Urea levels in plasma and erythrocytes of the southern fiddler skate, Trygonorhina fasciata guanerius

Plasma and erythrocyte samples from Trygonorhina fasciata guanerius were analysed for urea content. The erythrocyte urea concentration is higher than plasma urea concentration. The difference in concentration is probably due to the presence of urea normally bound to protein within the erythrocyte. Erythrocyte urea concentrations followed environmentally induced changes in plasma urea concentrations. Naturally occurring plasma urea concentrations, exceeding 500 mMolar, are some of the highest recorded for an elasmobranch.