Absence of costs of foraging excursions in relation to limpet aggregation

Summary1 Animal aggregation has been well studied, with many proximate and ultimate models proposed. Many of the proximate models have relied on cost–benefit arguments, but frequently aspects of cost have been inferred in relation to position in group. An expected cost to an animal that rests in aggregations, but departs to forage, is that being in the centre of a group may impede foraging opportunities, in that ‘inner’ animals cannot leave to forage until outer animals have done so. 2 Experiments with a limpet Cellana tramoserica , on a rocky shore in New South Wales, Australia, showed that central limpets remain central more often than would be expected by chance. In respect of costs due to access to foraging, we show that there is no effect of position (central vs. outer) in timing of foraging excursions or of feeding‐rate. We also show that an aggregated distribution of C. tramoserica at rest does not translate into an aggregated distribution of foraging animals. 3 We propose that aggregations of C. tramoserica are probably a function of how the group reassembles after foraging and is much dependent on processes happening at a small spatial scale. It is highly probable that assumptions about the effects of limpet aggregation on rocky shore ecology (i.e. in influencing patchiness of algae) may be premature.