groundwater dwellers that envenom their prey through stinger-like claws, drinking their liquified innards afterward (curiously, a centipede’s “fangs” are also modified walking legs: yet another case of convergent evolution). Their next-closest relatives are another group of oddballs, tiny filter-feeders called Cephalocarida, of enigmatic habits and little-known biology.
Related to these two, in turn, are the Branchiopoda (not to be confused with Brachiopoda, bi-valved filter feeders that are easily mistaken for genuine bivalves but in fact are not even molluscs—the main difference being that a bivalve has a “left” shell and a “right” shell, while a brachiopod has an “up” shell and a “down” shell), which include such familiar sights as Triops, brine shrimp and water fleas. But this is not to say that they are uninteresting: Branchinecta gigas, for example, is a titanic relative of the common sea monkeys that may reach almost nine centimeters in size and hunts by rolling itself into a C-shaped wheel, using its tail and antennae to detect prey and joining its swimming limbs to form a cage around it. B. raptor, a closely related species, has broader variety in its repertoire—it usually snatches up other fairy shrimp like a tiny, swimming mantis, but in colder weather it may lie flat on the bottom of its lake and snap at anything that happens to touch its highly sensitive tail appendages. It may also stop swimming and prowl the sediment for bottom-dwelling animals and, if sufficiently hungry, may even “dig” up detritus to flush them out from their burrows.
More distant relatives are the copepods, which are enormously diverse and include no small number of parasites—these are sometimes recognizable as crustaceans, but in many other cases (and especially in endoparasitic forms) they are so highly modified that they look like nothing so much as an undifferentiated lump of flesh, identifiable only through internal anatomy and sometimes by the egg sacs carried by the females. The cod worm, Lernaeocera branchialis, is one such example of a highly derived copepod—and it doesn’t settle for just any convenient tissue source either, instead branching into the heart of its fish host and drinking blood straight from its arteries. Sapphirina, a more copepod-like copepod, makes its home in salps (much like the amphipod Phronima). Its males have a curious, beehive-like pattern of hexagonal crystals in their cells—as with the feathers and scales of peacocks and some butterflies, the crystals selectively reflect a brilliant blue color, giving these animals their common name of sea sapphires. They are also capable of circularly polarizing light, though for what purpose is unknown (mantis shrimps are known to detect and use circularly polarized light in communication, but it’s unclear whether copepods have similar capabilities).
Copepods are in their turn closely aligned to barnacles, which also have no lack of parasitic forms—the ability of Sacculina to castrate male crabs is well known, and without seeing its ordinary-looking larva, you’d be hard-pressed to justify exactly why a branching network stemming from a plate-like central node should be classified as a barnacle. These two groups together form the class Maxillopoda; the remaining two classes of crustaceans are Ostracoda and Malacostraca. The former is home to seed shrimp (yet another bi-valved group—it turns out that shells are really useful in the deep sea), while the latter includes the kind of crustaceans that you’d want to eat, like crabs, lobsters, shrimp and woodlice.
…What, you’ve never tried woodlice?