COMPARATIVE TOLERANCE OF SMALL JUVENILE PALOURDES (TAPES-DECUSSATUS L) AND MANILA CLAMS (TAPES-PHILIPPINARUM ADAMS AND REEVE) TO LOW-TEMPERATURE

Small (3.24-31.70 mg dry weight) juvenile clams of a native British sp ecies (palourde, Tapes decussatus L., 3 size groups) and an introduced species (Manila clam, Tapes philippinarum Adams & Reeve, 4 size group s) were maintained either unfed or with low algae rations at 3 degrees C, 6 degrees C and 9 degrees C for up to 11 wk. The various size grou ps responded in a similar way within each experimental treatment. At 9 degrees C, all the juveniles survived for 11 wk. Unfed clams utilised biochemical reserves and lost weight and condition during this period . Increases in weight, reserves and condition in fed juveniles were gr eater in Manila clams than in palourdes. At 6 degrees C, all Manila cl am juveniles survived and fed animals showed a small increase in growt h and condition. All palourdes and unfed Manila clams lost weight and condition and the mortality of palourdes was 24-46% after 11 wk. At 3 degrees C, the majority (>88%) of juveniles of both species survived s hort (3 wk) periods without food. All juveniles utilised reserves and lost weight and condition, with a greater decline in palourdes. High m ortalities (>50%) of all palourdes and unfed Manila clams occurred aft er 3 wk. At all temperatures, a greater proportion of carbohydrate tha n lipid was used as an energy reserve. Total lipid content and the pro portion of polyunsaturated fatty acids (PUFAs) in the neutral lipids i ncreased in treatments in which juveniles grew and decreased where a l oss of organic weight was observed. In both clam species the proportio n of (n-6) fatty acids, principally 20:4 (n-6), in the phospholipids a ppeared to indicate a response to stress; juveniles in treatments in w hich total organic weight decreased contained proportionally more than juveniles which grew. Food cell consumption rates were lower at lower temperatures and at each temperature were higher in Manila clams than in palourdes. Respiration rates increased with temperature only in fe d Manila clams. In unfed palourdes respiration rates were higher at 3 degrees C than at 6 degrees C and 9 degrees C. Results are discussed w ith respect to recruitment and distribution of these clams in relation to winter survival of small juveniles.