relationship or that other components are important in determining the algal nutritional value. q
2000 Elsevier Science B.V. All rights reserved.
Keywords: Chaetoceros muelleri; Fatty acids; Light; Larvae; Mytilus edulis; Nutrients; PaÕloÕa lutheri; Rhinomonas reticulata; Skeletonema costatum
1. Introduction
Many marine animals rely on phytoplankton as a food source especially in their early stages of development. For a number of bivalve molluscs which are of commercial
interest, live phytoplankton cultures are routinely used as food for the larval and adult stages. From the plethora of phytoplankton species found in nature, only a small number
Ž .
have been used as food sources De Pauw et al., 1984 . In early research, the nutritional value of phytoplankton cultures in terms of biochemical composition was not considered
Ž .
Walne, 1963 . Instead, palatability and digestibility were suggested as being more significant. However, later studies, benefitting from advances in analytical methods,
emphasized the indisputable role of the algal biochemical composition in determining its Ž
. nutritional value Wikfors et al., 1984; Enright et al., 1986b . Factors influencing algal
Ž .
biochemical composition have been described by Leonardos 1998 . There is only general agreement on which biochemical constituents of phytoplankton
cells are most significant in determining their food value for planktonic herbivores. Numerous reports have examined biochemical classes of components, primarily protein,
carbohydrate and lipid and have reached different conclusions about the significance of Ž
gross biochemical composition Epifanio, 1979; Webb and Chu, 1983; Enright et al., .
1986a,b; Wikfors et al., 1992; Thompson et al., 1994 . However, since the observation Ž
. that certain long-chain polyunsaturated fatty acids PUFA are essential to many marine
organisms, considerable research has focused on the availability and the role of PUFA in Ž
. Ž
hatcheries Watanabe et al., 1983 and in the natural environment Sargent and Whittle, .
1981 . But there is remarkably little experimental evidence that diets containing more PUFA are superior for herbivores and there is now growing evidence that the impor-
Ž tance of dietary PUFA may have been overgeneralized Dickey-Collas and Geffen,
. 1992; Thompson et al., 1993, 1994 . This confusion may reflect true differences in the
nutritional requirements of various bivalve species. However, even minor changes in the culturing conditions employed in different laboratories may lead to different morpholog-
Ž ical, physiological andror biochemical properties of the algae in question Gallager and
. Mann, 1981; Thompson et al., 1993 . In most hatcheries and laboratory feeding trials
concerning the nutritional value of algae which essentially use the larvae as bioassay organisms, the algal biochemical composition is not constant over the whole time range
of the feeding trials. Thus, direct comparisons of results from different laboratories may
Ž be misleading. Although this conflict is commonly recognized by phycologists Baars,
. 1981 , its potential importance to mariculture, in general, has received comparatively
little attention. The present report tries to overcome some of these problems by employing continu-
ous algal culturing techniques which, in theory, should produce algal cells of constant
biochemical composition, thus rendering the experiments internally comparable; how- ever, in practice, some variation in the algal biochemical composition is to be expected.
Using monospecific diets, the growth performance of the larvae can be correlated with algal biochemical properties, without introducing differences in digestibility, cell size
and palatability which may have compromised previous studies using multispecies diets. Special emphasis was placed on algal fatty acids due to ever increasing evidence
suggesting the important role of these compounds as diet components of particular
Ž .
nutritional value Thompson et al., 1996 .
2. Materials and methods
