Discovery of a specialised anatomical structure in some physoclistous carangid fishes which permits rapid ascent without barotrauma

Abstract

Physoclistous fish are unable to rapidly ascend through the water column without significant risk of barotrauma via swim bladder hyperextension or rupture. Here, we report on the discovery of a highly specialised anatomical structure which permits some physoclistous fish species, the samson fish Seriola hippos and silver trevally Pseudocaranx georgianus, to vent swim bladder gas during ascent. Dissections of injected casts and X-ray imaging revealed the swim bladder ‘vent’ to consist of a membranous opening in the roof of the swim bladder which led to a flattened tube that bifurcated around the vertebral column and exited via a small, oval-shaped hole in the pharyngo-cleithral membrane underneath each operculum. Identification of these distinctive holes revealed that venting had occurred on ascent in 96 % of in S. hippos captured from depth. Decompression from simulated 30 m water depth in an experimental hyperbaric chamber revealed that venting in P. georgianus commenced when predicted swim bladder volume was approximately double that of its initial volume and ceased when the fish were again near neutrally buoyant. A homologous structure was identified in S. lalandi and S. dumerili. This structure represents an important step in the evolution of the teleost swim bladder which circumvents the normal physoclistous restriction on rapid ascents by eliminating, or minimising, barotrauma. Ecological benefits of this ability include an improved ability to capture prey, escape predators and perform ‘spawning rushes’. The increased resilience to barotrauma provided by the swim bladder ‘vent’ may also reduce the levels of post-release discard mortality for deep-water species which possess the structure.