What controls cannibalism in drilling gastropods? A case study on Natica tigrina

Abstract

Cannibalism is observed among the drilling gastropods in Recent and in the fossil record. However, the ecological factors triggering cannibalism are not well understood. While competition over food is considered a factor to initiate cannibalism in certain groups, it is yet to be tested for drilling gastropods.In an experiment with live naticid gastropods, Natica tigrina, we evaluated the effect of the following factors in controlling cannibalism: 1. availability of preferred prey, 2. size ratio of predator and prey, and 3. ontogenetic stage of the predators. Cannibalism is found to be quite rare when preferred bivalve prey (Cardium sp.) is present. Size ratio among predator and prey is observed to play the most crucial role; cannibalism is maximum in a mixed group of small and large gastropods. While the incidence of cannibalism is much lower in groups of similar body size, the intensity differs with mean size. Cannibalism is relatively common in groups of larger size compared to those of smaller size. This indicates an ontogenetic threshold in cannibalism; the naticids seem to acquire such behavior only at a specific ontogenetic stage.Our findings are corroborated by observed drilling pattern in the Recent shells collected from Chandipur-on-sea, India. While the preferred prey Cardium sp. has the highest drilling frequency, the shells of N. tigrina often bear drill holes; the smaller size class of Natica has the highest drilling frequency among all size classes. The smallest drill holes in Natica are larger than those found in Cardium; this pattern indicates that the Natica are less likely to attempt a cannibalistic attack in their early ontogeny. These behavioral patterns are also reported from a Miocene assemblage of N. tigrina from Poland (Zlotnik, 2001).It has been postulated that cannibalism should be observed in nutrient depleted conditions. However, our study demonstrates the relative importance of other ecological factors in guiding this behavioral trait. Moreover, the ontogenetic development of cannibalistic habit is closely linked to the evolutionary stable strategy (ESS). Therefore, such factors should also be taken into consideration while studying cannibalism in the context of drilling predation.