Print PageAlthough individuals tend to prefer forming groups with conspecifics, mixed-species groups occur in various taxa and are thought to be adaptive. The two main explanations proposed for the formation of mixed-species groups are increased defense against predators through dilution of individual risk and increased resource acquisition by following more experienced species or reducing intraspecific competition.
Here we investigate a mechanism proposed by Moller (1993) through which individuals reduce their host-specific parasite loads by joining mixed-species groups, either through reduced parasite transmission or reduced establishment success. As a study system we used two fish species native to the island of Trinidad, Poecilia reticulata and Poecilia picta , and their ectoparasites Gyrpdactylus spp. Both species coexist and form mixed-species shoals where their distribution ranges overlap.
We sampled fish from sites along a species composition gradient (P. reticulata-only, mixed-species, P. picta-only) in three rivers in the island Trinidad. At each locality we collected 40 individuals per species, and examined them individually for parasite load. We used a generalized linear model to test the effects of shoal composition, fish standard length, site salinity and river on the mean number of parasites in all individuals (mean abundance), infected individuals (mean intensity) and probability of being infected (prevalence).
Results/Conclusions
P. reticulata and P. picta individuals in single-species groups have significantly higher mean abundance (P. reticulata p < 0.0001 and P. picta p < 0.0001), mean intensity (P. reticulata p = 0.0075 and P. picta p < 0.0001) and prevalence (P. reticulata p < 0.0001 and P. picta p < 0.0001) of parasites than individuals in mixed-species shoals. Our results suggest that by forming mixed-species groups both fish species could gain similar advantages as single-species (i.e. reduced predation risk) groups but with a lower cost of parasitism from Gyrodactylus spp. infection. To our knowledge this is the first time that empirical support is provide for Moller's hypothesis
