Spermatocyte chromosomes of six species of Opisthobranchs (Mollusca, Gastropoda)

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Abstract
Chromosome numbers of six species of Opisthobranchia, five nudibranchs belonging to the Doridoidea, and one notaspidean, have been determined. The expected haploid number of 13 bivalents was found in Chromodoris luteorosea (Rapp, 1827), Hypselodoris messinensis (von Ihering, 1880), H. valenciennesi (Cantraine, 1841) and H. tricolor (Cantraine, 1835), while 12 bivalents were found in Discodoris atromaculata (Bergh, 1880). Berthella aurantiaca (Risso, 1818), the only notaspidean species analyzed in this study, possessed 13 bivalents, unlike the 12 previously reported for two other notaspidean species. The value of chromosome numbers used as a character for phylogenetic systematics in the Opisthobranchia is discussed.