Perciform fishes of the suborder Notothenioidei are the dominant fishes in the Ross Sea and in the Southern Ocean in general. They have evolved physiological adaptations which ensure their survival in ice-laden seawater at a temperature of -1.9°C and extended their habitat range. Among these adaptations are relatively high metabolic rates made possible by enzymes efficient in catalyzing energyproducing reactions at subzero temperatures; a restricted range of temperature tolerance; slow growth rates in spite of relatively high metabolic rates; antifreeze glycoproteins which reduce the freezing point of body fluids below that of seawater and prevent ice crystal growth; aglomerular nephrons which preclude the loss of small molecular weight antifreeze molecules in the urine and mechanisms (especially skeletal reduction and lipid deposition) promoting neutral buoyancy in some species even though swim bladders are absent. Species with different buoyancies are associated with the pelagic, cryopelagic, benthopelagic and benthic habitats in the Ross Sea. Physiological specializations, including buoyancy adaptations in many body systems, and the absence of competition from other fishes allowed this presumably monophyletic group to radiate into several habitats in the Southern Ocean.
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