The Chagos brain coral survives

by ZSL on

The Chagos brain coral survives – just.  It was not seen at all in 2017 after the mortality event of the preceding two years, and indeed it was not seen during the first week of diving during the present expedition in 2018.  But, on one recent dive on the ocean side off Moresby Island in the north of Peros Banhos atoll, several colonies were found between about 5- 15 m depth, which is its usual depth range.  It was still apparently absent on the closely adjacent Ile de la Passe and Ile Diamant.

This distinctive brain coral survived the massive warming and mortality event of 1998, and had again become reasonably common - until the 2015-16 warming and mortality.  Because we did not see it at all last year we had worried it had become locally extinct, at least in the northern atolls where we were visiting. It is still extremely rare, however, and in the first week of diving both in Diego Garcia atoll in the south as well as in Peros Banhos in the north, it has so far only been seen in that one site, and only with a handful of colonies. But, at least this does mean it is not extinct.

Ctenella chagius at Moresby Island
Colonies of Ctenella chagius spreading over older and much larger eroded parts of their original skeleton

No juveniles or small colonies of it have been seen yet.  The colonies we have seen appear to be remnants that have emerged from older, larger colonies in a Phoenix-like behaviour.  This happens like this:  a small, living patch of live coral appears on an old and eroded colony, giving the appearance of part of the eroding rock having come to life again.  This happens when a very few polyps were somehow protected during the warming, surviving perhaps because they were recessed deep in the colony or were more shaded. These surviving fragments have grown, multiplied and spread, and now form a layer of new colony over the old dead skeleton. These photos clearly demonstrate that this has taken place here. We have no evidence of this species reproducing yet, but presumably in due course these surviving and seemingly healthy survivors will be able to produce larvae of their own.  After the earlier mass mortality in 1998, we know many species took a few years before they become large enough or fit enough to reproduce, and this might be the case here too.

Ctenella chagius found by Moresby Island, Peros Banhos, BIOT
Colonies of Ctenella chagius spreading over older and much larger eroded parts of their original skeleton

The range of the Chagos brain coral is geographically very restricted, mostly limited to this archipelago and to some banks further west. It was a worry that local extinction for such a species could therefore mean a global extinction but we’re pleased to report, the Chagos brain coral does survive in this archipelago.

For more updates from this and future expeditions see @BIOTscience

This work was kindly funded by the Bertarelli Foundation as part of the Bertarelli Programme in Marine Science.

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