THE Notothenioidei, the dominant perciform suborder of Antarctic fishes, is a predominantly benthic group of about 75 species1. They lack a swim bladder, a feature consistent with their mode of life on the bottom of the sea1. Therefore few of them have been able to exploit the enormous biomass of planktonic crustaceans (krill) indigenous to the mid-waters of the Antarctic Ocean2,3. One notable exception, however, is the nototheniid Pleuragramma antarcticum Boulenger which has invaded this highly productive habitat. Unlike other members of this family, Pleuragramma spends its life swimming in the water2,4, at times beneath a cover of sea ice. Although Pleuragramma lacks a swim bladder, it has been observed to remain nearly motionless in the water when placed in an aquarium. Furthermore, its slow swimming speed and general body morphology (Fig. 1) appear to preclude the use of forward motion and associated hydrodynamic lift as a mechanism for maintaining position in the water column. In - 1.8 °C seawater, specimens of Pleuragramma weighed between 0.5 and 1 % of their weight in air indicating they were almost neutrally buoyant. We report here that this reduction in density results from the accumulation of lipid in intermuscular and subcutaneous sacs, and from a reduction in skeletal ossification.
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