Recent changes in molluscan diversity in the Gulf of Thailand: a paleontological approach

This paper reports a case history concerning the use of benthic molluscan thanatocoenoses as a record of living associations that preceded the modern ones. The investigated area encompasses the tidal flats and upper infralittoral bottoms facing the coastline of Phetchaburi province, approximately 150 km southwest of Bangkok, in the Northern Gulf of Thailand. Radiocarbon dating of molluscs in thanatocoenoses reveals that they have been affected by nuclear activities in the 1960s. The multivariate statistical treatment of faunal abundance data from 55 stations delineates 7 thanatofacies. The first one is widespread throughout the upper infralittoral zone and is largely dominated by Nuculana (Jupiteria) puellata, Timoclea (Chioneryx) scabra and Decorifer sp. Thanatofacies 2, recovered at a single station in the inner tidal flat, is dominated by Scapharca indica, Cerithidea cingulata and Cycladicama oblonga. Thanatofacies 3, also within the tidal flat at more sandy locations, is dominated by Corbula (Caryocorbula) lineata, Decorifer sp. and Mactra (Mactra) luzonica. The fourth is another typical intertidal thanatofacies, dominated by Pitar (Costellipitar) manillae, Anadara granosa and Arcopagia yemenensis. Thanatofacies 5 and 6 appear to be intertidal ecotones, representing different transitional aspects, between the infralittoral thanatofacies 1 and the strictly intertidal thanatofacies 4 and 5. The upper infralittoral thanatofacies 7 denotes sedimentary instability. The living associations obtained at the same locations appear to be markedly depleted in both species richness and total number of specimens, if compared to the co-occurring thanatofacies. The whole sea-bed area apparently underwent a dramatic decrease in benthic molluscan biodiversity during the last decades. This change seems to be related primarily to the intensive trawling in shallow water, practiced by local coastal villagers, possibly associated to shrimp farming effluent and sewage from coastal human settlements that affect the water quality.