Acquisition of a defended resource: a benefit of group foraging for the neotropical wrasse, Thalassoma lucasanum

Synopsis

The neotropical Pacific wrasse, Thalassoma lucasanum, forms groups of 30 to 300 individuals that overwhelm the defense of embryos by the Pacific sergeant major, Abudefduf troschelii, and thereby gain access to a food resource from which solitary individuals are effectively excluded by paternal defense. The duration of feeding by the wrasse groups and the fraction of embryos eaten in a nest are positively correlated with group size. The benefit of group foraging is probably derived from a reduction in the frequency with which group members experience attack by defending sergeant majors, as reflected in the positive correlation of group size with feeding duration. Large foraging groups only form when sergeant major embryos are present in nests, indicating that this is a primary benefit of participation in these groups for this wrasse. In contrast, in the Caribbean large foraging groups of the congeneric bluehead wrasse, T. bifasciatum, fail to form despite the presence of defended embryos of the Caribbean sergeant major, A. saxatilis. This might be due to the relatively low population densities of the Caribbean wrasse in comparison to those of the Pacific wrasse or to the relatively low densities of Caribbean sergeant major nests containing embryos at any time.