Belichnus Traces Produced on Shells of the Bivalve Lutraria lutraria by Gulls

Abstract

In February 2011, after a storm, thousands of adult, articulated, and still-living common otter shells Lutraria lutraria (L. 1758) were stranded on the North Sea beach of the Island Texel (NL). These 9 to12 cm long bivalves were rapidly found and consumed by both herring- and lesser black-backed gulls. Holes, irregular in outline, were observed in some 10% of the articulated shells of these bivalves. These holes were always smaller on the outside of the valves than on the inside and varied in size from 1 to 20 mm (outside) to 4 to 22 mm (inside). Often the other valve was crushed indicating consumption by gulls. We concluded that these holes were made by the gulls probing the shells; in a few cases, we observed that valves were broken starting from such a hole. Such traces are described in the literature as the ichnogenus Belichnus and were until now attributed to Stomatopoda only. We also suggest that a separate ichnospecies name for two Belichnus holes in one shell should not be used, as we see them simply as a double injury due to two blows. Our findings stress once more the importance of avoiding premature phylogenetic interpretation of traces and the use of a separate ichnotaxonomy.

Keywords:

• Belichnus
• Ichnotaxonomy
• Bivalves
• Birds
• Predators
• North Sea

ACKNOWLEDGMENTS

We thank directors of the Royal Netherlands Institute for Sea Research (NIOZ) for giving us space and facilities to continue work after our retirement. We are very grateful to the Texelse Courant for giving us permission to use Figure 1; to Arthur Oosterbaan and Pierre Bonnet for sharing with us their observations on gulls feasting on Lutraria shortly after stranding; to NIOZ-colleagues Magda Bergman, Rogier Daan, and Maarten Mulder for data on North Sea benthos; and to two anonymous referees and co-Editor-in-Chief Murray Gingras for very useful and encouraging remarks to improve our manuscript. The second author commented on earlier versions of this manuscript; we had many interesting discussions. Unfortunately, he died suddenly on November 13, 2011, after a last visit to his beloved beach.