262 E
.J. Cook et al. J. Exp. Mar. Biol. Ecol. 255 2000 261 –274
of certain fatty acids in the gonads of P . miliaris could be used to give an indication of the
predominant diet type of this species in the wild.
2000 Elsevier Science B.V. All rights reserved.
Keywords : Fatty acid composition; Nutritional implications; Psammechinus miliaris; Sea urchin
1. Introduction
Sea urchins are often thought of as herbivores Lawrence, 1975. Psammechinus miliaris
, however, has been reported to feed upon a wide range of organisms Lawrence and Lane, 1982 including hydroids, worms, echinoderms, diatoms Eichelbaum, 1909,
sponges, bryozoans, crustaceans Mortensen, 1943 and mussels Lawrence, 1975. The diet of sea urchins has been found to vary with locality and to be dependent on food
availability Vadas, 1977; Emson and Moore, 1998. Diet quality has also been found to influence somatic growth and gonadal growth and development in P
. miliaris Cook et al., 1998. It has been observed that P
. miliaris held in nets within cages stocked with Atlantic salmon Salmo salar contained significantly larger gonads than sea urchins held
in cages without salmon Kelly et al., 1998a. Similarly, natural settlements of P . miliaris
on commercial scallop lines in Loch Fyne have shown rapid somatic and gonadal growth pers. obs.. It appears that the sea urchins held in salmon cages were feeding upon
uneaten salmon food pellets Kelly et al., 1998a and that the sea urchins in Loch Fyne were feeding upon the encrusting organisms covering the scallop lines pers. obs.. The
precise diet, however, of the sea urchins in these two polyculture systems is unknown.
Animals are known to receive a considerable amount of lipid via their diet Sargent and Whittle, 1981 and the diet type has been found to alter the fatty acid composition of
certain herbivorous copepods, for example, Calanus spp. Graeve et al., 1994. It has been suggested, therefore, that certain fatty acids, or their ratios, can be used to provide
a more precise indication of an organism’s diet than, for example, analysis of the gut contents McGee, 1996. Arachidonic acid, 20:4n 2 6 and eicosapentaenoic acid EPA,
20:5 n 2 3 have been found in large quantities in macroalgal species, such as Porphyridium, Ascophyllum and Laminaria Paradis and Ackman, 1977. Stearidonic
acid, 18:4 n 2 3 and linolenic acid, 18:3 n 2 3 have also been highlighted as potential indicators of microalgae in the diet Mai et al., 1996. It has been suggested, therefore,
that herbivorous organisms would have a significant amount of these fatty acids in their diet Sargent et al., 1983. Docosahexaenoic acid DHA 22:6 n 2 3, however, has been
associated with carnivorous feeding Tagaki et al., 1986; Kharlamenko et al., 1995, although this fatty acid is also present in certain classes of flagellated microalgae
Sargent et al., 1995; Vashappilly and Chen, 1998; Mansour et al., 1999. The proportion of odd chained and branched fatty acids and the ratio of 18:1 n 2 9 18:1 n 2 7 fatty
acids has also been used to indicate the importance of bacterial input into the diet Parkes, 1987; Sargent et al., 1987; Kharlamenko et al., 1995; Pond et al., 1997. High
contents of branched and odd chain acids have been identified in the gonads of the mud-feeding sea urchin, Strongylocentrotus franciscanus Hayashi and Takagi, 1977.
E .J. Cook et al. J. Exp. Mar. Biol. Ecol. 255 2000 261 –274
263
The gonads of Psammechinus miliaris have been found to contain between 14 and 21 dry matter total lipid, with the highest values typically found in the period prior to
spawning Comely, 1979. Walker et al. 1998 found that the gonads of S . droebach-
iensis are used as a site for lipid storage during the remainder of the year. Variations in fatty acid composition, particularly in the proportions of 20:4 n 2 6, 20:5 n 2 3 and 22:6
n 2 3 in the gonads of 12 different species of sea urchin collected from various locations in Japan have been attributed to differences in their respective habitats, particularly the
type of food available Tagaki et al., 1986. This assumption was based, however, on observational studies of feeding habits in situ and unfortunately there have been no
studies involving the experimental manipulation of sea urchin diet to support this claim.
The aim of this study, therefore, was to determine the fatty acid profiles of the gonads of P
. miliaris fed either i a commercially manufactured salmon feed, or ii the macroalga, Laminaria saccharina for a period of 18 months and to compare the results
with fatty acid profiles of sea urchins collected from four sites on the west coast of Scotland. The fatty acid profiles of their known i.e., salmon feed and the macroalga, L
. saccharina and potential food sources i.e., mussels were also obtained, to increase our
understanding of the feeding habits of P . miliaris.
2. Methods
