Coral fluorescence predicts settlement response

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A recent study, conducted by The University of Texas at Austin, has demonstrated that the colour at which coral larvae fluoresces is an indicator as to whether or not the young coral will settle back onto the reef.

Larvae that fluoresce red are less likely to settle, and develop into coral polyps, than larvae that fluoresce with more green.

This is considered an important finding as it could help scientists monitor how corals are adapting to global warming.  It is expected that corals that don’t settle back onto their own reef will have a higher survival rate due to their dispersing to climes of a lower temperature.

Assistant professor of biology Mikhail Matz says, "By simply looking at the colour of a larval population, we may soon be able to say which larvae are going to be long-range dispersers and which will be short-range dispersers, under global warming, we expect a lot of evolution of this particular life history trait."

The study, which was conducted under laboratory conditions, crossed several different colour morphs of the small staghorn coral, Acropora millepora. 

The resultant larvae were exposed to a settlement cue – ground up crustose coralline algae – and then to stress factors such as light and heat. 

Scientists noted that the larvae inheriting the redder fluorescent colour were less likely to settle than the greener larvae, and while it’s clear that coral response to the settlement cue is under genetic control, it’s still not understood what role – if any – fluorescence has, and it may only be that the genes that determine settlement and colour are next to each other in the chromosome. 

Another view is that fluorescence has some genetic relation to the capacity for larvae to sense how close it is to the reef. Further research is to be conducted to investigate theses possibilities and the question as to why corals fluoresce in such spectacular colours remains unanswered.

"Bright, multicoloured fluorescence of reef-building corals is one of the most spectacular and least understood visual phenomena in the ocean," says Matz, "and we still have no idea what purpose it serves. But our discovery is a really good lead towards determining the function of fluorescence."

For further information please see the paper: C. D. Kenkel, M. R. Traylor, J. Wiedenmann, A. Salih, and M. V. Matz Fluorescence of coral larvae predicts their settlement response to crustose coralline algae and reflects stress Proc R Soc B 2011 : rspb.2010.2344v1-rspb20102344.