PATTERNS OF SHOREBIRD PREDATION TRACES ON ATLANTIC SURF CLAM VALVES

A growing dataset (>80) of irregular openings on the valves of Atlantic surf clams (Spisula solidissima) is attributed to shorebird predation (Belichnus isp.). Easily distinguishable from gastropod drillholes (Oichnus isp.), these traces have characteristic enlarged inner diameter (exit impact peeling). The majority of Belichnus traces occur slightly dorsally of the valve center (dorso-ventral distance from umbo/shell length ratio of ~0.2), with some proximal to the umbo and posterior edge. Fewer than 5% of shells exhibit outer surface peeling or two adjacent openings, which likely results from multiple beak impacts. Many shells preserve slight indentations along the ventral edge, where the valves were pried apart following the initial impact. In laboratory experiments, Belichnus-like breakage patterns can be created by point pressures of 1.50-1.75 kg/cm². Pressures >2.00 kg/cm² typically resulted in shell cracking or breakage, with similar patterns observed on many mollusks in the field (Revere Beach, Massachusetts; central coast of New Jersey), especially those of bivalves with softer and thinner shells than S. solidissima. The absence of Belichnus on large stretches of adjacent beaches may reflect the presence of specific bird species or predation patterns, especially on the largest 10-20% of surf clams. Research is ongoing at quantifying the 3D morphology of avian predation traces, exploring the relationships of Belichnus placement to the vital organs and muscles of the prey, and establishing diagnostic criteria that will distinguish it from traces of other predators and physical impact features (tool marks).