Lung and gill performance in gas exchange have been evaluated in eight species of air-breathing crabs with two different lung circulatory designs, those with portal systems and smooth lung linings, and those without portal systems and with invaginated and evaginated lung linings. In all species, the lungs were extremely effective in oxygen uptake whilst the performance of the gills was inferior. An exception to this was Gecarcoidea natalis, which has gills highly modified for aerial gas exchange; its gills and lungs were equally efficient in O2 uptake. The relative efficiencies of the lungs and gills in CO2 excretion differed between species, with the gills being the major site of CO2 loss in the more amphibious species and the lungs having an increasingly important role in the more terrestrial crabs. The presence or absence of lung portal systems was not found to correlate with either saturation rates or efferent oxygen concentrations, with both lung types being extremely efficient in O2 uptake. The lungs with portal systems showed a large increase in oxygen content in the first lacunar bed and progressively smaller increases in the next two; these lungs may, therefore, have some reserve for exercise.

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