Pêches planctoniques superficielles et profondes en méditerranée occidentale (Campagne de la «Thalassa » -janvier 1961 entre les îles Baléares, la Sardaigne et l'Algérois) VII. . THALIACÉS

In this high sea sector, we identified 7 species of Thaliacea: one Pyrosorne (Pyrosoma atlanticum), three Salps (Thalia democratica, Salpa fusiformis, Pegea confoederata) and three Doliolids (Doliolum nationalis, Doliolina mulleri. Doliolum denticulatum).If the distribution of the first two groups did not bring any new knowledge concerning their distribution or their behaviour in the Mediterranean Sea, it was not the same for the third group.Indeed, we demonstrated the existence of close relationships between the distribution of doliolids and local marine circulation. As observed by TRÉGOUBOFF (1965) in the Villefranche-sur-Mer area where the modification of the layers by the easterly wind brought doliolids down to 1 800 m deep, we found them at more than 2 500 m deep, brought from the surface to the bottom by a portion of the Atlantic waters that plunges around the North African coast.Moreover, Doliolum nationalis, a species whose ecology is still little known, turned out to be of great interest. A parallel between its vertical distribution and that of Cladocère Penilia avirostris on the same stations showed their common presence in low salinity formations. This reminded us of their coexistence in the Bay of Alger, which is subjected to a strong Atlantic influence, as well as their large concentrations in the mouth of the Rhône and Ebre rivers. These observations enable the association of Doliolum nationalis and Penilia avirostris in their predilection for diluted waters (fluvial or Atlantic) in the Mediterranean Sea and make this Doliolid a new ecological indicator.