In the Wadden Sea, shell repair frequency in the small gastropod Hydrobia ulvae varied from 2.8% to 11.2%. On tidal flats of the Mok, a small bay on the island of Texel, The Netherlands, in the Wadden Sea, higher repair frequencies varying from 11.8% to 41.8% were measured. The shelduck, Tadorna tadorna, a predator of Hydrobia, occurs here in densities far above average densities for the Wadden Sea. Shelducks ingest their prey whole and crush the shells of H. ulvae internally. Live specimens of H. ulvae were collected from shelduck feces. Those with intact operculum and only a damaged outer aperture rim of the shell were kept in aquaria and repaired their shell rapidly. This indicates that predators that ingest shelled prey can also leave repair scars on shells. Such scars, however, are indistinguishable from those resulting from failed predation by predators using such pre-ingestive shell breakage as decapod crustaceans.

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