STEELE, S.L.*; YANCEY, P.H.; WRIGHT, P.A; University of Guelph, Ontario; Whitman College, Walla Walla WA; University of Guelph, Ontario: Osmoregulation and Nitrogen Excretion in the Embryonic Little Skate (Raja erinacea)

Marine elasmobranchs maintain high levels of organic osmolytes in their tissues to counteract the dehydrating force of seawater. Some adult elasmobranchs are able to adapt to dilute seawater environments by adjusting tissue levels and excretion of nitrogenous osmolytes. Embryonic elasmobranchs undergo rapid development and feed endogenously, therefore they may not have developed the same osmoregulatory abilities as adults. We investigated whether embryonic skates (Raja erinacea) had similar osmoregulatory strategies as adult skates. Under control conditions, the ratio of urea: TMAO and other organic osmolytes was 2:1 in 4 month embryos and yolks, and 3:1 in 8 month embryos and yolks. The ornithine urea cycle enzymes ornithine transcarbamoylase (OTCase), carbamoyl phosphate synthetase III (CPSase III), and arginase, as well as the accessory enzyme glutamine synthetase (GSase), were measured in 3 month embryos and 8 month muscle tissue and all activity levels were approximately 5 to 10% of those found in adult elasmobranch livers. When exposed to dilute 75% (25 ppt) seawater versus 100% seawater (33 ppt), skate embryos at 4 and 8 months significantly increased excretion of urea to the environment for up to 24 h after exposure. There was a significant decrease in tissue TMAO (-20%) levels in 4 month embryos exposed to 75% seawater for 120h. Eight month embryos exposed to dilute seawater for 120h experienced a significant decrease in tissue urea (-18%) and other osmolytes (-29%). These findings indicate that embryonic elasmobranchs are able to maintain osmotic balance under dilute seawater conditions at very early stages of development.