Brad Wetherbee – Department of Biological Sciences, University of Rhode Island

Deep-sea sharks approach neutral buoyancy presumably as a means of reducing energy consumption in a nutrient poor environment.  In this study biochemical and physiological adaptations for buoyancy in deep-sea sharks of a variety of species and subgroups within species were examined.  The role of the liver as a hydrostatic organ was investigated by examination of liver size, the amount and composition of liver oil, coupled with buoyancy characteristics of the liver and whole shark.  Livers of deep-sea sharks average about 20% of the whole body weight, and consist of a mean of 80% oil. The liver oil of deep-sea sharks contains large proportions of the low-density lipid classes squalene and diacyl glyceryl ethers.  Near neutral buoyancy is maintained in deep-sea sharks through a variety of biochemical changes that occur during growth and maturation and differences among species.  Within a species of deep-sea shark, the proportion of squalene in liver oil tends to decrease as sharks increase in size, contributing to a more dense liver and liver oil.  However, larger sharks have a relatively larger liver that contains a greater proportion of oil in the liver, which compensate for decreased squalene content in oil, and sharks of all sizes approach neutral buoyancy. Liver oil of mature male sharks contains more squalene than liver oil of mature females.  Therefore males derive more hydrostatic lift from their livers, whereas females also rely on low-density lipids stored in eggs. Among species of deep-sea sharks there is variation in liver size, quantity of oil in the liver and the composition of the liver oil.  Species also differ in the amount of low-density matter contained in non-liver tissues, and species that have high concentrations of water and lipid in their muscle are those species with smaller livers, less oil and smaller amounts of low density lipid classes in their liver oil.  There appears to be strong selection pressure for neutral buoyancy in deep-sea sharks and nearly every individual deep-sea shark is close to neutrally buoyant.  However, the mechanisms used to obtain neutral buoyancy differ within and among species.

For pdf versions of publications resulting from this study click on the manuscript below. 

Wetherbee, B.M. and P.N. Nichols. 2000. Lipid composition of the liver oil of deep-sea sharks from the Chatham Rise, New Zealand .Comp. Biochem. Physiol. 125(B):511-521.

Wetherbee, B.M. 2000. Assemblage of deep-sea sharks on the Chatham Rise, New Zealand . U.S. Fish. Bull. 98:189-198.

Wetherbee, B.M. 1996. Distribution and reproduction of the southern lanternshark Etmopterus granulosus from New Zealand . J. Fish Biol. 49:1186-1196.