Comparative studies of the branchial morphology, gill area and gill ultrastructure of some thalassinidean mud‐shrimps (Crustacea: Decapoda: Thalassinidea)

Abstract

The morphology, gill area and branchial formulae of six thalassinidean decapods (Calocaris macandreae, Jaxea nocturna, Callianassa subterranea, Upogebia stellata, U. deltauraandU. pusilla) are reported. Additionally, the rarely‐encounteredAxius stirhynchusreceives brief attention. Gill formulae are presented; the simplest arrangements are found in the Callianassidae and Upogebiidae. The deeper‐burrowing, deposit‐feeding species that are regularly exposed to prolonged periods of hypoxia, i.e.Callianassa subterraneaandJaxea nocturna, had significantly larger gill areas than thalassinideans that occupied more oxygenated burrows (Upogebiaspp.,Calocaris macandreae). The increase in gill area was a result of flattening of the trichobranchiate gill filaments giving rise to a phylloid gill form. It is suggested that the efficiency of gas transfer, and hence diffusing capacity, was enhanced in the phylloid gill by the larger gill area and, because of the reduced cuticle thickness, by the shorter water‐haemolymph diffusion distance. The increased diffusion capacity of the phylloid gill is interpreted as a functional adaptation to the more severe physicochemical burrow water conditions experienced byCallianassa subterraneaandJaxea nocturna.