The parasite attached near this fish's eye makes it harder for it to get away from predators
The parasite attached near this fish's eye makes it harder for it to get away from predators (Dominique Roche)
Parasitic crustaceans cause significant drag on reef fish impeding their ability to swim fast and stay safe, researchers have found.
Coral-reef-dwelling fish like the bridled monocle bream (Scolopsis bilineatus) have a tough time keeping safe, says Binning.
Such small fish have to swim fast just to stay in one place as they are pushed this way and that by currents and waves in the shallow reef waters.
"You get areas on the reef where the water is moving quite dramatically," says Binning.
She says being able to swim fast is important if they want to stay hidden from predators in the coral reef, or if they need to actively escape a chasing predator.
But such fish also often have to contend with a relatively large parasitic crustacean called Anilocra nemipteri stuck on its head.
The parasite can be 15 per cent of the fish's total body length and hang around for up to two years, says Binning. It is presumed to suck the blood of the host and is related to another parasite that eats out the tongues of fish, says Binning.
Binning and colleagues investigated the impact of Anilocra nemipteri on Scolopsis bilineatus's physiology and on its streamlining and swimming performance.
Water is a lot denser than air, says Binning, so anything stuck on the fish can easily increase drag and slow it down.
The researchers collected fish from reefs around Lizard Island on the Great Barrier Reef and set up experiments designed to separate out the impact of the parasite on physiology and streamlining.
Binning and colleagues set up an underwater treadmill - a loop-shaped tank with a propeller in it that generates a current of known speed.
Because fish tend to swim against a current to stay in one place, this enabled Binning and colleagues to measure the speed the fish were swimming.
They were also able to measure the amount of oxygen the fish were consuming, which indicated how much energy the fish were using.
Binning and colleagues found that parasitised fish had a higher resting metabolic rate than non-parasitised fish.
"Even for doing nothing they need more energy use to survive," says Binning.
These fish also used more oxygen when swimming and were not able to swim as fast as non-parasitised fish, she says.
Binning says the parasite reduced the maximum swimming speed of the fish by half a body length per second.
The researchers then tested what happened when they stuck a plastic 'parasite' on a healthy fish.
"We glued on a fake parasite that had approximately the same weight, length and width as the normal parasite that you see in the wild," says Binning.
They found this had no effect on the fish's resting metabolic rate, but had the same effect on the fish's swimming abilities as a real parasite.
The plastic parasite reduced the fish's maximum swimming speed and caused it to consume more oxygen when swimming.
"They were really struggling, just as the fishes that had the parasite were," says Binning.
Levels of parasitism
Sampling shows that overall about 4 per cent of Scolopsis bilineatus around Lizard Island are parasitised by the crustacean.
Among juveniles this rises to about 10 per cent, says Binning, and in some habitats up to about 30 per cent of adult fish are parasitised.
Binning says it will be important to find out why fish in some habitats are more likely to parasitised. It could be, for example, because the water is warmer or contains more nutrients, she says.