ADAPTATIONS OF THE ANTARCTIC SILVERFISH PLEURAGRAMMA-ANTARCTICUM (PISCES, NOTOTHENIIDAE) TO PELAGIC LIFE IN HIGH-ANTARCTIC WATERS

Most fishes in Antarctic waters belong to the suborder Notothenioidei, highly developed perciform fishes which have been able to occupy not only a variety of benthic but also pelagic niches in Antarctic waters. One of the rare truly pelagic fish species, Pleuragramma antarcticum, plays a pivotal role in high-Antarctic food webs, due to its exceptio nal abundance. To investigate the life history of P. antarcticum more than 16 000 specimens were collected in the Weddell Sea during various cruises with RV 'Polarstern'. Apart from the more general life cycle adaptations with respect to reproduction, migrations, and feeding beha viour, P. antarcticum has developed a number of specific biochemical a nd physiological adaptations to cope with the environmental conditions in these permanently cold and highly seasonal Antarctic waters. Durin g its second summer P. antarcticum starts to accumulate large lipid de posits, mainly in the form of triacylglycerols, These low-density comp ounds provide the species with hydrostatic lift, an important factor f or a pelagic fish without a swim bladder. The lipid stores may also se rve as energy reserves. Highly polar brain gangliosides suggest wide-r anging neurophysiological adaptations to ensure proper functioning of the nervous system in icy waters. To avoid freezing in the presence of frazil ice P, antarcticum contains efficient antifreeze glycoproteins . A newly discovered glycoprotein acts as an additional antifreeze age nt. Although pelagic, adult P, antarcticum are rather sluggish, which is indicated by the small total gill area as well as blood physiologic al characteristics. Such behaviour diminishes routine energy costs. Bl ood viscosity is reduced and at least 2 major haemoglobins are found. Provided that these haemoglobins are functionally different, they indi cate a strong relationship between physiological and biochemical adapt ations of the oxygen transport system and life style. P, antarcticum r epresents a prime example of the complexity of adaptations necessary t o thrive in the pelagic realm of Antarctic shelf waters, a niche large ly unoccupied by other fish species.