An interspecies pair of C. trifasiatus (left) and C. melapterus (right) seen at Socotra. Credit: Tane Sinclair-Taylor
In recent years, there has been a flurry of published research into the causes of speciation in coral reef fishes. For many Indo-Pacific groups, geography seems to be the main driver of diversity, but it isn’t always well-known how this regional diversity was formed and when this diversification occurred. Such answers are best sought by studying subtle differences in genetic sequences—the greater the molecular difference, the greater the age at which specimens last shared a common ancestor. In a newly published article by Waldrop et al, these techniques were applied to a widespread and closely related subgenus of butterflyfish: Corallochaetodon. Four distinctive species make up this lineage, all occupying parapatric distributions spanning from the Red Sea to Polynesia. The names aren’t likely to be terribly familiar to the average reef aquarist, as these are all obligate corallivores (i.e. they only eat Acropora), but these are nonetheless some of the most ubiquitous members of their family to be found in healthy reef environments.
Biogeography of Chaetodon (Corallochaetodon) group. Credit: Walldrop et al 2016
Of the four, C. lunulatus of the Pacific Ocean and C. trifasciatus of the Indian Ocean are the two most widely distributed. This geminate species pair differs mostly in color, with trifasciatus having a bluer color to the sides and more orange along the caudal peduncle. The remaining species, C. austriacus of the Red Sea and C. melapterus of the Arabian Gulf, are dramatically more vibrant in color, having bright orange bodies and mostly black fins posteriorly. None of these are seen with any regularity in the aquarium trade, which is certainly in the best interests of these finicky fishes. As butterflyfishes tend to have a relatively long-lasting pelagic larval stage to their development, it’s not unexpected that they should show fairly widespread populations. Relatively few members of the family show the kind of regionalized endemism seen in more sedentary groups like Amphiprion or Chrysiptera; instead, we see most chaetodontid species spanning entire ocean basins or oftentimes encompassing the full breadth of the tropical Indo-Pacific.
The Pacific Ocean C. lunulatus. Credit: Paddy Ryan
In analyzing the genetics of Corallochaetodon, Waldrop et al found support for the Indian and Pacific Ocean populations splitting approximately 3 MYA, roughly correlating with a period some 2.6-2.8 MYA when sea levels dropped and the Sunda Shelf of Indonesia effectively isolated these two basins and the fish populations therein.… More:
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