The enigma of the eel life-cycle has become a little less enigmatic — at least as far as the Japanese eel, Anguilla japonica, is concerned. As described elsewhere in this issue (Nature 439, 929; 2006), Katsumi Tsukamoto has filled in a missing part of the picture of the species' ocean biology.
When they are mature, eels migrate from fresh water out into the open ocean, travelling thousands of kilometres to spawn. But the when and where of the actual spawning event, although suspected in general terms, have been hard to pin down. This is partly because of the technical challenges of isolating and identifying the tiny, transparent larvae just after they hatch. Tsukamoto has tackled those challenges to pinpoint the timing — which aptly coincided with a new moon in June — and strategic location of A. japonica spawning.
This fishing feat also meant that the newly hatched larvae could be photographed. As shown in this picture, teeth, jaws and pigmented eyes are evident within five days of hatching, when the larvae are still less than 5 millimetres long.
Thanks to the efforts in the 1920s of a Danish oceanographer, Johannes Schmidt, the spawning area of the European (A. anguilla) and American (A. rostrata) eels was identified as the Sargasso Sea in the western North Atlantic, spreading over some 5 million square kilometres. But it seems that the nursery of the Japanese eel, as identified by Tsukamoto, is much more compact — spawning is confined to a seamount northwest of Guam in the Philippine Sea, a location that enables the larvae to ride the northbound Kuroshio Current to their freshwater destinations in eastern Asia.
This discovery will be applauded by researchers specializing in eel biology, as well as by those intrigued by a life history that constitutes one of nature's wonders. But there is a commercial aspect too: eels are of great economic value, not least in Asia, and there is a pressing need for management and conservation of natural populations.
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Cotter, R. Spawning spot. Nature 439, 926 (2006). https://doi.org/10.1038/439926a