Ecological and geographical distribution of Mollusca of Lobos and Enmedio coral reefs, Southwestern Gulf of Mexico.

The Mollusca of Lobos Reef (21'28N, 97'13W) 100 km south-southeast of Tampico, Mexico, and Enmedio Reef (19'06N, 95'56W) 15 km southeast of Veracruz, were studied during late May and June 1973. Two hundred and ninty species (178 alive) were collected, consisting of 211 Gastropoda, 73 Bivalvia, 4 Polyplacophora and 2 Cephlapoda. Two hudred and twenty species (127 alive) were collected on Lobos and 219 (131 alive) on Enmedio. Wading, snorkeling and SCUBA were employed as collecting techniques in successively deeper waters about the reefs. Random observation was made for the larger motile and attached species, bottom samples were analyzed for micromolluscs, and rock samples were examined for boring species. The ecological distribution of molluscs within each of the reef biotic zones were studied in detail. The supratidal rocky shore habitat had the lowest species diversity (7 gastropods) and the highest percentage of species restricted to any one zone (100%). The Thalessia bed had the highest diversity (178 species), as well as the largest number of species restricted to one zone (61 species). Subtidal, hard substrate species extended through more of the reef biotic zones than most supratidal, hard substrate species or subtidal, soft substrate species. The geographical distribution of 245 species from Lobos and Enmedio reefs revealed that 97-98% of the molluscan faunas were tropical species. Most of them were wide ranging eurythermic tropical species. Of the 245 species analyzed 143 (58%) extended to Brasil, 119 (49%) extended to Bermuda, 98 (40%) extended into warm temperature waters, 52 (21%) extended to the offshore reef and hard banks in the northwestern Gulf, 22 (9%) extended to offshore North and South Carolina and 17 (7%) extended to waters outside the western Atlantic. Twenty species (8%) were restricted to the Caribbean Province proper (excluding the above extremes), and 5 (2%) are apparently new species, possibly endemic to the southwestern Gulf reefs.