The frequencies of heart (fH) and scaphognathite (ventilatory = fBC) pumping, and responses to hypoxia, hypercapnia and wetting (simulated rain), as well as oxygen consumption (MOO2), pre- and postbranchial haemolymph oxygen tension (POO2), oxygen content (COO2), carbon dioxide content (CCOCO2) and pH were measured in adult land crabs Coenobita clypeatus. There was a large increase in f8C in response to both hypoxia and wetting but a smaller increase in response to even severe hypercapnia. Some evidence suggests that ventilation via the scaphognathites may have been supplemented by a second (branchiostegal) pump when ventilatory requirement was high. fH was less responsive to either hypoxia or hypercapnia, but decreased with severe exposure to either. Haemolymph oxygen tensions were low (Pα,Oα,O2 = 14, PvOvO2 = 8) but haemocyanin oxygen affinity was high in vivo (P50 = 10 torr at 23°C) and postbranchial haemocyanin was 60–80% saturated. Oxygen content was also high allowing adequate oxygen release to the tissues despite the low oxygen tensions. ΔP50t = 0.37 torr/Δ °C, log ΔP50/ΔpH = - 0.84 torr/pH unit, both determined in vitro were lower than literature values for marine and littoral species. As in other terrestrial species, CCOCO2 and PCOCO2, (calculated) were high, as were both bicarbonate and non-bicarbonate buffering capacities. Water loss was less (0–08% body weight. h−1) in Coenobita than in other terrestrial crustaceans, this resulting from the protection of the adopted shell. Results obtained from Coenobita are compared with those from other terrestrial and littoral crabs to illustrate the influence of the adopted shell on the degree of modification needed to enter terrestrial habitats.

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