FREQUENCY AND DEGREE OF IMPOSEX AMONG SOM E STENOGLOSSATE PROSOBRANCHIA ALONG THE FRENCH COAST (ESPECIALLY BRITTANY AND NORMANDY) .2. THE SITUATION UNTIL SPRING 1992

The further development of imposex on the French coasts (namely Britta ny and Normandy) is analysed for 10 species of Meso- and Neogastropods (20 560 individuals). Beside the species already mentioned in 1988 (F ioroni et al., 1991 a) our description includes also Lamellaria perspi cua, Trivia arctica, T. monacha, Ocinebrina aciculata and Colus gracil is. New stages/types of imposex (Fig. 2) allow a new evolutionary syst em of imposex valid in all species. The frequency of imposex attains m ostly 100 % ; the VDS index is the best parameter for its intensity. D espite the official restrictions in the use of TBT (tributyltin), impo sex has still more advanced namely in the Muricidae and attains in sev eral species sterility and a shift of the sex ratio in favour of males (Ocinebrina aciculata, Nucella lapillus). Despite the general increas e of imposex, the localities distant from harbours have conserved thei r favoured position.