February 25, 2008
A beautiful black, white and yellow butterflyfish, much admired by eco-tourists, divers and aquarium keepers alike, may be at risk of extinction, scientists have warned.
The case of the Chevroned Butterflyfish is a stark example of how human pressure on the world’s coral reefs is confronting certain species with“blind alleys” from which they may be unable to escape, said Morgan Pratchett, a postdoctoral research fellow at the ARC Centre of Excellence for Coral Reef Studies and James Cook University.
In a study published in the journal Behavioural Ecology and Sociobiology, Pratchett and Michael Berumen, a postdoctoral scholar in the biology department of the Woods Hole Oceanographic Institution, warn that the highly specialized nature of the feeding habits of this particular butterflyfish – the distinctively patterned Chaetodon trifascialis– 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,” Pratchett said. “But they only eat one sort of coral – Acropora hyacinthus – and when that runs out, the fish just disappear from the reef.”
The team found it hard to believe a fish would starve rather than eat a mixed diet, so they tested C. trifascialis in tank trials on a range of different corals. The fish grew well when its favorite coral was available, but when this was removed and other sorts of corals offered, it grew thin, failed to thrive and some 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,”Pratchett said.
The A. hyacinthus coral, which the butterflyfish feeds on, is itself highly vulnerable to attacks by plagues of crown-of-thorns starfish (thought to be triggered by humans releasing excess nutrients onto the reef as sediment, fertilizer or sewage), 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,” Pratchett said. “It is estimated that up to 70 percent 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.”
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.
A previous case in which a coral-dependent fish vanished occurred in the case of Gobiodon,a specialized coral-dweller known only from one site, Kimbe Bay in Papua New Guinea, which was thought by scientists to have possibly become extinct after its habitat was destroyed.
Researchers consider that such extinctions are likely to occur as part of the global mass extinction of species now taking place, and that marine ecosystems may be particularly vulnerable in that small changes in habitat or water quality can have a big impact on their species.
Pratchett and Berumen said the study is one of the few so far to consider the evolutionary and ecological basis of dietary versatility, and has implications for the fate of specialized feeders throughout the animal kingdom.
Courtesy of James Cook University Media Office. James Cook University is Australia’s leading tropical research university. Areas of research focus include: biodiversity, sustainable management of tropical ecosystems, global warming, tropical agriculture, tropical health care in remote communities and cultural diversity.
The Woods Hole Oceanographic Institution is a private, independent organization in Falmouth, Mass., dedicated to marine research,engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the oceans and their interaction with the Earth as a whole, and to communicate a basic understanding of the ocean’s role in the changing global environment.