Genetic relationships between marine and marginal-marine populations of Cerithium species from the Mediterranean Sea

Results of isozyme electrophoresis were used to explore the genetic relatio nships between several Mediterranean morphs of Cerithium (Gastropoda: Proso branchia), for which taxonomy is currently uncertain because of high intras pecific variability and low interspecific differentiation.. The large speci es, classically known as C. vulgatum Bruguiere, 1789 was identified at four sites (two in the French Mediterranean and two in souther Spain). Two diff erent larval types were found in the French sites, but poecilogony could no t be demonstrated. Individuals collected from harbours were not genetically distinct from open-sea populations of classic C. vulgatum. However, a popu lation in the Embiez lagoon (French Mediterranean) which morphologically re sembles C. vulgatum did display distinct genetic traits, supporting its sta tus as a separate species. Of the small Cerithium species usually known as C. I rupestre, two sympatric species (C. "rupestre" Risso, 1826 and C, livi dulum Risso, 1826) were distinguished. Genotype frequencies within the anal ysed populations revealed much heterozygote deficiency. F-ST values (fixati on index measuring the effects of population subdivision) suggest a higher genetic differentiation for C. lividulum populations than for C. vulgatum p opulations. We assume that a high larval dispersal capability (via planktot rophy) allows a high gene flow between populations of C. vulgatum.