The phenomenon of parasitic manipulation—the ability of parasites to change the behavior of the host to their benefit—has long attracted the attention of scientists, science popularizers, and the general public. Unfortunately, despite the great interest in this phenomenon, little is known about the mechanisms of manipulation. The trematode Diplostomum pseudospathaceum, which lives in the lenses of fish eyes, suppresses their defensive behavior, making it more accessible prey for piscivorous birds—the definitive hosts of the parasite. This trematode makes the fish more active and forces the owner to stay closer to the water surface. For a long time it was believed that the reason for the change in the host's behavior was the deterioration of vision caused by the parasite.
A group of researchers from the A.N. Severtsov Institute of Ecology and Evolution RAS and Timiryazev Academy questioned this hypothesis. They studied the behavior of Dolly Varden (a fish from the salmon family) infected with D. pseudospathaceum in the light and in the dark. The fish were raised and infected with trematodes under controlled laboratory conditions to minimize the influence of environmental factors on the parasite-host relationship. It was assumed that if the problem really was a deterioration in vision, then in the dark the differences in behavior between control and infected fish should disappear. The behavior of the fish was filmed using infrared-sensitive cameras and analyzed “blindly,” meaning the researchers did not know which fish, infected or control, they were testing or which fish’s behavior they were analyzing in the video recordings. Only the autopsy showed whether the fish was infected or not. It turned out that both in the dark and in the light, the infected fish were more active, swam closer to the surface of the water and were caught in the net earlier, i.e., manipulations of the host’s behavior persisted in all light conditions. The results of the study confirm the ability of the parasite to manipulate the behavior of fish, but call into question the assumption of deterioration of the host's vision as the main mechanism of manipulation and hint that the parasite uses more sophisticated mechanisms to control its “prey”. The work was supported by a grant from the Ministry of Education and Science No. 075-15-2022-1134.
The article was published in the International Journal for Parasitology.
Gopko M., Tkachenko D., Shpagina A., Maximenko D., Mironova E. (2023) Is vision deterioration responsible for changes in the host’s behavior caused by eye flukes? International Journal for Parasitology, https://doi.org/10.1016/j.ijpara.2023.06.001.
A detailed synopsis of the article can be found on the popular science portal “Elements”.