Effects of photoperiod on egg production in Eurytemora affinis Poppe, 1880 (Copepoda: Calanoida) from the Seine Estuary (France)

The calanoid copepod Eurytemora affinis Poppe, 1880 is the most abundant species of the mesozooplanktonic community of the Seine estuary (France) in the lower salinity zone. Diapausing eggs of this species have been found in the sediment of this estuary but they are few in comparison with those found for other calanoid species. We thus decided to test the effect of photoperiod on diapausing egg production in E. affinis from the Seine estuary. For this, an experimental device has been developed. A combination of five photoperiods and two temperatures was used and the production of eggs was studied for the first and the second clutch. The production of diapausing eggs was demonstrated under photoperiodic conditions less than or equal to 12h of light either at 12 or 16 degrees C with a maximum under constant darkness (6%). A significant increase in the number of non-viable eggs was also observed between the first and the second clutch for all tested conditions and this may be due to the non-fertilization of these eggs. We also verified that the late naupliar instars are sensitive to the photoperiodic signal.The experimental approach has revealed that the production of diapausing eggs in the key species to the downstream Seine estuary, E. affinis, could be induced by short days, but photoperiod was probably not the only factor responsible for the induction of diapausing egg production. The percentage of diapausing eggs obtained under the induction experiment is similar to that obtained from the eggs extracted from the sediment of the Seine estuary (7%). The strategy of E. affinis in this estuary would be to constitute an egg bank, acting as long-term survival or in the case of adverse conditions. The role of geographical position (mainly latitude) and local conditions in the reproductive strategy of E. affinis is discussed.