The response of sea bream following abrupt hyposmotic exposure

Abstract

The response of a marine teleost, silver sea bream Sparus sarba, to abrupt hyposmotic exposure was investigated over a 120‐h period following direct exposure from sea water (SW, 33‰) to a hyposmotic environment of 6‰. Aspects of serum chemistry, stored metabolites and gill morphology were used to gain further insight into the biochemical, physiological and morphological alterations that take place following low salinity exposure of a marine fish. Rapid (<1 h) reductions in serum [Cl−] occurred while serum [Na+] exhibited only transient perturbations during initial exposure. Serum total [Ca] declined 24 h after exposure and returned to pre‐exposure levels by 120 h. Despite a tendency for muscle moisture to increase during the early stages of low salinity exposure to significant change could be detected. The response of the branchial chloride cell (CC) was rapid, with apical and fractional exposure area increasing after 6 h. The number of CCs exposed at the branchial surface reduced after 6 h but subsequently increased to elevated levels. Serum cortisol levels had increased three fold 1 h after hyposmotic exposure and stabilised at pre‐exposure levels within 12–24 h. Serum triiodothyronine (T3) levels exhibited a biphasic response, significantly elevating and decreasing after 3 and 6 h respectively. Significant post‐hyposmotic exposure elevations in serum glucose and protein occurred after 1 h, peaking at 3 h and returning to lower levels after 6 h. Total free ninhydrin reactive substances were significantly elevated 3 h post‐hyposmotic exposure, a phenomenon attributable to elevated levels of ammonia, alanine, arginine, glycine, isoleucine, lysine, methionine, phenylalanine, serine, taurine, threonine and valine. Of these substances, glycine, lysine, serine and taurine remained elevated for up to 12 h or longer. Serum urea levels elevated 1 h after exposure to 6%‰ and returned to SW levels 3 h post‐exposure. The relevance of these results is discussed within the context of current knowledge on the effects of hyposomotic adaptation on marine fish.