Rolling Cockles: Shell Abrasion and Repair in a Living Bivalve Cerastoderma edule L.

ABSTRACT

Live cockles were eroded from a tidal flat by a storm event and naturally transported to a nearby dike and beach. Their fate was observed regularly for two months. Some died from desiccation high on the shore, and some were consumed by oystercatchers. Others were caught by the byssus threads of mussels inhabiting the intertidal area, while a few tried to reburrow high on the shore. These cockles were supposedly rolled while being transported to the beach and were later observed being rolled by the waves just below the high water line. Hence, it was concluded that the shells were subjected to abrasion and that this was the cause of the holes that some individuals developed near the umbo of one or both of their (still articulated) valves. A few had repaired such holes, supporting the hypothesis that these holes were made during rolling transportation. Similar shell assemblages of articulated cockleshells partly with subumbonal; holes were later found elsewhere on the Wadden Sea coast of Texel. This “natural” experiment and the repair indicated that this abrasion occurred while the cockles were still alive, and not after their death. It may also help to better discriminate between biologically and physically produced traces on shells.

KEYWORDS:

• Shell abrasion
• Cockles
• Cerastoderma edule
• Shell repair
• Storm deposition

Acknowledgments

This is part of my ongoing low-budget project “taphonomy of recent shells,” (earlier presented as a poster, Cadée, 2013b). I am grateful to the facilities and office space the directors of NIOZ provided after my retirement. I am very grateful to my NIOZ colleague Thalia Watmough who carefully corrected my English and to Antonio Checa (Granada University) for comments on an earlier draft of the manuscript. I thank George Pemberton for asking me for a contribution to this Seilacher Memorial Volume. I have met Dolf only a few times, but admired his enthusiastic lectures, and his well-illustrated books and papers will remain a great joy to consult. I also thank the reviewers Anthony J. Martin and George Pemberton for their critical remarks.