Spatial variation in marine predation pattern across depositional environments: Insights from the drilling predation records of the recent bivalve fauna of Andaman Islands, India.

Biotic interactions, such as predation, is an important evolutionary force reshaping the shallow marine community in the geologic past. Benthic mollusks are ideal groups to study the deep time record of predation because they preserve the predation record (drilling and durophagous) on the shell and are known to respond to evolutionary triggers. The physical environment also influences shallow marine molluscan communities. It is important to evaluate the influence of physical factors on biotic interactions to understand the true mechanism of the evolution of shallow marine fauna in deep time. To evaluate the effect of abiotic factors on biotic interaction in the marine realm, we need a setting with distinct environmental patterns inhabited by diverse community with recognizable biotic interaction. The recent shallow marine bivalve community of the Andaman island provides such a setting.  

The Andaman island is part of Andaman–Nicobar archipelago in the Indian Ocean marking the western margin of the Andaman Sea. It represents a sediment-dominated accretionary wedge associated with a convergent margin. The east coast of Andaman island receives significant riverine input from Irrawaddy delta in the north and is located in the vicinity of well-developed reef system of south-east Asia. The east coast of Andaman also has diverse molluscan fauna. We collected samples from 19 localities along the east coast. These localities represent of diverse depositional environments and substrate types including muddy tidal flats, mangroves, sandy beach and coral reef. We collected grab samples from water depths between 1-15m. We characterized the drilling frequency (DF) and prey selectivity across environments, family and depth. Drilling predator–prey systems have been explored using cost–benefit models in modern and fossil assemblages. In this study, we also tried to establish a relationship between the benefit to cost ratio (BCR) of drilling attempts across depositional environments and compared them with global data.

We studied ~3000 bivalve specimens of which 291 have drill holes. The overall drilling frequency (DF) is 0.19. A total of 21 prey families were identified and the DF ranges between 0 to 0.5. Three families (Crassatellidae, Isognomonidae, Placunidae) show the highest DF. There is a significant variation in DF across environments. Coral sand shows the highest DF and muddy substrate shows the lowest.

The benefit to cost ratio (BCR) of drilling is positively correlated with predator size for the muddy substrate for the Andaman specimens. The Andaman BCR is lower than the global average for the shallow marine environment. In contrast, the BCR is higher for the mangrove community of the Andaman compared to the global data. The BCR declines with increasing predator size for the Andaman and global data. This implies a possible role of increasing metabolic cost of larger predators.

The record of predation in the geologic record has been thought to be influenced primarily by the interaction between organisms where the role of the physical environment is largely ignored. Our study demonstrates the significant influence of the physical environment in shaping predation intensity and prey selection which in turn changes BCR of predation events.