Testing the reproductive benefits of aggregation: the limpet Patella vulgata shows no evidence of synchrony in gonad development

Group living is ubiquitous, yet causal explanations are not fully tested. Evidence for a reduction in predation risk is clear, and there is support for reduction of risk from the abiotic environment. Potential reproductive benefits are less well understood, especially for non-lekking, externally fertilising animals which form medium-term aggregations. We used Patella vulgata to test the model that a reproductive benefit is derived from aggregation. We sampled grouped and solitary limpets fortnightly, from the point at which individuals were in a neuter state to the stage where the population was near spawning. Whilst aggregation increased overall as the population neared spawning, there was no difference between aggregated and solitary limpets in terms of sex ratio. There was also no difference in variability of gonad development between solitary and grouped animals, which means no synchrony in gonad development was necessary for externally fertilising animals to gain a benefit from aggregating. We suggest that causal explanations for an increase in limpet aggregation from autumn to winter are most likely to lie in the interaction of reduced grazing activity and increased predation pressure. Since limpets are a key component of rocky shore systems, understanding the processes determining their spatial arrangement has implications for our understanding of rocky shores.