Buoyancy function of the enlarged fluid-filled cranium in the deep-sea ophidiid fish Acanthonus armatus

Abstract

Acanthonus armatus Günther (family Ophidiidae) is a slightly negatively buoyant (mean weight in surface seawater =0.58% of air weight) benthopelagic fish without a swimbladder that has reduced tissues and components (muscle, bone, brain, gills, lipids) and a massive head. This species has an enlarged cranial cavity (10% of head volume) filled, except for the very small brain, with a low-density (specific gravity 1.008) fluid. The fluid provided static lift of about 0.30 g each for 4 specimens whose weights in water ranged from 1.0 to 2.6 g. Osmotic concentrations of the cranial fluid averaged 294 mOsm 1^-1 in 4 specimens and , in 2 specimens for which comparisons were possible, were 43 to 45% lower than the concentrations of the plasma and perivisceral fluid. Similarly, Na⁺ and K⁺ concentrations of the cranial fluid were lower than those of the plasma or perivisceral fluid. A. armatus has highly reduced heavy tissues and an increased content of dilute fluid, with a localization of even more highly dilute fluid in the enlarged cranium which partially offsets the relatively dense tissues of the head. Information on the morphology, behavior and ecology of the fish supports the interpretation of the buoyancy and chemical data. The conclusion is consistent with the hypothesis that obtaining and conserving energy and reducing predation are the important selective factors affecting the evolution of deep-sea fishes and that these factors have greatly influenced the chemical composition of the fishes.