Effects of the 26 December 2004 tsunami on littorinid molluscs near Phuket, Thailand

Abstract

A major effort is being undertaken to understand the effects of the December 2004 Indian Ocean tsunami on the marine environment. The emphasis has been on coral reefs, which are subtidal and offshore of the areas that were most heavily affected by the tsunami. The greatest losses of human life and destruction of human infrastructure occurred in the intertidal and supratidal regions where the waves had the greatest destructive force after they broke. We seek to determine the effects of the tsunami in the upper intertidal region, where the effects might be greatest, by using littorinid molluscs as an indicator group. An extensive 2002 survey of the distribution of littorinid molluscs at 50 sites in mangroves and on rocky shores in Thailand was recently published. Six of the study sites were on the Andaman Sea coast in the area of Phuket, Thailand, one of the regions most heavily hit by the tsunami. These sites (Fig. 1) were resurveyed between 13 and 18 April 2005, 3.5 months after the tsunami, to determine qualitatively what damage had occurred. Thirteen species of littorinids were recorded in the initial survey at the six sites; only seven species were recollected after the tsunami. The tsunami was not uniform, and depending on local topography affected the shorelines in different ways. The study sites included both rocky shores and mangrove areas (Table 1). Rocky shores at Kalim Beach and Nang Thong Beach were directly hit by the tsunami. While littorinid populations decreased considerably, the rocks themselves were not affected. At Nang Thong Beach the adjacent sandy shoreline was extensively modified (Figs 2, 3). The rocky shore of Mai ngarm Bay, Surin Island, has scattered mangroves (Rhizophora apiculata and R. mucronata ). The shore was in the lee of the tsunami and there were no visible effects. A site at an extensive mangrove at Laem Mai Kaew was heavily hit by the tsunami and was unrecognizable after the event; all littorinids at the site had disappeared. It should be noted that the site was at the seaward fringe of the mangrove, which was affected by the tsunami. The trees rapidly dissipated the force of the tsunami and there were no obvious effects shoreward of the mangrove fringe. Mangroves at Pak Meng Beach and Ta la Beach were largely unaffected by the tsunami. Even where littorinid species were present after the tsunami, densities appeared to have decreased. Even in areas where there were no apparent effects of the tsunami on rocks or mangroves, some of the littorinids were washed off the rocks or trees. Population reductions were likely to have been caused by the tsunami. However, it has been three years since the initial surveys were undertaken at the sites. As population densities are naturally variable, the reductions could have been due to another cause. For example, substantial natural declines in the population of Nodilittorina unifasciata (Gray, 1826) at Waterman Bay, Western Australia occurred over a two-year period. The fact that only seven of the 13 littorinid species originally found at the six study sites were present after the tsunami is not a cause for concern. The pattern found in the littorinids, as in the reefs, is of different effects in different areas. It is likely that more detailed studies would show that there are populations of the species remaining in the area that can recolonize areas affected by the tsunami. All of the species are widespread. The species with the smallest known distribution, Littoraria bengalensis, was described in 2001, and is known from the west coast of Thailand and also from India. All of the other species have much broader ranges. As far as is known, all of the littorinid species occurring on the west coast of Thailand have a planktonic larval stage, which will allow the ready dispersal of larvae into affected areas during the next spawning season. In summary, the immediate effects of the tsunami on littorinids in the Phuket area have been patchy, and it is likely there will be no major permanent changes to the distribution of species in the area.