Parasites a Drag for Fish
By Stephen Luntz
A parasite affects fish health more through the loss of its streamlined shape than through the blood it sucks, two PhD students at the Australian National University have found.
While it is not clear how widely applicable their results are to other parasites, they may have profound implications for the methodology of marine biologists.
Sandra Binning and Dominique Roche put bridled monocle bream, a coral reef fish, into a tank where water flows past at an adjustable rate, forming the equivalent of a treadmill for fish. The tank also allowed them to measure oxygen consumption.
“Oxygen consumption is a proxy for energy consumption: if a fish consumes more oxygen it is burning more fuel and will need to eat more,” says Binning. “Eating more means you may be more exposed to predation and have less time for other important activities, such as attracting mates.”
Unsurprisingly, infection with the crustacean Anilocra nemipteri increased the bream’s oxygen requirements. However, the difference was small at low speeds. When the parasite was removed their oxygen consumption returned to normal within 24 hours. “This is too fast for physiological effects such as anaemia,” Roche notes.
When the students glued a plastic model of the parasite to the fish’s head, the fish required the same additional oxygen as when they were infected by a real parasite. The finding has been published in Biology Letters.
“It’s similar to drag on a car with a roof rack. At slower speeds you don’t consume more petrol. But at higher speeds, the drag increases, meaning the engine has to work harder and uses more petrol,” says Roche.
The power required to overcome this air or water resistance increases with the velocity’s cube, so a doubling of swimming speed means that eight times as much energy is needed to overcome a drag effect.
Binning and Roche note that the tracking devices frequently used by scientists to observe animal behaviour are similar in size to A. nemipteri, and can thus distort research results. Roche cites a recent study of migratory eels that ended in failure, possibly because the tags prevented the eels from completing their journey.
Roche says the implications are dire for a warmer world. “In the future there will be more frequent and more intense wave action, which is a big stressor for coral reef fish because it increases their energy requirements. They have to swim even to stay in the same spot. This could prove too much for parasitised fish that are already working harder.”