Effects of salinity and temperature on the post-embryonic development of Eurytemora affinis (Copepoda; Calanoida) from the Seine estuary: a laboratory study

The species complex Eurytemora affinis is the most common calanoid copepod of northern-hemisphere estuaries. In the oligo-and meso-haline zones (salinity 2.5-25) of the Seine estuary, E. affinis reaches very high concentrations between March and June when temperature varies between 10 and 15 degrees C. To understand the population dynamics of E. affinis in the Seine estuary, we investigated the effect of a combination of temperatures (10-15 degrees C) and salinities (5, 15 and 25) on its postembryonic development. To take into account the individual variability, copepods were observed individually. Post-embryonic development time varies as a function of temperature and salinity and showed high individual variability. The increase in temperature from 10 to 15 degrees C decreased the mean postembryonic development time from 24 to 17 days. Increase in development time and mortality at salinity of 25 at both temperatures suggests that these conditions were the most stressful of those tested. No significant difference in development time or mortality was discerned between salinities of 5 and 15. In general, mortality was higher for copepodid stages than for naupliar stages. The individual variability increased during the development and was also very high: some individuals develop faster at salinity 25 than others at salinities of 5 and 15. Moreover, the distributions of the molting probability fitted well with a gamma density function, and thus we suggest this adjustment in models of population dynamics of copepods to tighten up the method of capturing individual variability. The population of E. affinis from the Seine estuary showed a faster development rate than conspecific populations from other estuaries at similar experimental temperatures and salinities. The responses of the post-embryonic development time to salinity and temperature show that E. affinis is well adapted to the low salinity zone of the estuary but can also survive at higher salinities.