An actuopalaeontological approach to the assessment of recent changes in benthic molluscan biodiversity: preliminary results in the Northern Gulf of Thailand

In the present paper, an attempt is made to use benthic molluscan thanatocoenoses as a record of living associations that have existed prior to the modern ones in the Northern Gulf of Thailand. The investigated area encompasses the tidal flats and upper infralittoral bottoms facing the coastline of Phetchaburi province, approximately 150 km southwest of Bangkok. The 14C dates from that area have shown that molluscs in thanatocoenoses appear to be affected by bomb radiocarbon and did live after the 1960s. The statistical treatment of faunal lists pertaining to 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.; it contains a total of 298 species and the average species richness per station is 60. Thanatofacies 2, recovered at a single station in the inner tidal flat, is dominated by Scapharca indica, Cerithidea cingulata and Cycladicama oblonga; the total number of species is 64. Thanatofacies 3, also within the tidal flat at more sandy locations, is dominated by Corbula (Caryocorbula) lineata, Decorifer sp. and Mactra (Mactra) luzonica; the total number of species is 32. The fourth is another typical intertidal thanatofacies, dominated by Pitar (Costellipitar) manillae, Anadara granosa and Arcopagia yemenensis; it comprises a total of 18 species. 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. It is apparent that the whole sea-bed area investigated in this study underwent a dramatic negative change 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. Other causative agents are likely the shrimp farming effluent and sewage from coastal human settlements that affect the water quality.