Original story by Bianca Nogrady, ABC Science
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.”
Nemo finds home
Meanwhile, a second study has found that one in five reef fish larvae successfully navigate their way back to the reef where they were born, using sound and smell to guide them.
The findings underline the important role of marine protected areas, report scientists in the Proceedings of the Royal Society Interface .
Lead author Professor Eric Wolanski of James Cook University says their modelling suggests reef fish larvae hone in on the familiar smell of their reef and once they get within two kilometres or so, and also use the long-range sound of snapping shrimp on the reef to guide them back home.
The model also suggests that the fish larvae that don’t quite make it back to their own reef will often ‘seed’ nearby reefs.
“What it also means is that reef which is under protection — marine protected areas — will supply a lot of fish larvae to reefs outside the marine protected area,” says Wolanski.
“About one-quarter to one-third will come back to the same reef but of the rest, those that are passing within 2000 metres of another reef will hear the sound of the shrimp on that reef.”