The European oyster showed constant increases in shell length and dry mass
during the cultivation periods (Pogoda et al. 2011). This implies a high ability of
dietary assimilation of the native European oyster, even when food availability is
low in summer (Rick et al. 2006) and a good adaptation to this offshore environment (Newkirk et al. 1995; Matthiessen 2001; Laing et al. 2006). In contrast,
seasonal variations with reduced growth rates in summer were observed for the
Pacific oyster. Interestingly, this species-specific difference is also clearly reflected
in parallel changes of the biochemical compositions.
The biochemical and elemental compositions were analysed to characterize the
nutritional condition and for a better understanding of related energetic processes.
Accumulation and depletion of metabolic energy reserves depend primarily on food
quantity and quality, environmental effects on metabolic processes, and reproductive activities (Beninger and Lucas 1984; Whyte et al. 1990; Ruíz et al. 1992).
Therefore, investigations focused on seasonal dynamics of the major energy storage
products, namely carbohydrates, proteins and lipids, on the compositions of lipid
classes and fatty acids as well as on carbon and nitrogen.
Both species utilized primarily glycogen as energy store during times of high
food availability. After the phytoplankton spring bloom, when reduced growth rates
were observed for C. gigas, glycogen contents were drastically depleted, whereas
lipid contents increased. In contrast, O. edulis kept on growing and accumulating
glycogen, while lipid contents remained relatively constant. These different
strategies may be explained by the earlier maturity of Pacific oysters and the
resulting conversion of carbohydrates to lipids to enhance the production of eggs,
which are rich in lipid (Gallager and Mann 1986; Whyte et al. 1990; De la Parra
et al. 2005).
In shellfish production, the condition index is commonly used to evaluate the
effects of the surrounding environment on these organisms. It is an adequate
parameter to describe the commercial quality, physiological state and health of
bivalve molluscs. The most commonly applied condition index (CI) is the ratio of
flesh mass to shell mass (e.g. Walne and Mann 1975; Davenport and Chen 1987).
Condition indices for all three offshore-cultivated bivalve species support the
positive results already observed for the growth performance at offshore sites. CI
values for Blue mussels, European and Pacific oysters indicated excellent conditions (Pogoda et al. 2011).
Seasonal variations in lipid class compositions of offshore-cultivated oysters
were essentially similar to those of nearshore-grown individuals (Pogoda et al.
2013). Triacylglycerols (TAG) are the main lipid stores in the investigated oysters
and serve as short-term energy reserves. Together with glycogen, they accumulate
during periods of high food availability and are depleted in periods of food paucity.
Accordingly, the amount of TAGs is a sensitive indicator of the nutritional condition of an animal (Fraser et al. 1985). Expressed as the ratio of phospholipids to
triacylglycerols (PL:TAG), values 1 indicate a good nutritional state (Abad et al.
1995; Caers et al. 2000) and European and Pacific oysters clearly improved condition during offshore-cultivation. PL and TAG levels reached equal proportions of
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during the cultivation periods (Pogoda et al. 2011). This implies a high ability of
dietary assimilation of the native European oyster, even when food availability is
low in summer (Rick et al. 2006) and a good adaptation to this offshore environment (Newkirk et al. 1995; Matthiessen 2001; Laing et al. 2006). In contrast,
seasonal variations with reduced growth rates in summer were observed for the
Pacific oyster. Interestingly, this species-specific difference is also clearly reflected
in parallel changes of the biochemical compositions.
The biochemical and elemental compositions were analysed to characterize the
nutritional condition and for a better understanding of related energetic processes.
Accumulation and depletion of metabolic energy reserves depend primarily on food
quantity and quality, environmental effects on metabolic processes, and reproductive activities (Beninger and Lucas 1984; Whyte et al. 1990; Ruíz et al. 1992).
Therefore, investigations focused on seasonal dynamics of the major energy storage
products, namely carbohydrates, proteins and lipids, on the compositions of lipid
classes and fatty acids as well as on carbon and nitrogen.
Both species utilized primarily glycogen as energy store during times of high
food availability. After the phytoplankton spring bloom, when reduced growth rates
were observed for C. gigas, glycogen contents were drastically depleted, whereas
lipid contents increased. In contrast, O. edulis kept on growing and accumulating
glycogen, while lipid contents remained relatively constant. These different
strategies may be explained by the earlier maturity of Pacific oysters and the
resulting conversion of carbohydrates to lipids to enhance the production of eggs,
which are rich in lipid (Gallager and Mann 1986; Whyte et al. 1990; De la Parra
et al. 2005).
In shellfish production, the condition index is commonly used to evaluate the
effects of the surrounding environment on these organisms. It is an adequate
parameter to describe the commercial quality, physiological state and health of
bivalve molluscs. The most commonly applied condition index (CI) is the ratio of
flesh mass to shell mass (e.g. Walne and Mann 1975; Davenport and Chen 1987).
Condition indices for all three offshore-cultivated bivalve species support the
positive results already observed for the growth performance at offshore sites. CI
values for Blue mussels, European and Pacific oysters indicated excellent conditions (Pogoda et al. 2011).
Seasonal variations in lipid class compositions of offshore-cultivated oysters
were essentially similar to those of nearshore-grown individuals (Pogoda et al.
2013). Triacylglycerols (TAG) are the main lipid stores in the investigated oysters
and serve as short-term energy reserves. Together with glycogen, they accumulate
during periods of high food availability and are depleted in periods of food paucity.
Accordingly, the amount of TAGs is a sensitive indicator of the nutritional condition of an animal (Fraser et al. 1985). Expressed as the ratio of phospholipids to
triacylglycerols (PL:TAG), values 1 indicate a good nutritional state (Abad et al.
1995; Caers et al. 2000) and European and Pacific oysters clearly improved condition during offshore-cultivation. PL and TAG levels reached equal proportions of
276
B.H. Buck et al.