PENETRATION AND CONSUMPTION RATES IN DRILLING GASTROPODS: A COST-BENEFIT APPROACH EVALUATING FEEDING STRATEGIES

Predation by drilling gastropods because it leaves distinct marks on the shells of prey allows the quantification of certain aspects of predator-prey interactions in situations where they cannot be observed directly. Such data have revealed a relationship between drill hole size and prey size, implying a positive relationship between the size of drilling predator and its prey. Although it is recognized that this relationship may be causally connected to feeding rate, detailed studies on controls on feeding rate are rare. Using the results of neontological experiments we calculated the rates of penetration and soft tissue consumption for a drilling gastropod preying upon a bivalve. The observed soft tissue consumption rate increase with increasing predator size, whereas the penetration rate remains constant. The latter result has been recognized in many other studies and appears surprising in light of the fact that as predators get larger the size of their boring organ increases which should give them a biomechanical advantage leading to a faster rate of penetration. To gain insight into this problem we applied a cost-benefit approach. In our model, the cost of feeding was the sum of consumption and penetration time, while the benefit was the caloric value of soft tissues consumed. We asked if increasing consumption rate or penetration rate has a greater impact on reducing the total time (equivalent to cost). Since we wanted to evaluate the relative effect of these parameters, we kept the excavation rate (the product of penetration rate and hole diameter) constant; with a constant excavation rate, an increase in penetration rate leads to a decrease in consumption rate, and vice versa due to the influence of drill hole diameter (= predator size). We modeled two equally probable feeding strategies with constant excavation rate; the first with constant penetration rate and the second with constant consumption rate. These feeding strategies predict different size relationships between drilling predator and its prey that are optimal from the cost-benefit perspective. Using the modeling approach we demonstrated that the first feeding strategy observed in nature has obvious advantages over the other providing an explanation for the observed size relationship between drilling predator and its prey.