1. Introduction
To produce seed of consistently good quality and at a competitive price is an important priority for the molluscan aquaculture industry. Spat rearing is a critical step.
Diet quality often results in variability in growth and quality with unacceptable overcosts for nursery and oyster growers.
Sufficient quantities of microalgae are required for feeding, which are costly for the hatchery–nursery. A number of papers on invertebrate nutrition have described the
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quantitative algal requirements Utting and Millican, 1995 . However, after supplying the recommended quantity, large variabilities are still observed for the subsequent
success of spat development. This led to the conclusion that the quality of algae is also Ž
. important. Polyunsaturated fatty acids PUFA , 20:5n y 3 and 22:6 n y 3, have been
Ž demonstrated to be essential for bivalves Langdon and Waldock, 1981; Helm et al.,
. 1991; Frolov and Pankov, 1992; Marty et al., 1992; Leal, 1994 .
It is quite difficult to provide with security a good phytoplanktonic diet to spat. Emulsions have been demonstrated to be a successful way to deliver essential fatty acids
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Lane, 1989; Robinson, 1992; Coutteau et al., 1996; Caers et al., 1998, 1999 . However, the absorption rates of micronutrients incorporated in emulsions as well as the relative
ingestion of the emulsion versus algae are still to be assessed. Likewise, the ability to synthesize or bioconvert sterols de novo is generally low or
absent, and varies among bivalve species. This implies that a dietary supply of sterol is Ž
necessary for bivalve growth Teshima, 1983; Teshima and Kanazawa, 1974; Teshima and Patterson, 1981; Teshima et al., 1979; Voogt, 1975; Trider and Castell, 1980;
. Gordon and Collins, 1982; Holden and Patterson, 1991; Napolitano et al., 1993 .
The sterol supply by microalgae is highly variable according to the species used ŽBerenberg and Patterson, 1981; Lin et al., 1982; Gladu et al., 1991; Tsitsa-Tzardis et
. al., 1993 . Sterol composition and associated quantity can also vary with algal culture
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conditions Gladu et al., 1991 . Consequently, the qualitative and quantitative variability of the sterol composition in microalgae used in nurseries will have consequences on spat
Ž phytosterol composition, and will modify growth and survival performance Wikfors et
. al., 1991 . Lipid emulsions appear to be a potential way to deliver essential sterols as has
been demonstrated. The aim of this paper was to estimate ingestion and absorption rates using sterols as
markers. To do this, we fed spat with algae, but lacking two sterols, i.e., stigmasterol and cholesterol, that were incorporated as markers in emulsions. The ingestion–di-
gestion of the emulsion was followed by measuring the incorporation of the two sterol markers in oyster spats.
2. Materials and methods
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