Juvenile gilthead sea bream (Sparus auratus

In teleost fish, as in other vertebrates, calcium is of key importance for numerous physiological processes. The skeleton of vertebrates consists mainly of calcium phosphate and calcium carbonate. It serves an important role as it determines body shape, protective aspects (scales, bone plates) and as a buffer internal reservoir for calcium and phosphorus. In teleosts, ~99% of the whole-body calcium fraction is incorporated into bones and scales (Flik et al., 1986). Indeed, calcium is also of major importance for many other physiological processes, such as vision, muscle contraction, vitellogenesis, signal transduction, blood coagulation and membrane permeability (Riccardi, 1999). In fish blood, calcium is either complexed (e.g. to citrate), protein bound or present as free ion. The free calcium fraction accounts for about half of the total calcium fraction and is the physiologically important fraction (Hanssen et al., 1991). Fish regulate their ionic plasma calcium level more strictly than their protein-bound calcium level, and this may relate to the fact that even minor disruptions in ionic calcium concentrations lead to severe stress and disturbance of calcium balance (Flik et al., 1995). Unlike terrestrial vertebrates, which depend solely on the diet as their calcium source, fish live in an environment with a readily available source of calcium. Seawater has a calcium concentration of ~10·mmol·l–1, whereas the total plasma calcium concentration of marine fish ranges from 2 to 3·mmol·l–1; thus, marine fish live in a hypercalcic environment and face an inward gradient of Ca2+. As calcium availability in the environment varies, fish have developed calcium regulatory systems that can react rapidly to changes in environmental calcium concentrations (Wendelaar Bonga and Pang, 1991; Bjornsson et al., 1999). Endocrine control of calcium metabolism in fish is regulated by both hyperand hypocalcemic hormones. Stanniocalcin (Lafeber et al., 1988; Wagner et al., 1998) acts as the major hypocalcemic (in fact anti-hypercalcemic, as it inhibits Ca2+ influx) hormone. Increased calcium levels in the medium induce hypercalcemic conditions and, by doing so, promote stanniocalcin release into the bloodstream, where it reduces the calcium influx in the gills and intestine. Prolactin (Kaneko and Hirano, 1993; Mancera et al., 1993; Flik et al., 1994) and PTHrP (parathyroid hormone related protein; Guerreiro et al., 2001) act as major hypercalcemic hormones. PTHrP is phylogenetically the predecessor of PTH, which appeared only after the water/land transition of vertebrates. Although recent reports indicate that fish express PTH (Danks The Journal of Experimental Biology 207, 4077-4084 Published by The Company of Biologists 2004 doi:10.1242/jeb.01254