Feeding Ecology of the Northern Moonsnail (Euspira heros) (Gastropoda: Naticidae) in Cape Breton, Nova Scotia

Abstract

Although the Northern Moonsnail (Euspira heros) is an important predator in the shallow, soft-sediment marine environments of Cape Breton, many aspects of its feeding ecology remain poorly understood. In this study I examined the diet breadth and prey selectivity of E. heros across its ontogeny at two localities: Port Morien Beach and Dominion Beach. I specifically asked: 1) Is the breadth of prey exploited by E. heros representative of prey availability?; 2) Does a moonsnail’s diet change across its ontogeny?; 3) Do moonsnails differentiate between different sizes of prey within a given species?; 4) Do moonsnails display site selectivity in where they drill their prey?; And 5) Does a predator of a given size produce the same borehole across prey taxa differing in shell thickness and composition. I addressed these objectives through beach collections of drilled and undrilled shells, and laboratory experiments to determine the relationship between borehole diameter and moonsnail size across prey species. Field evidence indicated that moonsnails have a broad diet consisting of 13 bivalve and 7 gastropod species, with some variation across locations based on prey availability. Larger moonsnails produced larger boreholes than smaller snails in laboratory experiments and, since this relationship did not vary across prey species examined, I used borehole diameters to assess ontogenetic changes in diet. With increasing size, moonsnails changed from a diet dominated by Soft-shelled Clams (Mya arenaria), Carpenter Tellins (Tellina carpenteri) and Gem Clams (Gemma gemma) to one focused on larger prey taxa including Atlantic Surfclams (Spisula solidissima) and conspecifics. In addition to the fact that larger predators tended to feed on larger prey items, larger snails also selected a wider range of prey sizes than smaller snails. Cost-benefit analysis showed that moonsnails may select prey based on energetic reward per unit drill time, although size constraints in handling prey also appear to strongly influence prey selection. Analysis of borehole site selection revealed that boreholes were frequently drilled in one particular area near the umbo in three bivalve species, but there was no preference for drilling one valve over the other. Borehole site selection was highly consistent within gastropod prey species as well, although the specific area selected varied among species. Through field surveys and lab experiments, I was able to demonstrate that E. heros has a broad diet of bivalves and gastropods, which is representative of prey availability, and changes across ontogeny. I have established that this naticid is size selective in the choice of its prey and is also selective in allocating drilling to a particular area of the prey shell. I have also determined that E. heros’ boreholes are consistent across different prey taxa. Subsequently, this study provides a more complete understanding of the diet breadth and feeding ecology of this naticid predator along the coastal shores of Cape Breton Island.