Mechanical size limitation and life-history strategy of an intertidal seaweed

Ocean waves impose large hydrodynamic forces on organisms living on rocky shores. Many intertidal organisms are structurally flexible, and the effect of this flexibility on the mechanical forces they experience is poorly understood. In addition, organisms in the rocky intertidal zone are generally much smaller than those in subtidal or terrestrial habitats. This study examined the influence of structural flexibility on the forces imposed by waves on the intertidal seaweed Pelvetiopsis limitata, including the potential for a mechanical size constraint on this species. A mechanical model of a seaweed on a wave-swept shore was developed on the basis of laboratory measurements of algal characteristics. Computer simulations predict that flexibility dramatically increases the forces imposed on the algae, particularly for large individuals. Along with field data on wave conditions and algal tenacity, this indicates that large algae are more likely to be dislodged than small ones throughout the year, and that very large individuals are expected to survive only during the summer and early fall. Measurements of thallus size in a population of P. limitata during a 2 yr period corroborated these model predictions. Algae in the field developed reproductive structures during the summer and early fall, when they were the largest and waves were the smallest. Thus, P limitata exhibits a life-history strategy in which it grows large when seasonal wave conditions permit, and reproduces before becoming dislodged by large winter waves, mitigating the reproductive consequences of a mechanical size constraint.