Parasitic infection of the eye lens affects shoaling preferences in three-spined stickleback

Abstract

The ability to compete with conspecifics and to adequately respond to visual stimuli of group mates are important prerequisites for profiting from group benefits such as confusion of predators and greater efficiency in acquiring food. By impairing their host’s physical abilities or making the host conspicuous, even non-contagious parasites that do not pose a direct risk of infection can interfere with group dynamics. Diplostomum pseudospathaceum, a widespread parasite of freshwater fishes, infects the eye lens and can impair the vision of its fish host. To test whether this eyefluke affects competitiveness and/or shoaling behaviour in three-spined stickleback (Gasterosteus aculeatus), experimentally infected fish were kept in mixed groups comprising infected and uninfected sticklebacks under limited food availability in semi-natural outdoor tanks. Change in body mass over time was measured and sticklebacks were given the choice to shoal with uninfected conspecifics or a mixed group in binary shoal choice experiments. Surprisingly, uninfected sticklebacks spent significantly more time with mixed shoals than with uninfected shoals while this preference was not found in infected sticklebacks. Infection did not significantly affect body condition or immune parameters indicative of stress level (relative spleen mass, granulocyte-to-lymphocyte ratio). The results suggest that sticklebacks can distinguish mixed from uninfected groups, but that they are also able to tolerate potential detrimental effects of infection. Whether uninfected fish can benefit from shoaling with infected but non-contagious conspecifics remains to be tested. Although the present data do not indicate a significant effect of infection on competitiveness, this should be examined further.