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 j uvenile oysters (spat) of a native species, European oyster, Ostrea ed ulis 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 degrees C for up to 11 weeks. The majo rity of O. edulis juveniles survived for 11 weeks at all temperatures. Unfed animals utilized biochemical reserves and lost organic weight d uring this period, Fed O. edulis showed a small weight loss at 3 degre es C, but increased in weight at 6 degrees C and 9 degrees C. At all t emperatures, O. edulis preferentially utilized lipid reserves in treat ments in which organic weight was lost. In contrast, C. gigas juvenile s preferentially utilized protein as an energy source, All C. gigas ju veniles survived at 9 and 6 degrees C, but high mortalities (> 95%) oc curred after 3-7 weeks at 3 degrees C, Both fed and unfed C. gigas juv eniles lost weight, reserves and condition at 3 degrees C. Juveniles g rew when fed at 6 and 9 degrees C. In both oyster species, the proport ion of phospholipid (n-6) fatty acids, principally 20:4 (n-6) increase d in those juveniles which showed a decrease in organic weight. Food c ell consumption rates were lower at lower temperatures and were higher in O. edulis at 9 degrees C than in C. gigas. The results are discuss ed with respect to recruitment and winter survival of small juveniles.