Fish parasite a drag

Using a fish treadmill and moulded plastic parasites, researchers from the ARC Centre of Excellence for Coral Reef Studies at The Australian National University have found that the energy cost to fish from externally-attached parasites is mostly due to drag, rather than physiological effects.

The findings, published in Biology Letters today, have implications for ecological tagging studies and the future of some coral reef fish.

Anilocra and algae attached to a bream. Photo by Dominique Roche.

Sandra Binning and Dominique Roche, both PhD candidates from the Research School of Biology in the ANU College of Medicine, Biology & Environment, and the ARC Centre of Excellence for Coral Reef Studies studied how parasitism affects the oxygen consumption of a Great Barrier Reef fish called the bridled monocle bream. These fish are often parasitized by Anilocra nemipteri, a type of crustacean that hooks onto the scales just above the bream’s eye.

Using a swim tank, which can be likened to a fish treadmill, the team made the fish swim and measured the amount of oxygen the fish was consuming.

“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,” said lead author of the study, Sandra Binning.

“For fish, eating more means you may be more exposed to predation and have less time for other important activities, such as attracting mates.”

As expected, fish without a parasite outperformed parasitized fish in all tests. When the parasite was removed, the fish was back to normal oxygen consumption only 24 hours later, suggesting the effects are quickly reversible.

To determine whether it was added drag that increased oxygen consumption, or if the parasite was somehow affecting the breams’ health, the team glued plastic models of the parasite to healthy fish.

“The model attached to the fish’s head simulates the drag caused by the parasite. When they’re not swimming fast, the fish are not affected,” said Binning.

However, at higher speeds the drag effects became more pronounced.

“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. It’s the same with the fish – there is more drag at higher speeds causing the fish to use a lot more energy,” said Dominique Roche.

This extra drag could spell trouble for parasitized fish, with global warming creating choppier water conditions due to increased extreme weather events.

“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 parasitized fish who are already working harder,” said Roche.

The results also have implications for ecological tracking studies in which radio or GPS transmitters are attached to animals so researchers can understand their migration movements.

“A tracking tag is similar to an ectoparasite stuck on the surface of a bird, a turtle or a fish. What our study shows is that drag effects are really important. If we want to see natural animal movement, especially in migrating species, then we need to invest more money into developing smaller tags,” said Binning.

The study Ectoparasites increase swimming costs in a coral reef fish  by Sandra Binning, Dominique Roche and Cayne Layton is published in Biology Letters.

More information:
Sandra Binning and Dominique Roche, CoECRS and ANU, ph +61 (0)2 6125 3828