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MARGINALIA

The Coelacanth: Act Three

Keith Thomson

Act Three: The Butler Didn't Do It

Act Three began with the capture in 1992 of a large coelacanth at a depth of about 50 meters off the coast of Quelimane, Mozambique, some 500 kilometers west of the Comores. A female, she carried no fewer than 26 near-term young. Then in 1995 a specimen was caught in a trawl at 150 meters depth off the southwest coast of Madagascar. Genetic studies showed that the Comoran and Mozambique specimens were strays from the same population. All three non-Comoran fishes had been unknown to local fisherman. All were caught over sandy bottoms, again suggesting atypical behavior (not conforming to the hypothesis, that is). So far, so good. However, nothing is more seductive—or dangerous—than a nice tidy hypothesis.

In July 1997 Arnaz Mehta Erdmann from Berkeley spotted a strange fish in a market at Bershati Market, Manado, at the north tip of Sulawesi (Celebes), Indonesia, 6,000 kilometers east of the Comores. She and her zoologist husband Mark remarked that it looked like a coelacanth, but, as he later told a Washington Post journalist, they assumed that the fish was already known from the Western Pacific. Once the mistake was realized, the search for another specimen was on. In July 1998 another one turned up, caught off the tiny island of Manado Tua.

Abruptly, our carefully crafted schemes are thrown into doubt. We have to reconsider some old questions. What is the full geographic range of the species; how does it live; and—the $64,000 question—how did the coelacanth survive? One of the presumed characteristics of "living fossils" is that their lineages change unusually slowly over time. Thus Latimeria looks superficially very much like a Paleozoic coelacanth, while the living descendants of one of the coelacanth's Devonian sister groups—Osteolepiformes—have diverged enormously (to include, inter alia, ourselves). However, Paleozoic coelacanths were mostly small fishes of shallow brackish and even freshwater ecosystems. Modern Latimeria, by contrast, is very large (up to 2 meters) and totally marine.

There are three main hypotheses to account for the persistence of a living fossil lineage and its morphological conservatism. The defining characteristics allow it to excel at something very specialized; the group shows a very broad general adaptiveness so that it has managed to withstand the slings and arrows of evolutionary fortune; or pure chance.

For coelacanths, the first hypothesis has prevailed—Latimeria chalumnae the species, and by implication its forebears, is a poor thing preferentially living in an unusually bleak ecological setting where competition is low. This hypothesis can accommodate the Indonesian record if, as reported, the island where it was found is also volcanic and geologically extremely young like Anjouan and Grande Comore (less than 1 million years). On the other hand, there seems little depauperate about the non-Comoran settings if they are worth the attentions of commercial fishermen. Instead, however, of adding patch upon patch to old hypotheses, let us imagine that all the known localities with coelacanths had been discovered in this year.

We would obviously conclude first that the fish is broadly, if thinly, distributed over at least the Indian and Western Pacific oceans, and perhaps beyond. Second, we would conclude that the fish lives, at least seasonally, in a wide variety of habitats and at a range of depths from less than 100 to more than 500 meters. Third, since they have all been found near land, a relation to a solid substrate is probably important in the life of Latimeria. Fourth, as two of the five regions where coelacanths have been found involve very young volcanic lava, this environment may be in some way specific to the fish. Fifth, we would start to hypothesize that they hover in bare submarine caves for particular reasons but venture into open water for others. This shift may be daily, as the fish has to feed, and may also have to do with reproduction or some other longer term life-cycle activity. Sixth, we know that the fish catches its prey in short bursts of speed, perhaps aided by its unusual jaw mechanism, but otherwise is a poor swimmer. However, we have no solid evidence to suggest that it uses ocean currents to assist in long-range migration/dispersion. Known specimens may, after all, have lived near where they were caught. Equally, we have no evidence that they do not. Seventh, numbers of individuals at any one location are probably low, at least on very small islands. But total population size, assuming a single species, could be reasonably large. Eight, we would continue to assume that its apparent rarity is partly an artifact of patchy distribution, possibly low population density at any one site and a mismatch between its preferred habitat(s) and the existence of indigenous fisheries at the right depths.

Against all this, how would we rate our previous hypotheses? On the whole, not well. We have led ourselves into flights of invention, trying to tell comprehensive "stories" in order to explain very limited data. In 1989 I ended a previous article about a (then) new development concerning the coelacanth with a reference to Thomas Henry Huxley's notion of scientific tragedy: the slaying of a beautiful hypothesis by an ugly fact. Little did we know….

© Keith Thomson




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