| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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here at the academy A Keenness for Echinoderms
Biologists choose to study different animals for different reasons. Some are pragmatic: geneticists favor the fruit fly because a generation, from egg to fly to egg again, takes only two weeks roundtrip. Neuroscientists study squid because of their spectacularly large neurons, which are as thick as spaghetti. For phylogeneticists, the echinoderms-a marine group which includes sea urchins and starfishes-have one very useful attribute: they are an ancient phylum with a huge fossil record. But one suspects a more visceral logic compels Rich Mooi, assistant curator and chair of the Academy's Department of Invertebrate Zoology and Geology, to study the echinoderms. They are, he explains, really, really weird. To fully appreciate the echinoderms, it helps to consider a less perplexing group first, like the vertebrates. Juxtapose the skeletons of a little brown bat, a human, and a blue whale. Now inspect the arrangement of the bones. Aside from the differences of scale, the body plans are roughly the same. And the animals' limbs are patterned similarly. Though specialized for different functions, it is clear they are embellishments of a single blueprint. Not so with echinoderms. "The echinoderms are so unfamiliar that the mind boggles at how to comprehend them," Mooi says. The sea urchin on his bench looks like a guava-sized Koosh Ball, one which, he explains, is likely equipped with venomous spines. A nearby fossilized sea lily, however, is a knee-high creature which looks most like a mop head mounted on a jointed bamboo pole. Loosely speaking, these organisms are phylogenetic kissing cousins. You'd just never know it looking at them. Unless you look very carefully, and in the right places. Which is precisely what Mooi has been doing for the better part of a decade, as he strives to sort out the tangled web of relatedness in this confusing phylum. Along with Bruno David at the University of Burgundy, Mooi has been devising a means of extracting common features from the welter of echinoderm morphologies. "In the past, the focus has been in finding the differences in [echinoderm] taxa. But what we're finding is that there are some fundamental principles of growth and morphology that allow you to see the commonality of structures." Uncovering these principles, says Mooi, tells us not only about the echinoderms, but also about evolution, and how it generates novel body morphologies. "This is a way of understanding how one organism can evolve into another, sometimes by making relatively small changes. Very seldom is something made of whole cloth: evolutionary novelty is a jury-rigging of what came before-you can only inherit what your parents gave you, but you can make changes to that." Mooi's professional trajectory is one that has wavered only fractionally from a laser-like focus on echinoderms. As an eight-year-old child, he briefly entertained thoughts of becoming an astronaut but quickly came around to his calling: "I decided I would be a marine biologist. I even drew up blueprints for my own research vessel." Mooi was recently afforded a rare opportunity to study some of the creatures in situ: observations made from a deep-sea submersible in the Bahamas added a behavioral dimension to his studies. And yielded some surprises. "We saw what looked like large sea urchins, sitting on top of [sea lily] stems. It looked for all the world like this urchin was plowing through a field of sea lilies, eating sea lily stems like candy cigarettes." The sighting, Mooi believes, explains why sea lilies have developed the ability to cast off parts of their stalk, "kind of like a lizard shedding his tail." Surprises are likely to keep coming. Echinoderms provide a wonderful model to study evolution's patterns, and the interesting questions won't all be snapped up any time soon. Says Mooi, "Within the big lab of the echinoderms, there is still plenty of bench space." Sharif Taha is a freelance writer living in San Francisco. |
