STEREOTYPICALLY BORING: AN ANALYSIS OF PREDATORY DRILL HOLE STEREOTYPY IN LITTORINA SAXATILIS FROM THE BAY OF FUNDY, NOVA SCOTIA

The naticid gastropod, Lunatia heros, is a major predatory gastropod within modern soft-sediment intertidal environments. However, naticid gastropod predation has not been described within a mixed rocky and soft-sediment intertidal zone. Economy Point, Nova Scotia provides a unique opportunity to observe and study the borings of Lunatia heros within the intertidal zone of the Bay of Fundy, where exposed bedrock and mud flats provide habitats for the predatory organism. Preyed shells collected along tidal channels included a variety of bivalves and gastropods, with Littorina saxatilis being the most abundant. Stereotypy of borings within Littorina saxatilis was measured using morphometric methods, including Bookstein Shape Coordinates analysis, angle measurements and predator-prey size comparison.

Bookstein Shape Coordinates analysis demonstrated that borings cluster within a relatively discrete morphospace associated with the body whorl adjacent to the aperture. Angle measurement indicated that the majority of boreholes are located in a narrow range, from 20 to 100 degrees, around the whorl from the aperture lip-whorl contact. Predator-prey size comparison showed a strong selectivity between predator size (using boring size as a proxy) and prey size (r = 0.69; p < 0.00001). This size stereotypy is most likely a result of the predator choosing its prey item to receive the greatest benefit, with the least amount of energy expended during foraging and grappling.

A high predatory intensity was calculated (attack frequency = 82.35%; success frequency = 99.55%) and is much higher than previous reports from soft-sediment environments along the western Atlantic (attack frequency = 5-30%). Possible explanations for these seemingly high numbers are likely a result of a bias in sampling technique; during collection, no living gastropod samples were removed, material collected was transported within the basin, and hermit crabs were abundant within the study area and selectively dwell within unbored shells.

Results presented here are consistent with findings from two other studies, which investigated naticid predation within northern latitudes of the western Atlantic. However, this study provides a localized, detailed examination of the most northern naticid habitat.