Tuesday, February 20, 2007

one-eyed cows of the sea

I'm not talking about the romantic sea cow, morphologically named, but the ubiquitious sea cow, ecologically named.

Shame on me for using such fancy words! I meant this: copepods are sometimes called the cows of the sea, not because they look anything at all like cows (they don't) but because of their somewhat bovine behavior. They are herbivores--grazers of phytoplankton--and they are very good at it. The fact that all of the carbon, photosynthetically fixed from carbon dioxide into useable sugars by tiny single-celled oceanic grass, is available to be transported on up the food chain to eventually build tuna and whales, is due in large part to the steady, efficient grazing of copepods.

But, as I said, they don't look anything like cows. They are small (millimeters) and crunchy (exoskeleton). The stereotypical copepod has long, sweeping antennae and one eye. A single eye. In the middle of its head.

For a long time that didn't seem too weird to me, any more than anything else in the routinely weird and wacky world of biology; it was just something to remember about copepods--like a rhinocerous having horns. But just recently, as I sat in class watching a professor draw a copepod on the board, I automatically corrected him when he drew two eyes--and it struck me, quite suddenly, that the cyclopsian nature of the copepod is totally and utterly bizarre.

Let's take a moment to talk about bilateral symmetry. It's another biological fact that we tend to take for granted--that if you drew a line down your middle, the left side would roughly match the right. It's by no means a rule of life--think of anemones and jellyfish, who display radial symmetry, or sea stars and urchins, who've taken it a step further to pentaradial symmetry.* But bilateral symmetry has been around for a pretty long time. The most primitive bilateral animals are the flatworms, and they have two eyes even though they don't have a coelom**.

So I started reading up on copepods, and I found out that the single eye of adult copepods is a "persistent" larval feature. Like most other aquatic crunchies (crustaceans), copepods have a larval stage called a nauplius. It looks rather simpler than the adult forms, and with successive molts it adds appendages and segments until it has the proper number to be considered a grown-up. The nauplius form is also simplified by having a single "median" eye--that is, an eye that is set nicely in the center of its head (like a cyclops) and not off to one side (like a pirate).

Most copepod nauplii simply retain this median eye as they mature into adult forms, but most other crustacean nauplii (and a few copepods) split their eye up into the two (or more) compound eyes of adults. At least, that's what we think happens, but this hypothesis has not received unanimous support, so I suppose some folks think that the adult eyes derive from some entirely separate tissue.

I haven't found any explicit evolutionary discussion of this phenomenon. I can only suppose, however, that the single eye of the nauplius is a derived character. That is, I expect that crustaceans evolved from a two-eyed ancestor, and as their larvae became more and more specialized, some developmental gene mutated and they ended up with only one eye.

I'd welcome any discussion, wild hypotheses, or research articles on the subject. Be warned, however, that searching for the words "copepod" and "eye" together yields a wealth of information about the very intimate relationships many copepods have with the eyes of other organisms. What I said about copepods as grazers is all very true, but the group has diversified incredibly, and they have also excelled at the parasitic lifestyle.

* Actually, echinoderms are weird when it comes to symmetry. Their larvae are actually bilaterally symmetrical, and it is only when they settle and turn into adults that they develop radial symmetry. Bilateral symmetry is partially preserved in the sea cucumbers, and it is actually one of the characters that makes echinoderms close cousins to chordates. Read more!

** A coelom is a body cavity, the hollow bit inside you that holds all your gooey organs; since they don't have a coelom, flatworms are just solid chunks of tissue. Being a solid chunk of tissue, they can't have circulatory systems or respiratory systems, and have to rely on diffusion to move gases and nutrients around their body. Relying on diffusion means you need a really large surface area to volume ratio, and it's this restriction that keeps flatworms flat. I used to be rather disdainful of the flatworms for being acoelomate; after all, the flatworm representative we meet most commonly in the lab, Planaria, is kind of cute but doesn't have much to recommend it. But then I met the marine flatworms, some of whom are more spectacular than nudibranchs, and I opened my mind to acoel virtues.

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