Summary

Australian snapper, Pagrus auratus is a high quality table fish but fisheries are declining and aquaculture has received significant attention over the last two decades. Juvenile snapper generally are found in estuaries which experience fluctuating water quality including daily and seasonal changes in salinity. Salinity can be reduced from normal ocean conditions by dilution following rainfall events or, in some estuaries such as St Vincents and Spencers Gulf in SA, the salinity can exceed seawater due to limited tidal exchange and high evaporation. Snapper are able to withstand the environmental changes; however prior to this study, the physiological mechanisms to do this were not well understood.

Like most fish, snapper have chloride cells located on their gills (filament and lamellar cells) which are responsible for excreting excess salt, sodium (Na+) and chloride (Cl-) ions, in order to maintain blood osmolality in a saline environment. However the changes in blood chemistry, including osmolality, and chloride cell size and density following rapid transfer of fish from ambient seawater (30 ppt) to hypersaline (45 ppt) and hyposaline (15 ppt) environments were not known and understanding the physical and physiological changes were the focus of this study.

Following rapid transfer of snapper from 30 ppt to 45 ppt, the serum osmolality and Na+, K+ and Cl- concentrations increased after 24 h however the changes were short lived and chemistry of the blood returned to normal levels after 7 days. In contrast, the serum osmolality, Na+ and Cl- decreased in fish following rapid transfer from 30 ppt to 15 ppt. These parameters were restored to normal levels after 7 days.

The ability for snapper to recover from rapid transfer from seawater to hyper- and hypoosmotic environments was related to their ability to alter gill chloride cells. In a hypersaline environment, the number of filament and lamellar chloride cells did not change but the size of the filament chloride cells increased after 72 h and were 1.4 times bigger than the initial size. In a hyposaline environment, the number of filament chloride cells decreased and the size of both filament and lamellar chloride cells also decreased after 72 h. Our results demonstrate that snapper can maintain their blood chemistry in a wide range of salinity and suggest that both filament and lamellar chloride cells are responsible for excretion of excess salt from snapper in hyperosmotic environments.