Juvenile gilthead sea bream (Sparus auratus L.; 10-40·g body mass) were acclimatized in the laboratory to full strength (34‰) or dilute (2.5‰) seawater and fed normal, calcium-sufficient or calcium-deficient diet for nine weeks. Mean growth rate, whole-body calcium and phosphorus content and accumulation rates were determined, as well as plasma levels of ionic and total calcium, cortisol and parathyroid hormone related protein (PTHrP; a hypercalcemic hormone in fish). When confronted with limited calcium access (low salinity and calcium-deficient diet), sea bream show growth arrest. Both plasma cortisol and PTHrP increase when calcium is limited in water or diet, and a positive relationship was found between plasma PTHrP and plasma ionic calcium (R 2 =0.29, N=18, P<0.05).Furthermore, a strong correlation was found between net calcium and phosphorus accumulation (R 2 =0.92, N=16, P<0.01) and between body mass and whole-body calcium (R 2 =0.84, N=25, P<0.01) and phosphorus (R 2 =0.88, N=24, P<0.01) content. Phosphorus accumulation is strongly calcium dependent, as phosphorus accumulation decreases in parallel to calcium accumulation when the diet is calcium deficient but phosphorus sufficient. We conclude that PTHrP and cortisol are involved in the regulation of the hydromineral balance of these fish, with growthrelated calcium accumulation as an important target.

SUMMARY Juvenile gilthead sea bream were exposed to diluted seawater (2.5‰salinity; DSW) for 3 h or, in a second experiment, acclimated to DSW and fed a control or calcium-deficient diet for 30 days. Branchial Ca2+influx, drinking rate and plasma calcium levels were assessed. Sea bream plasma parathyroid hormone related protein (sPTHrP) was measured, and mRNAs of pthrp, its main receptor, pth1r, and the calcium-sensing receptor (casr) were quantified in osmoregulatory tissues and the pituitary gland. When calcium is limited in water or diet, sea bream maintain calcium balance; however, both plasma Ca2+ and plasma sPTHrP concentrations were lower when calcium was restricted in both water and diet. Positive correlations between plasma sPTHrP and plasma Ca2+(R2=0.30, N=39, P&lt;0.05), and plasma sPTHrP and body mass of the fish (R2=0.37, N=148, P&lt;0.001) were found. Immunoreactive sPTHrP was demonstrated in pituitary gland pars intermedia cells that border the pars nervosa and co-localises with somatolactin. In the pituitary gland, pthrp, pth1r and casr mRNAs were downregulated after both short-and long-term exposure to DSW. A correlation between pituitary gland pthrp mRNA expression and plasma Ca2+(R2=0.71, N=7, P&lt;0.01) was observed. In gill tissue, pthrp and pth1r mRNAs were significantly upregulated after 30 days exposure to DSW, whereas no effect was found for casr mRNA expression. We conclude that in water of low salinity,declining pituitary gland pthrp mRNA expression accompanied by constant plasma sPTHrP levels points to a reduced sPTHrP turnover and that sPTHrP, through paracrine interaction, is involved in the regulation of branchial calcium handling, independently of endocrine pituitary gland sPTHrP.

In fish, vitellogenin is an important nutritional precursor protein produced solely in the liver and released into the blood where it binds calcium. In the gilthead sea bream

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