The effects of changes in salinity on osmoregulation and chloride cell morphology of juvenile Australian snapper, Pagrus auratus

The effect of rapid transfer of juvenile Australian snapper, Pagrus auratus from ambient seawater (30 parts per thousand) to concentrated hyperosmotic (45 parts per thousand) and diluted hyperosmotic (15 parts per thousand) environments on serum osmolality, serum [Na+], [K+], [Cl-], blood haematocrit and branchial chloride cell morphology was assessed during 168 h after transfer. Serum osmolality, [Na+], [K+] and [Cl-] increased after 24 h in 45 parts per thousand. In contrast, after 24 h in 15 parts per thousand, [K+] did not change but serum osmolality, [Na+] and [Cl-] decreased. The serum chemistry changes were transient and had returned to near initial levels after 168 h in 45 parts per thousand and 15 parts per thousand. Transfer from 30 parts per thousand to 45 parts per thousand and 15 parts per thousand did not affect blood haematocrit. Branchial chloride cells were identified in both filament and lamellar epithelia of snapper held in all salinity treatments by an immunocytochemical staining technique using an antiserum specific for Na+, K+-ATPase. In 45 parts per thousand, the number of filament and lamellar chloride cells did not change, but filament chloride cells were more abundant than lamellar chloride cells. In contrast, filament chloride cells had increased in size after 72 h and by 168 h after transfer from 30 parts per thousand were 1.4-fold larger than the initial size. In 15 parts per thousand, the number of filament chloride cells and the size of both filament and lamellar chloride cells had decreased after 72 h. Our results demonstrate that snapper can osmoregulate in a wide range of salinity and provide indirect evidence that both filament and lamellar chloride cells are responsible for excretion of excess salt from snapper in hyperosmotic environments. The ability for snapper to adapt rapidly and maintain homeostasis in a wide range of salinities supports the fact that snapper are a suitable species for land-based aquaculture in ponds, where rapid fluctuation in salinity can occur. Crown Copyright (c) 2007 Published by Elsevier B.V. All rights reserved.