Comparison of the metabolism of Acartia clausi and A. tonsa: influence of temperature and salinity.

In the Marseilles region (French Mediterranean coast), A. clausi is one of the most abundant copepod species of the Gulf of Fos while A. tonsa constitutes the almost exclusive copepod species of the Berre lagoon, a neighbouring semi-closed brackish area communicating with the gulf. As different ecophysiological capabilities to stand the various temperature, salinity and food conditions could explain why these two species do not coexist in the same environment, comparative experiments were performed on metabolism and feeding. The respiration and ammonia excretion of the two species were measured in different combinations of temperature (10, 15 and 20 degrees C) and salinity (15, 25 and 35 per thousand). For each temperature, at the salinity of 35 per thousand, respiration rates were less in A. clausi than in A. tonsa, the contrary being observed at the lowest salinity. At any temperature ammonia excretion was greater at the intermediate salinity in A. tonsa and least in A. clausi. In Acartia tonsa, Q(10) of respiration and excretion were minimum at the lowest salinity, while in A. clausi they were unaffected by salinity variation. The O:N atomic ratio (from respiration and ammonia excretion rates) was significantly more elevated in A. clausi (mean 21.2; range 13.6-28.7) than in A. tonsa (mean 11.3; range 4.2-25) suggesting a more proteinic oriented metabolism in the later. Feeding experiments where Dunaliella tertiolecta30 per thousand) or lagoon (<16 per thousand) salinity. The relationships between ingestion and food concentration in the two species were not significantly different. These different results are compared to other ecophysiological information concerning these Acartia species (survival tolerances, osmotic regulation, feeding behaviour) and are discussed in relation with the characteristics of their niches in the studied region.