AQUATIC SURFACE RESPIRATION, BUOYANCY CONTROL AND THE EVOLUTION OF AIR-BREATHING IN GOBIES (GOBIIDAE, PISCES)

The role of a buccal gas bubble, held while performing aquatic surface respiration (ASR; ventilating the gills with surface water during hyp oxia), was examined in benthic, intertidal Australian gobies (Favonigo bius tamarensis, F. exquisitus, Pseudogobius olorum, Chlamydogobius sp ., Mugilogobius paludis, Cryptocentroides cristatus and Arenigobius bi frenatus). Analyses of the forces of lift and weight of the head and b ody during ASR indicate a hydrostatic role for the bubble, During ASR, lift from the bubble was sufficient to provide neutral or positive bu oyancy to the head, anchoring the mouth at the water surface, A buoyan cy role was confirmed by experiments demonstrating the ability of some species to alter bubble volume, to compensate either for different bo dy positions or for water densities (salinities), Use of the bubble fo r aerial respiration by Cryptocentroides, Mugilogobius, Chlamydogobius and Arenigobius, was confirmed in hypoxia by the presence of blood-fi lled capillaries in the buccal subepithelium (mean air-blood barrier l ess than 30 mu m) in areas of the buccal cavity that contacted the bub ble. Blood-filled capillaries were rare or absent in normoxia in all s pecies except Mugilogobius. Cutaneous respiration was inferred from th e presence of blood-filled capillaries in the dermis and epidermis of emersed portions of the head in Mugilogobius, Chlamydogobius and Areni gobius. The buccal bubble has respiratory and hydrostatic roles and th ere is support for the hypothesis that ASR and the buoyancy regulation (air-gulping) required to perform it effectively are prerequisite ste ps in the evolution of air-breathing in these gobies.