Genetic Variation and Divergence in the Genus Carcinus (Crustacea, Decapoda)

The extent of genetic differentiation of the decapod crustaceans Carcinus maenas (L.) and C. aestuarii Nardo was studied in nine nearshore locations comprising various European marine and brackish‐water habitats, (Norway, Germany, France, Spain, Italy and Yugoslavia), using starch gel electrophoresis. The heterozygosity for all 19 allozyme loci was 0.026–0.016 (C. maenas) and 0.011–0.003 (C. aestuarii); one locus (Pgm) was the most polymorphic. The populations had rather low levels of genetic variability, comparable to those found in other decapods. In general, a light clinal variation in allele frequencies was discovered at the Pgm locus of C. maenas as latitude decreased. Because of similar morphologies and a closed genetic relationship (I = 0.89), the two forms should be considered subspecies. Therefore, the designation of the Atlantic subspecies C. maenas maenas is proposed, and the term of the Mediterranean subspecies C. maenas aestuarii is recommended.