Ancient Fish that Went Its Own Way
Ancient Fish that Went Its Own Way
Eons Ago, Indonesian Coelacanth Diverged From Related Species
Sanders, Public Affairs
Berkeley scientists who last year discovered a new population of coelacanths in Indonesia now report that it is a totally separate species that split from the only other known population of the ancient fish some four to six million years ago.
Until an Indonesian fisherman landed a coelacanth last July and brought it to the attention of a Berkeley scientist, this "living fossil" was thought to dwell only in deep water around the Comoro Islands off southeast Africa.
The new report on the coelacanth appears in a recent issue of Proceedings of the National Academy of Sciences. The authors are Mark Erdmann, a postdoctoral fellow at Berkeley; Roy Caldwell, professor of integrative biology; David Hillis of the University of Texas, Austin; and his colleagues Mark Holder and Thomas Wilcox.
"There's little question of it's being two species," said Caldwell. The species was named Latimeria menadoensis earlier this year, in contrast to the Comoran species, Latimeria chalumnae.
The date of divergence of the Indonesian and Comoran species was arrived at by comparing DNA sequences from the two fish, and estimating how long it would have taken for the differences to evolve.
Because such "molecular clocks" are uncertain, Caldwell admits the species could have diverged much earlier.
"We based our estimates on the rate of evolution of tetrapods like salamanders," he said. "But in fact, coelacanths may be as closely related to sharks as to tetrapods. Since sharks evolve much more slowly, the coelacanth populations could have diverged as long as 16 million years ago."
This suggests that some geologic disturbance, perhaps even the formation of the Indian subcontinent, severed whatever connection there was between the two populations. Today the two known populations are separated by about 10,000 kilometers.
Caldwell suspects there are other undiscovered populations of the ancient fish scattered about the margins of the Indian and Pacific Oceans between Indonesia on the east and Africa on the west.
The report on the Indonesian coelacanth comes as German scientist Hans Fricke arrives this week in Manado, Sulawesi, with a submersible called "Jago" to search for the homes of these coelacanths. He and Erdmann together will explore the slopes of volcanic islands like Manado Tua, where the only known specimens of the Indonesian coelacanth were caught, at depths of 100 to 400 meters. Though Erdmann has examined two coelacanths accidentally caught by local fishermen, no other fish have been landed since July 30, 1998.
Fricke, a coelacanth expert, has led six expeditions to the Comoro Islands to obtain one-of-a-kind footage of these creatures in their natural habitat.
The conclusions of the Berkeley and University of Texas team differ markedly from those of a group, headed by a French scientist, which earlier this year compared DNA from the Indonesian and Comoran coelacanths and declared them separate species. In their paper last March, they named the Indonesian fish discovered by Erdmann Latimeria menadoensis.
The authors of that paper, led by Laurent Pouyaud of the Institut de Recherche pour le Developpement, incorrectly estimated the divergence time at 1.3 million years ago.
They also compared a small number of Comoran coelacanths with the Indonesian species and found nine separate distinguishing characteristics that indicated to them that the two are separate species.
Erdmann and Caldwell compared the two Indonesian fish caught so far to 16 Comoran coelacanths and found no single characteristic to distinguish the two.
Despite the fact that morphological comparisons alone do
not unequivocally indicate they are separate species, the
DNA differences clearly suggest that they are distinct, the
authors of the new paper conclude.