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We analyzed the composition and community structure of soft-bottom mollusks of Isla San Francisco (24°48'N, 110°34' W) and its possible relationship to some environmental variables. We selected 18 sampling sites in thesummer of 2012, 36 samples were collected, 18 corresponded to biological samples and 18 to sediment. Thetemperature, depth, sediment type, and organic matter were used as environmental indicators. We analyzed the abundance and diversity of the malacological benthic community as ecological descriptors. Two thousand, seven hundred and seventy-three specimens were registered belonging to 36 bivalve species distributed in 11 families and five orders. The gastropods were the most represented group with 44 species distributed in 25 families and six orders. The highest values of richness and abundance were found south of the island in carbonate sediments of coarse sand and shallow bottoms. The most abundant bivalves were Tellina eburnea with an average size of 5 mm H followed in importance by Gary helenae. The most abundant gastropod was the micromollusk Cystiscus politulus. The species with the highest frequency of occurrence (> 50%) were: Laevicardium substriatum (58%), Crepidula aculeata (58%); Gari helenae (50%) and Lucina undatoides (50%). The mean values of diversity index was 2.7 bits per ind. and the evenness were high (> 0.8) in most of the sample sites, the minimum value recorded was 0.65 influenced by the abundance of the bivalve Tellina eburnea that was the dominant species. We found a strong association of mollusks with deposits of calcareous algae, pieces of coral and echinoderms.

Palabras clave: diversity; bivalves; Golfo de California; Soft bottom

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