Buoyancy role of urea and TMAO in an elasmobranch fish, the Port Jackson shark, Heterodontus portusjacksoni

The Port Jackson shark is denser than seawater, like most other elasmobranchs. Its small liver of relatively low lipid content, does not contribute significantly to buoyancy. Like other elasmobranchs, Port Jackson sharks accumulate high levels of urea and methylamines (trimethylamine oxide [TMAO] and betaine) in their body fluids as important balancing osmolytes. We calculate from the solute composition of plasma and muscle fluid, using molecular weights and partial molal volumes of solutes, that urea and TMAO in the body fluids of the Port Jackson shark contribute significant positive buoyancy. The high, positive partial molal volumes of urea, TMAO, and betaine provide this positive buoyancy despite their high molecular weights. The accounted solutes in plasma provide a net lift of about 2.4 g L(-1) relative to seawater; positive lift is contributed by Cl- (3.6 g L(-1) urea (2.8 g L(-1)) and TMAO (1.7 g L(-1)), and negative lift is contributed by Na+ (-1.8 g L(-1)) and protein (-3.5 g L(-1)) For muscle fluid, the accounted solutes contribute 6.4 g L(-1) lift; positive lift is contributed by Cl- (1.2 g L(-1)), urea (2.9 g L(-1)), TMAO (3.4 g L(-1)), and betaine (0.3 g L(-1)). We conclude that urea and methylamines are not only osmotic-balancing and counteracting solutes in elasmobranchs, but contribute significantly to positive buoyancy.