Infralittoral macrobenthos of the Patras Gulf and Ionian Sea: Opisthobranch Molluscs

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      Surveys carried out in the summers of 1981 and 1982 revealed 19 macrobenthic species, the most varied assemblage of infaunal and epifaunal opisthobranchs yet reported. The study-area consisted of 4 sectors: the Patras Gulf, Zakinthos and Kefallonia islands, and the Ionian Sea coast of the northern Peloponnese. The Patras Gulf sector contained the finest sediments; sand was the dominant type in the other 3 sectors. The infaunal carnivore Philine aperta was the most prevalent opisthobranch in the study-area as a whole, followed closely by the herbivorous Haminea navicula and the carnivorous Weinkauffia diaphana. Some species, like Cylindrobulla fragilis, were purely herbivorous. Others were micro-carnivores, feeding on infaunal foraminiferans, annelids and bivalved molluscs (e.g. Cylichna cylindracea, Rhizorus acuminatus, Acteon tornatilis, Aglaja tricolorata and Retusa truncatula).
      Biogeographically there are 4 categories represented in the samples: (a) an indigenous Mediterranean component (27.8%), (b) a Mauretanian component (16.7%), (c) a Lusitanian component (27.8%), and (d) a group of eastern Atlantic cosmopolitan species (27.8%), of which 2 species (11.1%) penetrate as far as the Black Sea. There is no evidence that the epifaunal species are more widespread than the infaunal species. No Lessepsian immigrants could be identified.
      A significant positive correlation between opisthobranch species-diversity and mean grain-size was found only in the Kefallonia samples. A comparison between the Jaccard and Matching coefficient methods of expressing ecological affinity between opisthobranch species showed the superiority of the former method for our data. This was because the Matching coefficient method gave undue weight to the number of conjoint absences in the samples. Foremost among the examples of significant similarities was the association between Bulla striata and Cylindrobulla fragilis, between Weinkauffia diaphana and Atys jeffreysi and between Rhizorus acuminatus and Retusa agg. In the first pair there is probably a trophic cause for the association, but in the other examples it is probable that the protagonists are correlated not with each other directly but with some abiotic factor.