Eider, shelduck, and other predators, the main producers of shell fragments in the Wadden Sea, palaeoecological implications

Seventy five percent by weight of the > 2 mm carbonate fraction of Wadden Sea sediments consists of fragmented shells, thirty percent > 8 mm and forty five per cent in the 2-8 mm fraction. Eiderducks (Somateria mollissima) feed mainly on mussels (Mytilus edulis) and cockles (Cerastoderma edule). Shells are crushed internally to fragments with a size-range from < 0-1 to 8 mm, twenty per cent were < 1 mm, sixty per cent 2-8 mm. One-third to one-half of the fragments in the 2-8 mm fraction in the sediments are due to eider predation alone. Other birds, crabs and fish probably produce the remaining fragments of this fraction. Shelduck (Tadorna tadorna) feed on the small gastropod Hydrobia ulvae; a varying amount (seventeen to thirty two per cent by weight) of shells was found intact in their faeces, but the remainder is fragmented. Around forty per cent by weight of Hydrobia shells in the Wadden Sea sediments is broken. This can be attributed to shelduck and other predators (e.g. knot) feeding on Hydrobia. Fragments in the > 8 mm fraction may also be produced by predators (shore crabs, oystercatchers). Physical destruction plays a minor role in the Wadden Sea. Shell fragmentation cannot be used as a measure of water turbulence. The high percentage of shell fragments indicates high predation pressure. However, the use of shell fragmentation to estimate predation pressure in fossil faunas is not possible, because some predators leave one (oystercatchers) or both valves (Asterias) intact. Despite high fragmentation fidelity of the death assemblage to the living fauna of the Wadden Sea is high. Physical destruction would leave only fragments of durable skeletons with low fidelity to the living fauna.