Energy maximisation vs. time minimisation in the foraging of Patella vulgata

1. Recent field recording of grazing activity in Patella vulgata has shown that limpets maintain an almost continuous grazing rate during the whole foraging excursion. Since previous assessment of the foraging strategy of this species, based on the assumption that grazing rate varied between foraging and travelling, led to conclude that this species adopts a time minimization strategy, we tested a new model based on a constant grazing rate whilst travelling. 2. Two opposite strategies were tested: (a) maximization of the energy gain (Emax), and (b) minimization of the time spent away from the home scar (Tmin). Predictions based on these two strategies, temporal constraints on foraging activity and on estimates of energy input and costs incurred by limpets, were compared with the behaviour of a population of limpets inhabiting a vertical wall at Menai Bridge (UK). 3. According to strategy (a) limpets would forage for the entire duration of their potential activity phase (nocturnal low tide), whereas according to strategy (b) they would only spend away from home the time necessary to balance energy expenditure, which roughly corresponds to one-quarter of the entire duration of the potential activity phase. Moreover, according to the energy maximization strategy, duration of grazing would be independent of size, while according to the time minimization strategy a positive correlation of grazing activity would be expected with size. 4. The comparison of empirical data with the outputs of the model showed that neither a pure time minimization nor the energy maximization hypothesis can provide good estimates for the behaviour of the studied population of P. vulgata, which appears somehow intermediate between the two strategies. The possible explanations for this result are discussed in relation to the biology of the species, the assumptions of the model, and the difficulties in estimating the critical parameters.