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

## TL;DR

A fish with a swimbladder was observed at 7259 meters deep in the Japan Trench, showing it can maintain buoyancy under extreme pressure.

## Contribution

This is the first in situ observation of a swimbladder functioning at such extreme depths in the hadal zone.

## Key findings

- The swimbladder provided buoyancy at 74.4 MPa with an estimated energy cost 200 times less than lipid-based buoyancy.
- Oxygen in the swimbladder could theoretically support aerobic metabolism for over a year.
- Swimbladder function depends on minimal oxygen diffusion across its wall at extreme pressures.

## Abstract

A macrourid, Coryphaenoides yaquinae sp. inc., was observed to be attracted to bait and exhibiting normal foraging behaviour during a period of 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. During normal metabolism, 192 days would be required to fill the swimbladder. At these depths, oxygen is very incompressible, so changes in volume during ascent or descent are small. However, swimbladder function is crucially dependent on a very low rate of diffusion of oxygen across the swimbladder wall. The oxygen in the swimbladder could theoretically sustain aerobic metabolism for over 1 year but is unlikely to be used as a reserve.

Highlighted Article: The deepest ever observation of a fish with gas-filled buoyancy exhibiting normal behaviour shows that gas secretion and retention function at the extreme hydrostatic pressures of hadal ocean trenches.