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A home from home for coelacanths

The discovery of specimens of the coelacanth, Latimeria chalumnae , in Indonesian waters raises questions about the geographical distribution and conservation status of this remarkable fish.

On page 335 of this issue1 Erdmann et al. report the existence of living coelacanths off the northeast coast of Sulawesi, Indonesia, some 10,000 km east of their presumed natural home of the Comoros archipelago in the western Indian Ocean (Fig. 1). The circumstances of the initial discovery are astonishing — Erdmann's wife happened to see the instantly recognizable form of a coelacanth being carted through a fish market; Erdmann and colleagues later saw and described a live Indonesian specimen.

Figure 1: Latimeria west and Latimeria east.
figure1

The only natural home of the coelacanth was assumed to be the Comoros archipelago off Madagascar; three specimens caught elsewhere in the western Indian Ocean (capture dates shown) are assumed to be strays swept away from the Comoros by the powerful Mozambique current. Erdmann and colleagues' announcement1 of finds of coelacanths off Sulawesi, Indonesia, considerably extends the fishes' known geographical range. Given the distance between the Comoros and Sulawesi, the direction of ocean currents and the distinct geological history of the two regions, it is unlikely that the Indonesian specimens are strays.

The first living coelacanth was caught in 1938 in a shark gill-net off the mouth of the Chalumna River, South Africa, and was named Latimeria chalumnae by J. L. B. Smith2. The catch raised great excitement, for two reasons. First, this was the only surviving species of a lineage of fishes — the coelacanths — that originated in the Devonian, about 360 million years ago, but was thought to have become extinct in the Upper Cretaceous some 80 million years ago, the date of the youngest fossil recovered. So here was a true ‘living fossil’. How had the lineage survived for that time without leaving any trace in the fossil record?

Second, the theory of relationships prevalent in the late 1930s held that the coelacanth was a direct descendant of Devonian fish-like ancestors of the tetrapods (land-living vertebrates, including ourselves). By studying Latimeria it was expected that we might understand something about the physiology, behaviour and ecology of our own remote ancestors during the transition from life in water to life on land.

From modern evolutionary analysis3 it seems that the coelacanths are more distantly related to tetrapods than first thought, and that they are their modern cousins rather than their sisters (lungfishes are now taken to be the closest living relatives of tetrapods). But Latimeria may still hold the answers to several questions. It is the only living animal to have a functional intracranial joint: that is, a complete division running through the braincase and separating the nasal organs and eye from the ear and the brain. Many Devonian fishes and primitive tetrapods had a similar intracranial joint, but the functioning and biological significance of this structure remain obscure. Latimeria's paired fins move in a fashion that is unlike the coordination seen in most fishes but is exactly the same way as we move our arms and legs, and studies of its nervous system may help trace the beginnings of tetrapod locomotion. The ear of Latimeria is also interesting, in that there seem to be sensory areas that are precursors of structures responsible for hearing in air4.

These points of scientific interest, along with the general curiosity value of a living fossil, have caused great demand for Latimeria. Given an estimated Comoran population of just 500, and the species’ low fecundity (it is a live bearer), this has been cause for concern5. The possible existence of an Indonesian population may relieve that anxiety, but equally may also mean that other fishing grounds have to be monitored.

For reasons shown in Fig. 1, it is likely that the Indonesian population is quite distinct from that in the Comoros. DNA and protein profiling should provide the unequivocal answer. If the populations are distinct, such work may also allow estimation of the time that they separated. If they are not distinct, then the implication is that the species has a very broad geographical distribution (not unknown in some deep-sea fishes6) and that we can expect coelacanths to occur elsewhere in the Indian Ocean, and maybe in the Pacific.

Comoran coelacanths live at about 180 metres, below the 18 °C isotherm, and inhabit submarine caves formed through recent volcanic activity7. We don't know the exact ecological preference of the Indonesian coelacanths, but these too were caught around a volcanic island known to have submarine caves, and at about the same depth. One theory for the scarcity of Latimeria is that the species is competitively inferior and takes refuge in newly established and remote areas8. Hitherto, the only known occurrence in the Comoros supported this view. But if more populations are discovered, the less likely it becomes. It could just be that Latimeria is a widespread deep-sea fish, which may explain the absence of the coelacanth lineage from the fossil record for so long because deep-sea sediments carrying remains of fossil fish are extremely rare.

The Sulawesi fishermen seem to be well acquainted with the coelacanth; for instance, they have a name for it. It is amazing how the Indonesian occurrence of such a large (adults are up to 1.6 metres in length) and distinctively shaped fish should have escaped the world's attention for so long. The next step will be to estimate the size of the population. We must hope it will be large. At present Latimeria is classified under category ‘K’ (‘insufficient known to justify conservation action’). However unfortunate many conservationists think that is, the new discovery underlines how little we really know about the coelacanth in particular and oceanic life in general.

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Forey, P. A home from home for coelacanths. Nature 395, 319–320 (1998). https://doi.org/10.1038/26328

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