Abstract
Predation and parasitism are two of the most important sources of mortality in nature. By forming groups, individuals can gain protection against predators but may increase their risk of being infected with contagious parasites. Animals might resolve this conflict by forming mixed-species groups thereby reducing the costs associated with parasites through a relative decrease in available hosts. We tested this hypothesis in a system with two closely related poeciliid fishes (Poecilia reticulata and Poecilia picta) and their host-specific monogenean ectoparasites (Gyrodactylus spp.) in Trinidad. Fish from three different rivers were sampled from single and mixed-species groups, measured and scanned for Gyrodactylus. The presence and abundance of Gyrodactylus were lower when fish of both species were part of mixed-species groups relative to single-species groups. This is consistent with the hypothesis that mixed-species groups provide a level of protection against contagious parasites. We discuss the importance of potentially confounding factors such as salinity and individual fish size.
Highlights
Forming groups is an adaptive strategy seen in many animal species [1]
We found that P. reticulata and P. picta individuals in mixedspecies groups had a lower abundance of Gyrodactylus and were less likely to be infected than conspecifics in single-species groups
For P. picta these differences became apparent only after controlling for specific conductivity. Both fish species experience predation risk at all sites [33] and both species are known to be infected by hostspecific contagious parasites [29,34]
Summary
Forming groups is an adaptive strategy seen in many animal species [1]. Individuals that join a group of conspecifics often gain benefits that increase their fitness compared to those that do not join a group; group formation involves interactions that are costly to the individual. Two major benefits of group forming behaviour are a decrease in an individual’s probability of being preyed upon and an increase in the probability of acquiring food [2]. A major cost that individuals experience when joining a group is increased contagious parasite transmission [3]. As group size increases, increased contact between suitable hosts [4] results in higher mean number of contagious parasites per individual and higher percentage of infected individuals [3,5,6]. Mobile parasites need to search or ambush their hosts; as group size increases this behaviour will likely decrease a host individual’s probability of receiving a mobile parasite attack [7,8]
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