Ontogenetic Shifts of Predatory Strategies by the Carnivorous Gastropod Acanthina monodon (Pallas, 1774)

ABSTRACT Predatory strategies used by carnivorous gastropods may change during their ontogeny. In muricid gastropods attack mechanisms include an accessory boring organ (ABO), radula, labral tooth and/or pedal muscle. However, these mechanisms and their patterns of use in relation to the ontogeny of the carnivorous gastropod remain uncertain. We studied the occurrence of shifts in predatory strategies through the ontogeny of the gastropod Acanthina monodon preying on the mytilid Perumytilus purpuratus. Our results showed a direct relationship between predator and prey size. During attack, the small-sized snails (up to 18 mm shell length, SL) used exclusively the ABO, and medium-sized snails (18–20 mm SL) shifted to using the radula for rasping shells. Meanwhile, the largest-sized snails (> 20 mm SL) used the radula, but also occasionally the pedal muscle and labral tooth, to attack their prey. The site selectivity on the mytilid shell varied according to predatory mechanisms used. The small-sized gastropods used the ABO to drill the center of the prey valves, whereas mid-sized and large-sized snails used the radula and the labral tooth on the valve edges. Occasionally, large-sized snails also used ABO. Shifts in predatory strategies and attacked areas are influenced by the development and consolidation of structures involved in the attack as predator size increased. The incorporation of trace elements during rachidian teeth growth may enable major resistance to friction against carbonate prey valves, as well as the appearance and development of the labral tooth, play relevant roles in the predatory mechanism shifts, which allows the consumption of larger prey. These patterns described evidences the gastropod's predatory behaviour in terms of energetic gain while minimizing the risk of the predator itself being preyed on.