Influence of salinity on the distribution of Acartia tonsa (Copepoda, Calanoida)

The calanoid copepod Acartia tonsa is the most abundant copepod species in the Berre Lagoon near Marseilles, France. This species can be found in a wide range of environments and therefore should be capable of colonizing the neighboring region of the Gulf of Fos. We test the hypothesis that differences in salinity between the Berre Lagoon and the Gulf of Fos prevents colonization of the latter by A. tonsa. The degree of tolerance of this species for changes in salinity was tested through laboratory experiments carried out with samples taken during a year long cycle. Its resistance towards different kinds of osmotic shocks was tested, and the optimal salinity for the population of A. tonsa of the Berre Lagoon was determined. The euryhalinity of A. tonsa was confirmed with these experiments, although it was demonstrated that this species shows high mortalities if the instantaneous change in salinity is greater than 10-15 parts per thousand. Other complementary experiments were undertaken to define the maximum and minimum values of salinity on which this species could survive. The maximum salinity was 72 parts per thousand, and the minimum was lower than 1 parts per thousand. The optimal adaptation was found between 15 and 22 parts per thousand. (C) 1999 Elsevier Science B.V. All rights reserved.