Fragmentatie van Ensis directus-schelpen door zilvermeeuwen

[Fragmentation of Ensis directus-shells by Herring gulls] The American razor clam Ensis directus is a successful non-indigenous invader in the Wadden Sea since the late 1970s. It often shows mass mortalities in the winter half-year (also known from its native area), of which the cause is poorly understood. During such mass mortalities Ensis directus leaves partly its burrow and such protruding Ensis, unable to re-burrow, become an easy prey for Herring gulls during low tide. In March 1999 I studied Ensis-shellhandling and -fragmentation by Herring gulls along part of the Wadden Sea dike on Texel. Herring gulls carried the collected specimens to a dry place (Fig. 1), where they started shaking the shell vigorously, dropping it in between such periods of shaking to consume the protruding meat. In some cases they were able to get at the meat without damaging the shells, in other cases this shaking caused characteristic shellfragmentation (Fig. 2) ranging from small fragments broken from one or both valves near the middle, or one valve broken near the middle, to both valves broken near the middle, but the remnants of the anterior part still adhering to each other by the ligament. Shell hammering plays a subordinate role. No other predators are known to produce similar shellfragments of Ensis. Shellhandlingtime was only 1 to 2 minutes.