Spatial and temporal variability of intertidal rocky shore bivalves and gastropods in Sichang Island, East coast of Thailand

Abstract

Spatial and temporal variability of intertidal rocky shore bivalves and gastropods in Sichang Island, east coast of Thailand were studied over 12 months (March 2008 to February 2009). The study area experiences mixed tides with the largest tidal range of 3.6 m above MTL during spring tides. Shores are exposed to the prevailing northeast monsoon in the winter (November to February), resulting in stronger wave action during this season than in summer(March to May) and rainy season (June to October). Transects were positioned at five different heights from mid shore to high shore (2.5 to 3.5m above MTL) dominated by rock oysters in the mid shore. At each height, abundance of bivalves and gastropods were recorded within fifteen replicate quadrats (25 x 25 cm) on a monthly basis. A total of 2 bivalves and 10 species of gastropods were identified. In the high shore, the littorinid Echinolittorina malaccana dominated. Dense beds of the bivalve Isognomon nucleus were abundant in crevices at 2.5 and 2.75 m above MTL, whilst a large number of snails (Planaxis sulcatus) were found near the midshore. Below this, the rock oyster Saccostrea cucculata was dominant, with a variety of mobile gastropods (Cellana grata, C. toreuma, Patelloidea saccharina and Siphonaria japonica). The general sequence of zonation patterns were clear over time, but the vertical extent of some mobile species varied in accordance with the monsoon effect and increasing tidal range in winter, especially on the more exposed of the two sites. The high shore littorinids, E. malaccana moved up shore and were more abundant near the supralittoral line in winter suggesting that the seasonal monsoon effect and tidal range were important factors determining the distribution of some species of mobile gastropods, allowing them to exploit higher tidal levels.