An unusual distribution of Oithona nana Giesbrecht (1892) (Crustacea : Cyclopoida) in a bay: The case of Toulon Bay (France, Mediterranean Sea)

The distribution of the Cyclopod Oithona nana in Toulon Bay (France, Medite rranean Sea) was studied. This bay has an artificial breakwater which creat es two sub-ecosystems, an inner little bay (heavily polluted) and an outer large bay (less polluted). We set up six sampling stations in the inner bay and nine outside, along a littoral/pelagial gradient. Zooplankton hauls we re taken with a 90 mum mesh plankton net (with flowmeter). The sea surface temperature, conductivity and salinity were also measured. The abundance of O. nana was much higher in the little bay than outside. There were also ve ry few males in the little bay, and almost none in the large bay, in contra st to the abundant females in the sampling stations adjacent to the little bay. The sex-ratio was always in favour of females and there were more adul ts than copepodites. These results indicate that the ecological status of t he little bay favours the development of O. nana, and we suggest that this species may be used as a biological indicator of such perturbed systems.