Molluscan predator-prey interactions and its relationship to physiographic variables: A case study from Andaman Islands, India

Abstract

Predation is a major driving force for the evolution of an ecosystem through natural selection. It is responsible for transferring energy from one trophic level to the next. In the marine ecosystem, drilling and durophagous predation are common types of predations where molluscan prey, especially bivalves, are preyed upon by shell crushing and drilling predators, respectively. Carnivorous gastropods create complete and incomplete drill holes marking successful and failed drilling predation attempts respectively. Unsuccessful durophagous predation often leads to the development of repair scars in molluscs. Although the pattern of such predation intensities through time has been studied to understand its effect on the structure of marine ecosystems, its relationship with the physical environment is relatively less explored. This project attempts to investigate the variation in predation intensity in bivalve prey with reference to environmental variables such as depth, habitat type, salinity, and pH using samples collected from multiple locations along the east coast of Andaman Islands India. Drilling (DF) and repair scar (RF) frequencies are proxies to infer predation intensity. The study demonstrates a low degree of predation intensity for both drilling predation (DF =0.22) and durophagy (RF=0.005). The variation in DF is not explained by latitude, depth, environment, or size. Among the oceanographic variables, salinity shows a significant positive correlation with DF. The depth and habitat explain variation in the size of the prey and the predator. The bivalve size decreases with increasing depth. Among the environments, mangrove and muddy substrates are characterized by the largest size of the prey and predator, respectively. The predator size inferred from OBD (Outer borehole diameter) shows a significant positive correlation with prey size, implying a strongly selective prey choice by the predator. The magnitude of the selectivity is highest in medium depth and the muddy substrate. Our study highlights the importance of physical variables such as salinity and nature of substrate in controlling drilling predation intensity and predatory behavior in tropical marine ecosystems.