Some aspects of osmotic and ionic regulation in Adriatic sturgeon Acipenser naccarii. II: Morpho‐physiological adjustments to hyperosmotic environments

Summary A salt water exposure trial with juvenile Adriatic sturgeon, Acipenser naccarii, (5 months old; mean (± SD) weight = 56±28 g) was initiated by direct transfer from freshwater (FW) to brackish water (BW) of medium (310 mOsm.Kg-1= 11 %o) or high salinity (640 mOsm.Kg-1= 23 %o). Survival at 6 weeks and homeostatic regulation of plasma osmolality and ion concentrations demonstrated that A. naccarii of this age and/or size class possess the ability to adapt to hyperosmotic environments. Regulation of the osmotic status of body fluids was associated with differences in the number of mitochondria-rich (MR) cells on the gill lamellae, whereby sturgeon in high salinities exhibited reduced MR cell numbers as compared with those in FW. Measurement of Na+, K+-ATPase activity in crude gill homogenates from the three groups of sturgeon revealed higher activity in sturgeon at salinities of 310 and 640 mOsm.Kg-1 relative to those in FW; significantly higher in the sturgeon at 640 mOsm.Kg-1. There were no differences in H+-ATPase activity amongst the groups, as measured on the same crude homogenates, but there was a significant increase in the ratio of Na+,K+-ATPase to H+-ATPase in the sturgeon in water at 640 mOsm.Kg-1 relative to those in FW. There was a significant negative linear correlation between gill MR cell number and Na+,K+-ATPase activity. Apparently successful adaptation to BW was also indicated by similar low levels of serum Cortisol, and similar rates of resting oxygen consumption, in all groups. Nonetheless, a growth study on triplicate groups of 40 tagged sturgeon in FW or BW at 600 mOsm.Kg-1 (20 %o) revealed that animals in BW grew less well and exhibited less efficient feed conversion. The results indicate that although sturgeon exhibit some morpho-physiological adjustments to hyperosmotic environments and are able thereby to regulate plasma ions and osmolality in BW at 310 and 640 mOsm.Kg“1, they do not grow as well in BW at 600 mOsm.Kg-1 as they do in FW and, in fact, died when disturbed by heavy activity near the tanks.