Damselfish ‘algal gardens’ harbour coral disease

From ABC News in Science, 25 June 2014:

The unique damselfish practice of cultivating their favourite type of algae on coral reefs contributes to an increase in coral disease, Australian researchers have found.

The study, published in the Proceedings of the Royal Society B, also suggests that overfishing of other fish species may contribute to an increase in the numbers of damselfish, which in turn may boost coral disease.

Damselfish are known for their habit of ‘farming’ a particular species of algae found on coral reefs — often to the detriment of the coral itself, says lead author and PhD candidate Ms Jordan Casey from the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University.

“They weed out the stuff they don’t like, they farm the good stuff, they keep it at a certain level where it continues to grow, and they’re constantly engaging in very aggressive behaviour that keeps other fish outside of their territories so they can’t feed on this very good palatable feeding algae,” says Casey.

Previous research has suggested that this farming behaviour, which leads to algal dominance over coral, has implications for the health of the coral but no one had explored what was going on at the microbial level.

To explore how the practice affects coral, the team looked at the DNA of microbial populations present in areas of the Great Barrier Reef populated by two different species of damselfish. They found a much greater abundance of bacteria linked with coral black band disease inside the damselfish territories than outside their range.

In a healthy reef environment, the damselfish’s influence — which can also benefit the coral by keeping coral-eating fish away from their territory — would be kept in check by other factors such as predators.

“Several studies recently have shown that in fished reefs, there is a higher abundance of these territorial damselfish,” says Casey.

“If this fishing activity is actually increasing the abundance of these damselfish, human impacts may be indirectly also increasing the level of coral disease because we’re showing these reservoirs of coral disease and potential pathogens are cultivated by damselfishes.” Read more.

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