Function and metabolism
Discussion
Supplementation improves condition index
Supplying Skeletonema costatum to oysters during six weeks in ponds
largely improved the condition index. This was largely explained by
an increase in glycogen content. This result confirms those previously
obtained in similar conditions (Baud et al., 1995) and are consistent with
those showing that biochemical composition of oysters was influenced
by seasonal variation of food availability (Deslou-Paoli & Héral, 1988;
Ruiz et al., 1992).
Glycogen accumulation is frequently observed in bivalves fed diets
rich in carbohydrates (Castell & Trider, 1974; Trider & Castell, 1980)
which are the main source of energy for bivalves (Gabbot, 1975; Ruiz
et al., 1992). In the present study, glycogen accumulation in oysters is
probably related to the high carbohydrate content of Skeletonema costatum
(Parsons et al., 1961). Note that the storage of glycogen by bivalve flesh
is more efficient when diets supply a large amount of PUFAs (Trider &
Castell, 1980). So glycogen accumulation in oysters could be improve
by the high PUFA proportion in Skeletonema costatum.
Lipid content of oysters fed microalgae remained unchanged. This
result is in good agreement with those obtained in clams fed Skeletonema costatum (Baud et al., 1990). It is consistent with the fact that
lipid content increases in oyster flesh only during gametogenesis
(Gabbot, 1975; Ruiz et al., 1992; Pazos et al., 1996).
Supplementation with Skeletonema costatum changes fatty acid
composition of oyster lipids
A large part of the changes m fatty acid composition of oysters lipids
can be explained by a direct accumulation of algae fatty acids into total
lipids of oysters. This results is in good agreement with those obtained
in juvenile oysters (Waldock & Nascimento, 1979; Langdon & Waldock, 1981). Thus supplemented oysters showed higher proportion
of 20:5 (n-3), 16:1 (n-7) and 14:0 and lower proportion of 20:4 (n-6),
18:2 (n-6), 22:6 (n-3), 16:0 and 18:0 than controls because Skeletonema
costatum had high proportions of 20:5 (n-3), 16:1 (n-7) and 14:0 and a
low proportion of 20:4 (n-6), 18:2 (n-6), 22:6 (n-3), 16:0 and 18:0.
The decrease in 22:6 (n-3) in oyster lipids strongly suggested that oysters
have low ability for elongation and desaturation of dietary PUFAs.
Although Skeletonma costatum provided large amounts of 20:5 (n-3), this
fatty acid was not elongated and desaturated into 22:6 (n-3). This
result is consistent with those of Waldock & Holland (1984) who
demonstrated that juvenile oysters were able to elongate and further
desaturate dietary long-chain fatty acids but not in sufficient amount
to sustain optimum growth.
In contrast, oysters seem to be able to elongate 16 carbon monounsaturated fatty acids into the corresponding 18 carbon fatty acids. This
hypothesis is strongly supported by the increase in the proportion of
119
Discussion
Supplementation improves condition index
Supplying Skeletonema costatum to oysters during six weeks in ponds
largely improved the condition index. This was largely explained by
an increase in glycogen content. This result confirms those previously
obtained in similar conditions (Baud et al., 1995) and are consistent with
those showing that biochemical composition of oysters was influenced
by seasonal variation of food availability (Deslou-Paoli & Héral, 1988;
Ruiz et al., 1992).
Glycogen accumulation is frequently observed in bivalves fed diets
rich in carbohydrates (Castell & Trider, 1974; Trider & Castell, 1980)
which are the main source of energy for bivalves (Gabbot, 1975; Ruiz
et al., 1992). In the present study, glycogen accumulation in oysters is
probably related to the high carbohydrate content of Skeletonema costatum
(Parsons et al., 1961). Note that the storage of glycogen by bivalve flesh
is more efficient when diets supply a large amount of PUFAs (Trider &
Castell, 1980). So glycogen accumulation in oysters could be improve
by the high PUFA proportion in Skeletonema costatum.
Lipid content of oysters fed microalgae remained unchanged. This
result is in good agreement with those obtained in clams fed Skeletonema costatum (Baud et al., 1990). It is consistent with the fact that
lipid content increases in oyster flesh only during gametogenesis
(Gabbot, 1975; Ruiz et al., 1992; Pazos et al., 1996).
Supplementation with Skeletonema costatum changes fatty acid
composition of oyster lipids
A large part of the changes m fatty acid composition of oysters lipids
can be explained by a direct accumulation of algae fatty acids into total
lipids of oysters. This results is in good agreement with those obtained
in juvenile oysters (Waldock & Nascimento, 1979; Langdon & Waldock, 1981). Thus supplemented oysters showed higher proportion
of 20:5 (n-3), 16:1 (n-7) and 14:0 and lower proportion of 20:4 (n-6),
18:2 (n-6), 22:6 (n-3), 16:0 and 18:0 than controls because Skeletonema
costatum had high proportions of 20:5 (n-3), 16:1 (n-7) and 14:0 and a
low proportion of 20:4 (n-6), 18:2 (n-6), 22:6 (n-3), 16:0 and 18:0.
The decrease in 22:6 (n-3) in oyster lipids strongly suggested that oysters
have low ability for elongation and desaturation of dietary PUFAs.
Although Skeletonma costatum provided large amounts of 20:5 (n-3), this
fatty acid was not elongated and desaturated into 22:6 (n-3). This
result is consistent with those of Waldock & Holland (1984) who
demonstrated that juvenile oysters were able to elongate and further
desaturate dietary long-chain fatty acids but not in sufficient amount
to sustain optimum growth.
In contrast, oysters seem to be able to elongate 16 carbon monounsaturated fatty acids into the corresponding 18 carbon fatty acids. This
hypothesis is strongly supported by the increase in the proportion of
119