Near anoxia and sulphide as possible factors influencing the spatial distribution of Acartia tonsa and Acartia clausi: comparative evaluation of egg tolerance

In many coastal and estuarine areas the planktonic copepods Acartia tonsa and Acartia clausi show a spatial separation with A. tonsa restricted to brackish waters and confined environments and A. clausi inhabiting areas more influenced by sea water. The hatching and viability of A. tonsa and A. clausi eggs exposed to anoxia and anoxia/sulfide (conditions that are frequent in bottom waters of the most confined areas) was evaluated to determine if these stress factors play a role in the distribution of these species. Subitaneous eggs, spawned by laboratory reared organisms, were incubated in near anoxia (<7.59×10−3 mmol O2 L−1) or anoxia/ sulfide (∼1 mmol L−1) for different periods (1, 4, 8 and 15 days), then transferred to normoxic conditions. The exposure of the eggs to near anoxia and sulfide appears to induce the same response (quiescence) in both species. Exposure times ≤8 days to near anoxia or anoxia/sulfide did not affect egg viability, while 15 day exposure caused significant declines in hatching success of both species. A significant difference between the effects of near anoxia and anoxia/sulfide was observed when incubation lasted 15 days; hatching of eggs exposed to sulfide being higher than that of eggs exposed to near anoxia for both species. No significant differences were observed between the two species in hatching success of eggs exposed to both near anoxia and anoxia/sulfide (with the exception of eggs incubated in near anoxia for 4 days). The results indicate that the impact of anoxia and sulfide on the eggs of the two Acartia species cannot be a factor explaining the spatial distribution in coastal and brackish environments of these copepods. Feeding experiments on A. clausi were also performed. Suitability of different algal species to rear this copepod was evaluated and the results were compared with data previously obtained for A. tonsa. Differences in feeding needs between A. clausi and A. tonsa are discussed.