Respiration and Circulation in Land Crabs: Novel Variations on the Marine Design

Abstract

Both the “true” crabs (Brachyura) and hermit crabs (Anomura) include species that show numerous behavioral, morphological, and physiological specializations permitting terrestrial life. This paper examines respiratory and circulatory adaptations for air breathing in these land crabs. Respiratory specializations include modification of gas exchange structures for air breathing (gills and elaborated branchial chamber linings), ventilatory mechanisms permitting effective air pumping, an elevated hemolymph oxygen capacity, and a primarily CO2- rather than O2- sensitive ventilatory control system. The qualitative aspects of hemolymph oxygen transport and metabolic rate are apparently unchanged from that of marine crabs. While the basic cardiovascular morphology of land crabs appears similar to that of marine forms, there is considerable elaboration of the vasculature of the branchial chamber lining, which in some species includes a unique double portal system. Cardiac output is lower in land crabs (probably related to their higher hemolymph O2 capacity), but insufficient data on hemolymph pressures prevent comparisons with marine forms. In general, land crabs have modified (sometimes extensively) existing structures and processes found in their marine relatives rather than evolving structures for terrestrial life de novo . Accordingly, land crabs present a useful model for the evolution of terrestriality, showing that even subtle anatomical changes can result in the large changes in physiological function necessary for the terrestrial invasion.