ABSTRACT Branchial complements, gill morphology, gill surface areas, gill volumes, branchial volumes, and branchial space of the stream-dwelling parastacid crayfish Astacopsis franklinii and the burrowing Parastacoides tasmanicus are compared. A reduction in gill number in Parastacoides results from the absence of pleurobranchiae on the fifth, sixth, and seventh thoracic segments and the extreme reduction of the pleurobranch on segment 8. There is a tendency for gill filaments to be longer in Parastacoides and the terminal spines on the filaments of the podobranchiae are longer, hooked rather than straight, and occur more frequently in this species. Most of the respiratory exchange area is provided by the podobranchiae. Linear regression equations (log transformed data) describing the allometric relationship between body size and gill area, gill volume, branchial volume, and branchial space are provided for both species. Parastacoides has a significantly greater size-related gill area and branchial volume than Astacopsis and, since there are no differences in gill volume, there is also a significant increase in branchial space. Much of this additional space appears to be created towards the posterior of the branchial chamber. The differences between the two species are interpreted as adaptations for the burrowing, amphibious life-style adopted by Parastacoides. We suggest that they serve primarily to facilitate oxygen uptake, reduce the tendency for the gills to become clogged, and aid in flushing of the branchial chamber in hypoxic waters with a very high particle content; they are seen as providing secondary benefits to assist survival during the often prolonged periods when the burrows lack free water.
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