Evolutionary history and adaptive significance of respiratory structures on the legs of intertidal porcelain crabs, genus Petrolisthes

Semiterrestrial and terrestrial crabs have evolved multiple strategies for aerial respiration. An uncommon strategy for aerial respiration is seen in porcelain crabs, genus Petrolisthes, where decalcified areas on the meral s egments of the walking legs are used as respiratory structures. Here, the e volutionary history and adaptive significance of these structures in porcel ain crabs is examined. Interspecific variation in leg membrane size is from 0% to 60% of the surface area of the meral segment. Leg membrane relative size is positively correlated with body size across species but not within one species, Petrolisthes cinctipes. Phylogenetic analyses suggest that leg membranes are ancestral to one of two eastern Pacific Petrolisthes clades. Comparative analyses using phylogenetic independent contrasts indicate a r elationship between leg membrane relative size and body size that is phylog enetically independent. In large-bodied intertidal species, whole-animal la ctate accumulation during aerial incubation is 200%-300% higher when the le g membranes are obscured, indicating that the leg membranes are functional respiratory structures in these species. Thus, it is possible that leg memb ranes have facilitated the evolution of larger body sizes by providing addi tional respiratory surfaces to accommodate the associated higher metabolic demands.