International Journal of Comparative Psychology

Fish typically choose shoalmates with similar phenotypic characteristics to themselves, thus creating shoals for which predators have difficulty identifying and attacking one specific individual. And while shoaling should provide similar anti-predator benefits to both males and females, the two sexes do not always make the same shoaling decisions. Here we explore the effect of phenotype on sex specificshoaling in three varieties of zebrafish (Danio rerio) and the closely related pearl danio(Danio albolineatus). We hypothesized that males and females of each type of zebrafish (wildtype, golden mutants and leopard mutants), as well as male and female pearl danios, would choose to shoal rather than be alone and, when given a choice of shoalmates, would shoal with fish of their own phenotype rather than dissimilar fish. As expected, our results show that most fish preferred to shoal rather than be alone. However, while both sexes of wildtype zebrafish responded identically to shoaling decisions, male and female mutant zebrafish and pearl danio fish differed in their response to such choices. When given a choice of shoalmates, wildtype zebrafish of both sexes showed no discrimination between different D. rerio strains, although they did choose to shoal with wild type conspecifics rather than pearl danios. The shoalmate preferences of the mutant zebrafish revealed that males showed no discrimination between shoals of their own variety and wildtype shoals, while mutant females preferred shoals of their own strain. Similarly, male pearl danios showed no discrimination between shoals of their own species and shoals of wildtype zebrafish, while pearl danio females preferred their own species. These results demonstrate the complex influence of sex and phenotype on shoaling behavior.