Factors influencing the rate of gas loss from the Physoclist swimbladder

Abstract

The aim of this study was to provide a rational account of the control of gas loss from the swimbladder of a physoclist fish Pollachius virens (L). Where possible the work has also Included a study of vertically migrating mesopelagic fish and of deep-sea demersal types. The bladder loses gas by two major routes, directed loss through the oval in buoyancy adjustment and random diffusion loss through the bladder wall. The oval Is controlled by two distinct mechanisms - muscular and vascular, and the effect that each of these has on gas loss has been examined and quantified as far as possible. The structure, histology and pharmacology of the oval were reexamined in terms of control of gas loss. The oval membrane in all anacanthines is formed from an extension of the tunica interna of the bladder and this was shown to be the layer most impermeable to oxygen. It is a good design point that the adjustable oval should be formed from this layer and this is in contrast to the arrangement found in eels. The vascular control exerted by the oval plexus proved more difficult to assess. The effect of drugs on the plexus was confirmed, and preliminary estimates of maximum flow rates were made from latex-injected preparations using the Poiseuille equation. The flow rates in the oval were estimated using radiolabelled microspheres and a reference organ technique in a resting and an actively resorbing fish. A significant increase in the flow rate was noted when the bladder was artificially overinflated. The permeability to oxygen of sections of the bladder wall was estimated and low oxygen permeability was correlated with the presence of purine crystals in the tissues. It was found that substantial purine deposits occurred in the submucosa of the tunica interna and this layer had the lowest permeability to oxygen. Results by several authors indicate that the oxygen permeability of the bladder wall is inversely proportional to purine content. The purine content of the whole bladder wall was estimated in a number of species and found to be proportional to the mean depth of occurrence of the animals. The maximum depth of neutral buoyancy was calculated in some fish using the permeability data. They were shown to have high rates of gas loss and in vertically migrating mesopelagic fish the bladder is probably Inadequate to maintain neutral buoyancy at all depths. The effects of fins and ventilation thrust in reduction of sinking rate are considered and the elastic collapse of the bladder at depth may act as a safety device ensuring some static lift. These three factors may contribute to maintenance of a stable position in the water column. The physiological and ecological significance of these observations is discussed.