COMPARATIVE-STUDIES OF THE BRANCHIAL MORPHOLOGY, GILL AREA AND GILL ULTRASTRUCTURE OF SOME THALASSINIDEAN MUD-SHRIMPS (CRUSTACEA, DECAPODA,THALASSINIDEA)

The morphology, gill area and branchial formulae of six thalassinidean decapods (Calocaris macandreae, Jaxea nocturna, Callianassa subterran ea, Upogebia stellata, U. deltaura and U. pusilla) are reported. Addit ionally, the rarely-encountered Axius stirhynchus receives brief atten tion. 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 hyp oxia, i.e. Callianassa subterranea and Jaxea nocturna, had significant ly larger gill areas than thalassinideans that occupied more oxygenate d burrows (Upogebia spp., 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 efficie ncy of gas transfer, and hence diffusing capacity, was enhanced in the phylloid gill by the larger gill area and, because of the reduced cut icle thickness, by the shorter water-haemolymph diffusion distance. Th e increased diffusion capacity of the phylloid gill is interpreted as a functional adaptation to the more severe physicochemical burrow wate r conditions experienced by Callianassa subterranea and Jaxea nocturna .