DISTRIBUTION OF MOLLUSCS IN MANGROVES AT SIX SITES IN THE UPPER GULF OF THAILAND

Abstract

Molluscs were studied in mangroves at six sites in the upper Gulf of Thailand from March to June 2004. At each site, density, diversity, and biomass of molluscs were measured in replicate samples made in four habitats: Avicennia and Rhizophora tree zones, and the unvegetated mudfl at and channel. There is no Rhizophora zone at the site at Ang Sila. A moderate diversity of 47 species (31 gastropods and 16 bivalves) was collected. There was considerable variation in population characteristics between sites and between habitats at a site. Mangrove tree habitats tended to have greater diversity than unvegetated habitats. Mean density varied from 9.4/m 2 at Ban Num Chieo to 841.5/m 2 at Bangpoo, a difference of almost two orders of magnitude. There was no clear pattern of density in the different habitats. Three habitats (mudfl at, Avicennia and Rhizophora tree zones) each had one site where there were no molluscs found. The greatest densities were recorded in three different habitats: 1,120.6/m 2 in the channel at Laem Chabang; 1,235.8/ m 2 in Rhizophora at Khlong Khon; and 3,289.6/m 2 in Avicennia at Bangpoo. The high densities in these three habitats were contributed entirely by three different species, one at each of the habitats. The greatest biomass was found at different sites from the greatest density. A single species of bivalve was responsible for the high biomass at the three sites with the greatest biomass; the dominant species differed at the three habitats. Three assemblage groupings were found. The largest comprised molluscs from all Avicennia and Rhizophora habitats and the channel habitats at Bangpoo and Bang Ya Phreak. All fi ve mudfl at and two channel habitats formed a second group, and the Leam Chabang channel formed an isolated group. Gastropods were mostly epifaunal (24 species) or arboreal (7 species); most bivalves were infaunal. All 16 bivalves were fi lter feeders, and 23 gastropods were herbivores. In general, fi lter feeders and detritivores dominated density and biomass. This is consistent with the ecological role of molluscs being intermediate between the mangroves as primary producers and the higher trophic levels in the food web. Marine species dominated the salinity preference.