WHAT TRIGGERS CANNIBALISM IN DRILLING GASTROPODS: A CASE STUDY ON NATICA TIGRINA

Cannibalism is observed among the drilling gastropods in Recent and in the fossil record. However, the ecological factors triggering cannibalism is 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. Similarly, optimal foraging theory is proposed to explain such behavior; it predicts a critical net energy gain for the predator to adopt this behavior.

In an experiment with live naticid gastropods, Natica tigrina, we evaluated the effect of following factors to initiate cannibalism: 1. Availability of preferred prey, 2. Size ratio of predator and prey, 3. Ontogenetic stage. Although cannibalism is found to be quite rare when preferred prey (Cardium sp.) is present, the relative abundance of the prey does not affect the frequency of such attempts. Size ratio is observed to play the most crucial role; cannibalism was found to be maximum in a mixed group of small and large gastropods. While the incidence of cannibalism is much lower in groups of similar size, the incidence differs with mean size. Cannibalism is relatively more 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 some 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 Natica tigrina often bears 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.

It has been postulated that cannibalism should be observed in nutrient depleted conditions such as at times after mass extinctions. However, our study demonstrates the relative importance of other ecological factors in guiding this behavioral trait. Therefore, such factors should also be taken into consideration while studying cannibalism in the context of drilling predation.