A CASE OF SPECIFIC PREY SELECTION BY DRILLING PREDATORS FROM MIOCENE OF KUTCH, INDIA
All the specimens used for this study were collected from an exposure near Rampar village in Kutch region of Western India. The beds yielding the specimens belong to the Lower Chhasra formation of Early Miocene age. The prey group consists of four species of oyster bivalve (Ostrea latimarginata, Ostrea angulata, Crassostrea gigensis, Hyotissa hyotis), Chlamys sp. and Placuna lamellata. The shapes of the drill holes indicate the naticid gastropod to be responsible for such drilling predation.
The overall drilling frequency is 20% where as the species level frequency is as high as 35%. Ostrea angulata has the highest incidence of successful drill hole. There is quite a high incidence of incomplete drill holes; while the assemblage level frequency is 41%, the species level frequency is as high as 57%. Ostrea latimarginata has the highest incomplete drilling frequency. For Ostrea angulata, the drilled individuals are larger than the undrilled ones. The average size of the shells with incomplete drill holes is larger than those with complete drill holes for all the groups. There is no significant correlation between outer borehole diameter (OBD) and the prey size of Ostrea angulata. However, the average OBD size for complete drill holes is significantly higher compared to the incomplete drill holes implying the increase in success rate of a predator with size and possibly ontogeny. We have also found incidence of strong site stereotypy where the predators prefer a specific central area on the shell. The specimens also showed a preference by the predator for left valve while drilling.
Our study of this molluscan assemblage demonstrates preferred selection of prey in terms of taxonomy, size, site and valve by the predatory gastropod.