Comparative low temperature tolerance of small juvenile European, Ostrea edulis L., and Pacific oysters, Crassostrea gigas Thunberg

The present investigation was carried out to measure the survival of juvenile oysters (spat) of a native species, European oyster, Ostrea edulis L., and an introduced species, Pacific oyster, Crassostrea gigas Thunberg, at typical winter seawater temperatures in the British Isles. Two size groups of each species were maintained either unfed or with low algae rations at 3, 6 and 9^oC for up to 11 weeks. The majority of O. edulis juveniles survived for 11 weeks at all temperatures. Unfed animals utilized biochemical reserves and lost organic weight during this period. Fed O. edulis showed a small weight loss at 3^oC, but increased in weight at 6^oC and 9^oC. At all temperatures, O. edulis preferentially utilized lipid reserves in treatments in which organic weight was lost. In contrast, C. gigas juveniles preferentially utilized protein as an energy source. All C. gigas juveniles survived at 9 and 6^oC, but high mortalities (> 95%) occurred after 3-7 weeks at 3^oC. Both fed and unfed C. gigas juveniles lost weight, reserves and condition at 3^oC. Juveniles grew when fed at 6 and 9^oC. In both oyster species, the proportion of phospholipid (n-6) fatty acids, principally 20 : 4 (n-6) increased in those juveniles which showed a decrease in organic weight. Food cell consumption rates were lower at lower temperatures and were higher in O. edulis at 9^oC than in C. gigas. The results are discussed with respect to recruitment and winter survival of small juveniles.