Growth performance and osmoregulation in the shi drum (Umbrina cirrosa) adapted to different environmental salinities

In order to investigate the ability of shi drum (Umbrina cirrosa) to be reared at diverse locations, growth and osmoregulatory performance were assessed at full-strength seawater (40 psu), nearly iso-osmotic water (10 psu) and low salinity water (4 psu). At the end of the 84-day experimental period, fish reared at 4 psu displayed shorter mean fork length, lower mean body weight, lower specific growth rate and higher food conversion efficiency than fish reared at 10 or 40 psu. The effect of salinity on growth performance was also reflected by changes in plasma triglycerides and cholesterol, with fish reared at 4 psu exhibiting the lowest mean concentrations, while there was no significant difference in mean plasma glucose concentrations among treatments. Plasma osmolality was lower at 4 psu from day 42 onwards, while there was no significant difference in mean plasma K+ and Cl− concentrations. Plasma sodium and gill Na+/K+-ATPase activity showed minimum values on day 42 at 4 psu, but at the end of the experiment there was no difference among groups. Pavement cells, mucus cells and chloride cells were identified by histology on the gill epithelium. In shi drum reared at full seawater, mucus cells contained a mixture of acid and neutral mucins, whereas in fish adapted to hypo-osmotic environment neutral mucins were mainly observed. There was a significant increase in chloride cell number over the course of the study in all fish, but there was no difference among the three experimental salinities. Finally, in fish reared at 40 psu salinity, chloride cells increased in size significantly compared to fish adapted to 4 psu, whereas at 10 psu after 42 d there was a significant reduction in chloride cell size. These results indicate that shi drum reared from full-strength seawater to iso-osmotic salinity do not face any osmoregulatory imbalance, while fish reared in hypo-osmotic water displayed osmoregulatory impairment and low growth performance.