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

The assessment of biodiversity changes occurred during the last decades currently requires the availability of yearly observations throughout the considered time span. 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 mollusks in thanatocoenoses appear to be affected by bomb radiocarbon and did live after the 1960s. The faunal lists pertaining to 52 stations were selected for statistical treatment, involving Qmode cluster analysis and non-metric Multi-dimensional Scaling Ordination (MDS) on a Bray-Curtis similarity matrix of transformed species abundance data. The analysis 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; the average station heterogeneity, expressed by the Shannon-Weaver index (H’) is 3.29. Thanatofacies 2, recovered at a single station in the inner tidal flat, is dominated by Scapharca indica, Cycladicama oblonga and Cerithidea cingulata; the total number of species is 64 and H’ is 4.27. 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 and average values per station of species richness and H’ are respectively 23 and 3.67. The fourth is another typical intertidal thanatofacies, dominated by Pitar (Costellipitar) manillae, Anadara granosa and Arcopagia yemenensis; it comprises 18 species in total with an average species richness of 10 and H’ averaging 1.69. Pitar (Costellipitar) manillae and Anadara granosa denote fresh water influence. Thanatofacies 5 through 7 appear to be intertidal ecotones, representing different transitional aspects, between the infralittoral thanatofacies 1 and the strictly intertidal thanatofacies 3 and 4. They exhibit moderate to low average values of both species richness and H’. 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 average number of species per station drops from 60 to 3, from 64 to 0, from 23 to 1, from 19 to 0, from 18 to 1, from 10 to 0.3 and from 9 to 0 in those areas where respectively thanatofacies 1 through 7 were recovered. The decrease of the total number of specimens goes accordingly. It is apparent that the whole sea-bed area investigated in this study underwent a dramatic negative change in benthic mollusk biodiversity during the last decades. This change seems to be related primarily to the intensive trawling in shallow water, practiced by local coastal villagers, which resuspends an enormous quantity of mud resulting in very high turbidity that clogs the gills of many benthic invertebrates; moreover, trawling kills seabed organisms by crushing or burying them and prevents larvae from settling. Other causative agents are likely the shrimp farming effluent and sewage from coastal human settlements that affect the water quality.