The implications of divergence in spatial nesting patterns in the geminate Caribbean and Pacific sergeant major damselfishes

Abstract

Male Caribbean Abudefduf saxatilis, and Pacific, A. troschelii, sergeant majors nest in colonies, but nest densities in the Pacific are much greater than in the Caribbean. The relatively dense within-colony aggregations in the Pacific have apparently been favoured by the screening effects of defence by peripheral males against foraging groups of the wrasse, Thalassoma lucasanum, resulting in lower rates of predation on embryos guarded by males with centrally placed nests. Males with centrally placed nests were also more likely to spawn successfully than were those with peripheral nests, suggesting that aggregated nesting has been further reinforced by female preferences for males with nests in which risk of predation on embryos is lowest. In the Caribbean, where similar predation on embryos was absent, relatively slight within-colony spatial aggregation of nesting was observed. This tendency might be maintained by female preferences for males with close nearest-neighbour distances because males that failed to spawn had longer nearest-neighbour distances than those that did spawn, but the difference was only marginally significant. Alternatively, because the Caribbean and Pacific species are thought to have been derived from a single population following the uplift of the isthmus of Panama, it is possible that spatial aggregation represents a residual tendency, originally favoured by selection under ancestral conditions.