Fish with parasites attached to their heads have a preference for turning left or right when given a T-intersection, research from The Australian National University (ANU) has revealed.
This preference is known as lateralisation and is present in humans as well as many other animals.
The fish used in the study were the bridled monocle bream.
Mr Dominique Roche, a PhD candidate in the ANU Research School of Biology, says: "There has been some evidence that lateralisation is plastic, meaning it can change depending on the circumstances."
Parasites often attach themselves to bridled monocle bream just above one of the fish's eyes.
The study aimed to find test whether having the parasite attached to the fish's head had any influence on lateralisation and whether it could be changed.
The bream were caught from Lizard Island on the Great Barrier Reef, some with and some without parasites, and swam a maze that resembled a T-intersection.
Mr Roche says: "The population as a whole didn't show a preference to turn one way or the other.
"However, at an individual level, some fish showed a turning preference, with parasitised fish showing a much stronger preference than their unparasitised counterparts."
He continues: "If they have a parasite, they definitely choose a side."
Turning preference was much less obvious when the parasite was removed.
Mr Roche says: "This is one of the first instances where lateralisation has been shown to be plastic and change so rapidly."
According to the research, lateralised fish are quicker at responding to threats so have an advantage against predators.
Mr Roche says: "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."
However, not all of the parasitised fish reacted the same with some turning one way and others turning the other.
Ms Sandra Binning, who collaborated with Mr Roche on the study, says: "This is a good thing - the parasites are quite big and a predator could spot them easily.
"If all parasitised fish always turned towards their parasite, eventually predators would be able to anticipate their reaction, and parasitised fish would lose the advantage of reacting quickly."
This result can help to better understand if human preference for using the right or left side of the body depends on circumstances and surrounding environment.
The research is published in the journal Behavioural Ecology and Sociobiology.