The last decades in the Gulf of Thailand: a dramatic reduction in soft-bottom mollusk biodiversity

Several campaigns at sea conducted in the Gulf of Thailand from 1996 to 2003 provided a wide database on mollusk soft-bottom assemblages. The shallow seafloor facing the Phetchaburi coastal plain was sampled at about 150 locations, while 25 were investigated in the Pak Phanang Bay (Nakhon Si Thammarat). All shells extracted from samples were identified to species level; this led to the publication of a suite of papers where a total of 695 taxa have been described and figured, thus obtaining the most detailed review on the marine mollusks of this area ever published. Among these species, 24 have been proposed as new to science. A brief analysis of shell material coming from 113 samples provides an overview of the taxonomic composition and diversity of the soft-bottom mollusk fauna in the Gulf. The 609 species recognized in thanatocoenoses of both areas belong to 111 families (47 of bivalves, 3 of scaphopods, 61 of gastropods); the most represented are, among bivalves, Galeommatidae (40 species), Tellinidae (32), Veneridae (23) and Arcidae (15); among gastropods, Turridae (54), Odostomiidae (45), Turbonillidae (36), Vitrinellidae (16), Triphoridae (17) and Epitoniidae (15). The number of species per sample spans from 5 to 254 in Phetchaburi, from 4 to 27 in Pak Phanang; the number of specimens per sample (with abundance values referred to a standard volume of 5 l) varies greatly from 3 to 16897 and from 10 to 2275 respectively. Conversely, mollusk live associations include only 115 species belonging to 49 families (25 of bivalves, 3 of scaphopods, 21 of gastropods); the best represented are, among bivalves, Tellinidae (10 species), Veneridae (8), Arcidae (7) and Mytilidae (7); among gastropods, Turridae (7), Vitrinellidae (3) and Columbellidae (3). The number of species per sample is comprised between 1 and 28 in Phetchaburi, 1 to 4 in Pak Phanang. The number of specimens per sample spans from 1 to 80 in Phetchaburi, from 1 to 395 in Pak Phanang. Sampled thanatocoenoses bear a 14C age of about 30 years, allowing some considerations about the evolution of the marine environment and its biodiversity occurred in the last decades. In Phetchaburi the loss of both diversity and richness is evident; this delineates the surviving of a very disperse and poor mollusk association, at least partly caused by anthropic pressure on sea bottoms. In Pak Phanang Bay, instead, the less marked observed variations can be ascribed to the progressive confinement of the bay due to the closure of its northern entrance rather than to human exploitation of the sea bottom.