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