IONIC AND OSMOTIC REGULATION CAPABILITIES OF JUVENILE GULF-OF-MEXICO STURGEON, ACIPENSER OXYRINCHUS DE SOTOI

The salinity tolerance, and hydromineral regulation capabilities of th ree size groups (small 110-170 g; medium 230-290 g, large 460-700 g; n = 48 for each group) of 13-month-old juvenile Gulf of Mexico sturgeon were investigated. Fish (n = 6 for each salinity) were transferred di rectly from freshwater (FW) to a series of experimental salinity treat ments (0, 5, 10, 15, 20, 25, 30, and 35 parts per thousand (ppt)). Fis h were also acclimated in brackish water (20 ppt) for 2 weeks and tran sferred to a salinity of 34 ppt. In this condition juvenile Gulf of Me xico sturgeon adapted to saltwater (SW) and maintained their :hydromin eral balance. FW adapted sturgeon (n = 6) had an average blood hemotoc rit of 28.2 +/- 0.8%, plasma osmolality of 260.7 +/- 1.6 mOsm kg(-1) H 2O, and plasma ion concentrations of 135.7 +/- 1.2 mM l(-1) Na+, 106.9 +/- 1.9 mEq l(-1) Cl-, and 2.9 +/- 0.1 mM l(-1) K+. In SW adapted stu rgeon (n = 8) blood parameters averaged 26.9 +/- 0.7% for hematocrit, 294.2 +/- 2.3 mOsm kg(-1) H2O for osmolality, 152.0 +/- 1.7 mM l(-1) N a+, 149.2 +/- 1.4 mEq l(-1) for Cl-, and 3.1 +/- 0.1 mM l(-1) K+. The method of transfer (abrupt or slow acclimation) directly affected fish survival and the time they took to achieve ionic and osmotic regulati on. This SW adaptation appears to be related to body size, the larger the fish the easier the adaptation process. A threshold size of about 170 g was apparent for the fish to adapt to saltwater after 2 weeks of acclimation. Chloride cells were present in both FW and SW adapted st urgeon with SW and brackish water fish having chloride cells significa ntly (P < 0.05) more numerous (561 +/- 53 and 598 +/- 45 cells mm(-2)) and larger in size (41.0 +/- 3.85 and 34.2 +/- 4.49 mu m(2)) than FW adapted sturgeon (10 +/- 1.0 cells mm(-2) and 22 +/- 2.53 mu m(2)). Fe w chloride cells were observed in the opercular membrane, however, non e were found in the pseudobranch and spiracle. (C) 1998 Elsevier Scien ce Inc. All rights reserved.