Abundance and active patch selection modulate reproductive connectivity and fitness of pea crabs living on sand dollars

Abstract

Connectivity is paramount for population stability, but the mechanisms underlying the distribution of populated patches and how they affect reproductive connectivity and individual fitness remain elusive. Here, we mapped the distribution of sand dollars Encope emarginata as habitat patches for obligate-commensal pea crabs Dissodactylus crinitichelis, at several sites in Brazil. At occupied patches, we assessed whole-crab population structure and the fitness of ovigerous females. By doing so, we obtained population snapshots under varying environmental conditions that portrayed the natural variation of connectivity and fitness in the region. While sand dollar supply did not limit the size of crab populations, overall crab abundance limited reproductive connectivity and the potential for offspring production. However, except for cases where connectivity was extremely low and high, active crab aggregations enhanced reproductive potential despite the overall random distribution of sand dollars. Crab interactions within hosts, likely controlled by larger females, added to reproductive connectivity by increasing the frequency of mating pairs. Differently from the population-level case, effects of crab abundance on individual fitness were dual and only detectable when abundance was lowest (positive) or highest (negative), so that fitness remained high at intermediate crab abundance, decreasing when it became either too low (e.g. Allee effects) or too high (e.g. energetic costs of intraspecific competition).