Function and metabolism
because it is relatively easy to produce in large quantities in this area
from the underground sea water. It has been found that the composition of this sea water is particularly suited to the culturing of this algae
(Baud et al., 1990; Baud & Bacher, 1990). A previous study showed that
the supply of Skeletonema costatum to oysters in ponds improved the
condition index because of the increase in the flesh weight of oysters
due to enhanced glycogen levels (Baud et al., 1995). However, no data
was obtained regarding changes in the lipid and fatty acid compositions
of oysters. According to the previous studies on adult and juvenile
oysters, significant changes in the fatty acid composition of oysters can
be expected by feeding oysters with Skeletonerna costatum. This diatom
typically possesses a fatty acid composition characterized by a high
content in 20:5n-3 and 16 carbon polyunsaturated fatty acids (Ackman
et al., 1964; Chuecas & Riley, 1969; Volkman et al., 1989; Dunstan et
al., 1994; Bergé et al., 1995). It was of interest to investigate these
changes because the aroma of oysters is strongly related to their fatry
acid composition. Indeed, many aroma compounds in seafood arise
from oxidation reactions involving fatty acids (Josephson, 1991).
This study examines the chemical changes (focusing on the lipid and
fatty acid compositions) of oysters fed with Skeletonema costatum during
six weeks in ponds. The fatty acid composition of total lipid extract (0,
1, 2, 3, 4 and 6 weeks) was determined to estimate the retention of
fatty acids.
Materials and methods
Skeletonema costatum culture
Pure cultures of Skeletonema costatum diatoms were grown in batches in
outdoors concrete tanks (80m 3 ) under continuous aeration as described
by Sauriau et al. (1997). The tanks were filled with underground salt
water pumped from the subsoil reserve in the Bourgneuf Bay. Skeletonema costatum diatoms were harvested when the population was in the
exponential growth phase (3-4 days after the start of the culture). The
cells were harvested by continuous centrifugation with a Robatel Slab
540 centrifuge using an inlet flow of 3.5 m 3 . h"
1 and a rotation rate of
2 200 tr. min - 1 .
Oysters
Oysters (Crassostrea gigas) were purchased from a commercial hatchery in
Bourgneuf Bay. The weight of “shell + flesh” and flesh and the condition index of oysters were measured from 30 oysters and found to be
63.5 ± 10.6g, 9 9 ± 0.4g and 50.5 ± 10.6 g, respectively. The condition
index (CI) was calculated using the formula CI = (w d x 1 000)/ (w t -w s ),
where w d = flesh dry weight, w t = oyster roral weight and w
s
= shell dry
weight (Lawrence & Scott, 1982). Then, the oysters were divided into
two groups. Each group was placed into off-bottom trays in the concrete
raceway (3 m x 0.6 m x 0.5 m deep). The raceways were filled of sea
113
because it is relatively easy to produce in large quantities in this area
from the underground sea water. It has been found that the composition of this sea water is particularly suited to the culturing of this algae
(Baud et al., 1990; Baud & Bacher, 1990). A previous study showed that
the supply of Skeletonema costatum to oysters in ponds improved the
condition index because of the increase in the flesh weight of oysters
due to enhanced glycogen levels (Baud et al., 1995). However, no data
was obtained regarding changes in the lipid and fatty acid compositions
of oysters. According to the previous studies on adult and juvenile
oysters, significant changes in the fatty acid composition of oysters can
be expected by feeding oysters with Skeletonerna costatum. This diatom
typically possesses a fatty acid composition characterized by a high
content in 20:5n-3 and 16 carbon polyunsaturated fatty acids (Ackman
et al., 1964; Chuecas & Riley, 1969; Volkman et al., 1989; Dunstan et
al., 1994; Bergé et al., 1995). It was of interest to investigate these
changes because the aroma of oysters is strongly related to their fatry
acid composition. Indeed, many aroma compounds in seafood arise
from oxidation reactions involving fatty acids (Josephson, 1991).
This study examines the chemical changes (focusing on the lipid and
fatty acid compositions) of oysters fed with Skeletonema costatum during
six weeks in ponds. The fatty acid composition of total lipid extract (0,
1, 2, 3, 4 and 6 weeks) was determined to estimate the retention of
fatty acids.
Materials and methods
Skeletonema costatum culture
Pure cultures of Skeletonema costatum diatoms were grown in batches in
outdoors concrete tanks (80m 3 ) under continuous aeration as described
by Sauriau et al. (1997). The tanks were filled with underground salt
water pumped from the subsoil reserve in the Bourgneuf Bay. Skeletonema costatum diatoms were harvested when the population was in the
exponential growth phase (3-4 days after the start of the culture). The
cells were harvested by continuous centrifugation with a Robatel Slab
540 centrifuge using an inlet flow of 3.5 m 3 . h"
1 and a rotation rate of
2 200 tr. min - 1 .
Oysters
Oysters (Crassostrea gigas) were purchased from a commercial hatchery in
Bourgneuf Bay. The weight of “shell + flesh” and flesh and the condition index of oysters were measured from 30 oysters and found to be
63.5 ± 10.6g, 9 9 ± 0.4g and 50.5 ± 10.6 g, respectively. The condition
index (CI) was calculated using the formula CI = (w d x 1 000)/ (w t -w s ),
where w d = flesh dry weight, w t = oyster roral weight and w
s
= shell dry
weight (Lawrence & Scott, 1982). Then, the oysters were divided into
two groups. Each group was placed into off-bottom trays in the concrete
raceway (3 m x 0.6 m x 0.5 m deep). The raceways were filled of sea
113