In Antarctic notothenioid fishes large amounts (3% w/v) of small molecular weight glycoproteins lower the freezing point of the body fluids a few tenths of a degree below the freezing point of seawater (-1.9°C), enabling these animals to avoid freezing1. Although these glycoproteins have molecular weights of 2,600-23,500 and would be expected to be filtered into the urine, they remain in the blood because the kidneys of these fishes contain only aglomerular nephrons2,3. Unlike the situation in most fishes, urine formation is the result of secretion rather than filtration and reabsorption. On the other hand, the peptide antifreezes in Northern Hemisphere fishes such as the winter flounder4, Pseudopleuronectes americanus, are retained by the glomerular kidney even though inulin, of comparable weight, is rapidly filtered from the blood into the urine5. The Antarctic eelpout (zoarcid), Rhigophila dearborni, which is unrelated to either the Antarctic notothenioids or P. americanus, also uses a peptide antifreeze (molecular weight 6,000) which is maintained at a concentration of 3% (w/v) in the blood plasma. We report here that the lack of antifreeze in the urine of R. dearborni probably reflects the fact that the glomeruli are not functional and cannot filter. We support this conclusion with morphological and physiological evidence and relate our findings to the conservation of biological antifreeze necessary for life in ice-laden polar waters.
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