CLASSIFICATION AND INTERPRETATION OF THE ESTABLISHED MEDITERRANEAN BIOCOENOSES BASED SOLELY ON BIVALVE MOLLUSKS

Based on the bivalve fauna known from an extensively studied zone, alo ng the infralittoral in the Patras Gulf, a biotope approach was attemp ted using numerical taxonomy methods. The efficiency of various combin ations of similarity measures and clustering techniques is discussed f or this type of data (single group - binary). Four main station groups were delimited which were further assigned to the biotopes of known M editerranean biocoenoses. The four groups correspond to: (a) the bioto pe of the Posidonia beds (HP) and that of coarse sands and fine gravel s under the influence of bottom currents (SGCF). The limits of these t wo biocoenoses are not clear-cut. (b) The biotope of the coastal detri tic bottoms (DC) usually occurring in deeper areas but met in the infr alittoral under environmental stress. (c) A transitional zone where de generating Posidonia meadows are gradually replaced by Cymodocea and Z ostera, facies of the muddy sand in sheltered areas biocoenoses (SVMC) . This degradation can be attributed to various disturbing factors inc luding natural environmental changes. (d) The biotope of fine well-sor ted sands (SFBC). A typical malacofauna is present with the addition o f some species. Regardless of the differences, Peres & Picard's (1964) scheme is more appropriate in the Mediterranean ecosystems than any o ther northern European scheme.