In situ observation of a macrourid fish at 7259 m in the Japan Trench: swimbladder buoyancy at extreme depth.

A macrourid, Coryphaenoides yaquinae sp.inc. was observed attracted to bait and exhibiting normal foraging behaviour during 80 min within view of a baited video camera on the sea floor at 7259 m; the deepest ever observation of a fish species with a swim bladder. The buoyancy provided by an oxygen-filled swim bladder at 74.4 MPa pressure was estimated to be 0.164 N, at a theoretical energy cost of 20 kJ, 200 times less than the cost of equivalent lipid buoyancy. 192 days would be required to fill the swimbladder during normal metabolism. At these depths oxygen is very incompressible so changes in volume during ascent or descent are small. However, swimbladder function is crucially dependent on very low rate of diffusion of oxygen across the swimbladder wall. The oxygen in the swimbladder could theoretically sustain aerobic metabolism for over one year but is unlikely to be used as a reserve.