On the buoyancy of some deep-sea sharks

Abstract

Fish of five species of deep-sea squaloids ( Centrophorus squamosus, Centroscymnus coelolepis, Dalaticis licha, Deania calcea and Etmopterus princeps ) and one-deep sea holocephalan ( Hydrolagus affinis ) were all found to float when brought to the surface and placed in surface or laboratory sea water. However, by taking account of the effects of salinity, temperature and pressure differences between this seawater and that in which the animals lived, it is shown that all these animals must have been very close to neutral buoyancy at the bottom of the sea. Every one of these fish had an enormous oily liver and the lift which this gave almost exactly compensated for the weight in sea water of the rest of the animal. These livers contained large amomits of the hydrocarbon squalene which is not a convenient material to have as a metabolic reserve but which, with its low specific gravity (0.86), is particularly suited to give lift, being 80 % more effective per unit weight for this purpose than cod-liver oil. It is calculated that because of this unusual oil such fish not only obtain the lift needed for neutral buoyancy more economically in terms of the weight of oil required, but also in terms of the metabolic energy which has to be used to provide the oil-store responsible for buoyancy. It is argued that these fish must carefully regulate the oil content of their livers so as always to balance exactly the weight in sea water of their other tissues. The mechanism whereby they do this is not known.