REACTIONS OF GOBIOID FISHES TO HYPOXIA - BUOYANCY CONTROL AND AQUATIC SURFACE RESPIRATION

Laboratory observations of three genera of Eleotridae (Hypseleotris-two species, Philypnodon-two species, Gobiomorphus) and 11 genera of Gobiidae (Redigobius, Pandaka, Gobiopterus, Parkraemeria, Favonigobius-two species, Pseudogobius, Arenigobius-two species, Cryptocentroides, Mugilogobius-three species, Bathygobius, Chlamydogobius) revealed four distinctive ways for survival in hypoxia. Each involved aquatic surface respiration (ASR) in which > 50% of fish ventilated their gills with surface water when O2 fell below 2.1 ppm. Control of buoyancy was an important element in three of these strategies. Firstly, midwater eleotrids and gobies used ASR while swimming slowly at the surface maintaining near neutral buoyancy. Secondly, 11 species of benthic, negatively buoyant gobies performed ASR in the shallows resting on the substrate propped up on the pelvic fins with the head arched upwards to the water surface. Nine of these species held a bubble in the buccal chamber which was frequently exchanged. Thirdly, four of the above bubble-gulping species frequently took a buccal bubble of sufficient size to attain a vertical position at the surface while performing ASR. In both positions the bubble was held only when fish were using ASR and it appeared to function in providing lift to the head or body, facilitating ASR, and in addition, adding O2 to buccal water prior gill ventilation. In one benthic goby, the percentage of fish using a buccal bubble increased with an increase in salinity. Fourthly, one benthic goby used anaerobic respiration for several hours prior to using ASR.