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 ha bitats 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 warr eni is an apparently amphibious freshwater crab from southern Africa, Cardi soma hirtipes is an air-breathing gecarcinid crab with some dependency on f reshwater, and Gecarcoidea natalis is an obligate air-breathing gecarcinid endemic to Christmas Island in the Indian Ocean. All three species have wel l-developed lungs but retain gills and show seasonally different activity p atterns that, in the gercarcinids, especially G. natalis, include long-dist ance breeding migrations. The three species were better at breathing air th an water, but P. warreni was the best at breathing water. Cardisoma hirtipe s is essentially an obligate air breather and appears to experience faculta tive 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. Geca rcoidea natalis outperforms C. hirtipes by virtue of a unique haemolymph sh unt 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 greate r haemolymph oxygenation. Despite showing decreased arterial-venous Delta P o-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 mor e adept at walking than C. hirtipes. The breeding migrations of C. hirtipes and G. natalis were completely aerobic, but G. natalis walk farther and pr obably 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 fact or in the biology of air-breathing crabs. The annual migrations include cos ts other than locomotion, for example, burrow construction and intermale co mpetition. Estimates of costs that consider walking alone will underestimat e the metabolic and stored fuel requirements for successful reproduction.