Parasite infection alters schooling behaviour: deviant positioning of helminth-infected minnows in conspecific groups

Abstract

The formation of polarized swimming groups is a successful antipredator adaptation of many fish species because the confusion created by multiple synchronously moving targets reduces the attack success of predators. However, maintaining synchrony and spatial position within such groups requires the functioning of complex sensory and motor systems, so the ability to school effectively could be sensitive to disease or infection with debilitating parasites. As many predators overcome this confusion by attacking visually distinct individuals in groups, infection-associated traits that impair schooling ability might increase predation on parasitized school members, with consequences for the survival of both host and parasite. We report here on a study designed to investigate the effects of parasite infection on the schooling behaviour of European minnows, Phoxinus phoxinus. Over a series of trials done in a flow-pool, minnows infected with plerocercoid larvae of the cestode Ligula intestinalis consistently exhibited increased nearest neighbour distances when compared with uninfected conspecifics in the same school, suggesting that infected fish may be unable to maintain close spatial positions within schools. Positive relations were found between the area covered by a school and both the nearest neighbour differences of uninfected and infected fish in the school, but these relations differed with infected fish exhibiting increasingly larger nearest neighbour differences than uninfected fish as schools became more dispersed. Infected fish also occupied peripheral positions more often than would have been expected if individuals assorted randomly within schools. Our results indicate that infected fish in schools exhibit positioning behaviour that deviates from that of uninfected conspecifics. As schooling is known to be a primary antipredator response of minnows, the altered behaviour of parasitized individuals within polarized groups could increase their susceptibility to certain predators, with potential consequences for parasite transmission.