Why not be larger in wave action sites? Energetic constraints due to body size in hermit crabs

Coping with wave action is energetically demanding. Therefore, the endurance of animals exposed to waves should depend on a long-term balance between energy intake and the energy needed to withstand waves. Since the hydrodynamic forces of lift and drag increase with body size, we expected to find smaller individuals in wave-exposed areas. We sampled the body size distribution of hermit crabs along a gradient of wave action intensity. Then, we experimentally estimated energy budget (assimilated energy, maintenance metabolic rate, and the cost of coping with waves) as a function of body size in individuals exposed to bidirectional water flow at two different intensities for 21 days. We used the exponents of the assimilated energy and of the energetic cost of coping with waves to compute the theoretically predicted optimum and maximum energetic size according to the Sebens model. We found that the cost of coping with waves increased with wave intensity and body size due to the higher energetic demands of physiological maintenance and of remaining attached to the substrate. Although assimilated energy increased with body size under both wave regimes, this increase was less pronounced in the individuals exposed to the stronger wave regime compared to those in the weaker wave treatment. Furthermore, under the stronger experimental wave regime energetic constraints increased with body size, and individuals collected in the field were smaller than the maximum size predicted by the model for a similar wave regime. Together, these results suggest that hermit crabs may exhibit an adaptive submaximal growth strategy to mitigate the hydrodynamic costs of wave action.