Australasian Science: Australia's authority on science since 1938

Parasites Turn Fish

By Stephen Luntz

Parasites can make fish left- or right-finned, or at least with a tendency to turn one way, according to researchers at the Australian National University.

PhD students Dominique Roche and Sandra Binning are studying the behaviour of Anilocra parasites, which commonly attach just above one eye of coral reef fish. The pair has previously shown that the parasites affect fish by creating drag at high speed, not by sucking too much blood (AS, March 2013, p.7).

Now they have found that the parasites change the fishes’ preferences when it comes to turning at a T-intersection. “The population as a whole didn’t show a preference to turn one way or the other,” says Roche. “However, at an individual level, some fish showed a turning preference, with parasitised fish showing a much stronger preference than their unparasitised counterparts. If they have a parasite, they definitely choose a side.”

It might be expected that any turning preference would relate to the side on which the parasite attached, but this did not occur. While fish with parasites were individually consistent in their preferences, some turned towards the side with the parasite while some turned away. Parasite removal returned fish to their state before attachment.

Roche and Binning have an explanation for the apparently curious behaviour. “Having a preferred side gives the fish an advantage,” Roche says. “Lateralised fish are quicker at responding to threats.

“We’ve shown previously that parasitised fish swim slower than unparasitised fish. Given that our parasitised fish don’t swim very fast, it makes sense that they need to react faster to predators to give themselves a head start and have a better chance of escaping.”

Always turning towards the parasite might be counterproductive, Roche suggests, because predators might work this out and be ready. “Some predators are capable of exploiting escape responses, such as snakes that force prey to go towards their mouths.”

If lateralisation improves the response to a threat it is puzzling that all fish do not have a preferred direction to turn. “There are disadvantages as well,” Roche says. “If a predator is coming from the right and you always go right it is better to take a moment to think. If parasitised you need to respond fast because you are slower to start off with. For a fast swimmer it can be worth assessing the situation.”

Roche says they are considering testing whether plastic pseudoparasites used in previous studies produce the same lateralisation. He adds that it is “hard to extrapolate to humans, but the one thing we thought was most interesting is how plastic lateralisation can be”.