No abstract

The anatomy and the histology of the digestive tract of young and adult sea-bream is described from studies using light and scanning electron microscopy. The dentition in the juvenile (25-30 mm long) comprises all canine-like teeth. to which plate teeth and transition elements are added in the adult.The oesophagus shows a multi-layered mucosa in the upper part, and single-layered regions in the lower part. The multi-layered regions are formed by epithelial cells, mucus-secreting cells and by cells rich with eosinophilic granules.The Y-shaped stomach, clearly distinguishable. has a single-layered columnar epithelium under which, in the cardiac and fundic portion, gastric glands, comprised of all similar cells, are present. The pyloric region is characterized by four caeca, to the base ofwhich the ductus pancreaticus and the ductus hepaticus discharge.The pancreas is composed of small masses spread along the upper intestine; in the adult, pancreatic infiltrations can be seen in the liver.The intestine is short (relative length 0.5-0.6). The intestine epithelium consists of columnar cells intercalated with mucus-secretingcells. A funnel-like valve marks the passage to the intestine terminal region, characterized by a mucosa of cells with an abundance of vacuoles full of eosinophilic granules.

A salt water exposure trial with juvenile Adriatic sturgeon, Acipenser naccarii, ( 5 months old; mean (* SD) weight = 56f28 g) was initiated by direct transfer from freshwater (FW) to brackish water (BW) of medium (310 mOsm.Kg-' = 11 %) or high salinity (640 mOsm.Kg-' = 23 960). 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 mitochondriarich (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 3 10 and 640 mOsm.Kgrelative to those in FW; significantly higher in the sturgeon at 640 mOsm.Kg-I. 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-' 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-I (20 %) 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-', they do not grow as well in BW at 600 mOsm.Kg-' as they do in FW and, in fact, died when disturbed by heavy activity near the tanks.

Specific growth rates, exercise respirometry, and swimming performance were compared in young-of-the-year Adriatic sturgeon (Acipenser naccarii) maintained in freshwater (FW) or acclimated to brackish water (BW) that was slightly hypertonic to sturgeon plasma, at a salinity of 11 g·L–1. Specific growth rate was significantly (17%) lower in BW than in FW. Sturgeon in BW also had a significantly (30%) higher standard metabolic rate than those in FW. In both groups, the relationship between swimming speed and oxygen uptake was described equally well by a linear or exponential equation, with a power relationship between swimming speed and net cost of locomotion and a linear relationship between tailbeat frequency and swimming speed. However, sturgeon in BW exhibited higher mean total oxygen uptake, net costs, and tailbeat frequencies than the FW group at any given swimming speed. There were, however, no differences in aerobic scope or maximum sustainable swimming speed between the FW and BW groups because the BW group exhibited a compensatory increase in active metabolic rate and maximum tailbeat frequency. The results indicate that FW is a more suitable environment than mildly hypertonic BW for young-of-the-year Adriatic sturgeon.

In this study the first experimental trials of acclimation to salinities of the Italian sturgeon Acipenser naccarii are described. Preliminary observations regarding some morphological features of the gut, kidney and gills following exposure to 20 and 30‰ salinity are reported, and compared with those displayed by many teleosts in hyper-and hypoosmoregulation. Results are discussed with regard to the osmoregulatory mechanisms and to the physiological limits of adaptability of this species in the second year of its life cycle. Within the Acipenseridae, Acipenser naccarii is generally considered a euryhaline species, but nothing is known about its mechanisms of osmotic homeostasis. This species could be a good candidate for aquaculture in fresh and brackish waters owing to the recent success obtained in artificial reproduction.1995 The Fisheries Society of the British Isles