Butterflyfish facing the risk of extinction

Washington, February 25 : Scientists have warned that the chevron butterflyfish are facing the risk of extinction.

Dr Morgan Pratchett, an expert at the ARC Centre of Excellence for Coral Reef Studies and James Cook University, feels that the case of the colourful, triangular butterflyfish indicates how human pressure on the world's coral reefs is confronting certain species with 'blind alleys' from which they may be unable to escape.

Working with Dr Michael Berumen of Woods Hole Oceanographic Institution (USA), Dr Pratchett has found that the highly specialized nature of the feeding habits of this particular butterflyfish make it an extinction risk as the world's coral reefs continue to degrade due to human over-exploitation, pollution and climate change.

"The irony is that these butterflyfish are widespread around the world, and you'd have thought their chances of survival were pretty good," Dr Pratchett said in a study report, published in the journal Behavioural Ecology and Sociobiology.

"But they only eat one sort of coral - Acropora hyacinthus - and when that runs out, the fish just disappear from the reef," he added.

The researchers said that they were finding it hard to believe that a fish would starve rather than eat a mixed diet, and thus tested C. trifascialis in tank trials on a range of different corals.

They observed that the fish grew well when its favourite coral was available. However, in the absence of their favourite coral, they grew thin, failed to thrive and some even died.

"We call these kinds of fish obligate specialists. It means they have a very strong dietary preference for one sort of food, and when that is no longer available, they go into decline. We still don't have a satisfactory scientific explanation for this, as it seems like rather a risky tactic in evolutionary terms - but it must confer some advantage provided enough of its preferred food is available," Dr Pratchett said.

In their report, the researchers have also revealed that the A. hyacinthus coral, which the butterfly fish feeds on, is itself highly vulnerable to attacks by plagues of crown-of-thorns starfish-which is thought to be triggered by humans releasing excess nutrients onto the reef as sediments, fertilizer or sewage.

They say that the coral is also vulnerable to storms and to the coral bleaching caused by the heating of ocean surface waters to 32 degrees or more, which is thought to be linked to global warming.

"Although extremely widespread, the Chevroned butterflyfish may be at considerable risk of extinction following ongoing degradation of coral reefs around the world, because the coral itself is exceptionally vulnerable. It is estimated that up to 70 per cent of the world's coral reefs are now badly degraded, which usually involves the loss of this particular coral - and, when it goes, the C. trifascialis also disappear from the reef," Dr Pratchett said.

"To make matters worse, butterflyfishes are one of the main families of coral reef fishes being targeted by aquarium collectors. However, the specialized coral-eaters are clearly not suitable for keeping in aquaria - and often die because they cannot obtain their main food source," he added.

The researchers claim that their study is one of those few projects which consider the evolutionary and ecological basis of dietary versatility, and that it has implications for the fate of specialised feeders throughout the animal kingdom.

ANI