Locomotion, respiratory physiology, and energetics of amphibious and terrestrial crabs

The transition from breathing air to breathing water requires physiological and morphological adaptations. The study of crustaceans in transitional habitats provides important information as to the nature of these adaptations. This article addresses the physiology of air breathing in amphibious and terrestrial crabs and their relative locomotor abilities. Potamonautes warreni is an apparently amphibious freshwater crab from southern Africa, Cardisoma hirtipes is an air-breathing gecarcinid crab with some dependency on freshwater, and Gecarcoidea natalis is an obligate air-breathing gecarcinid endemic to Christmas Island in the Indian Ocean. All three species have well-developed lungs but retain gills and show seasonally different activity patterns that, in the gercarcinids, especially G. natalis, include long-distance breeding migrations. The three species were better at breathing air than water, but P. warreni was the best at breathing water. Cardisoma hirtipes is essentially an obligate air breather and appears to experience facultative hypometabolism during immersion. Cardisoma hirtipes has a haemocyanin with a high affinity for O-2 that facilitates loading from air but makes 30% of the Hc bound O-2 inaccessible. The gecarcinids but not P. warreni show increased diffusion limitation for O-2 over the lung during exercise. Gecarcoidea natalis outperforms C. hirtipes by virtue of a unique haemolymph shunt from the lung into the gills. Paradoxically, it is modifications of the gills for aerial O-2 uptake in G. natalis that allow for relatively greater haemolymph oxygenation. Despite showing decreased arterial-venous Delta Po-2, P. warreni increased the arterial-venous Delta [O-2] with no recourse to anaerobiosis during 5 min exercise. In the short term, P. warreni is more adept at walking than C. hirtipes. The breeding migrations of C. hirtipes and G. natalis were completely aerobic, but G. natalis walk farther and probably faster. Seasonal changes in underlying metabolism of G. natalis are strongly implied, including variations in hyperglycaemic hormone, variable basal metabolic rates, and a diel alkalosis present only in migrating crabs. The persistent dependence on water for reproduction is a determining factor in the biology of air-breathing crabs. The annual migrations include costs other than locomotion, for example, burrow construction and intermale competition. Estimates of costs that consider walking alone will underestimate the metabolic and stored fuel requirements for successful reproduction.