Massaal aanspoelen van lege wulkenschelpen, schanserwaard Texel

The whelk Buccinum undatum no longer lives in the Dutch Wadden Sea, but empty shells are still present and a few can usually be found along the shore. In December 1995 suddenly many empty shells were cast on a beach in the southern part of the island Texel, where this had not occurred before in the last 30 years. This sudden mass occurrence is probably related to reconstructions in 1991 of the nearby harbour of the Netherlands Institute for Sea Research (NIOZ) (Fig. 2). A dike was build between the tidal flat and the harbour after which a sandbar is building up on the tidal flat behind the harbour (Fig. 2b) from material eroded from the tidal flat in front. In December 1995 strong E and SE winds caused extremely low tides and enhanced erosion of the deeper tidal flats with subsequent transport of shells including the whelks. More than 500 whelks were collected, 200 were relatively intact and could be used for a closer inspection. They were all large (Fig. 1) with an average size of c. 65mm. Probably all had been occupied by hermitcrabs as indicated by epizoans such as Hydractinia echinata. The boring polychaete Polydora ciliaca was present in 99% of the shells. Shell repairs occurred in 68% of the shells, sometimes shells had more than one repair scar. In total 204 repair scars were observed in 200 shells i.e. 1 per shell on average. This repair frequency is much higher than the 9% found in 2 other common gastropods in the Wadden Sea: Littorina littorea and Peringia ulvae (Cadée 1994). These were related to unsuccessful attacks of crab predators. This suggests an additional cause for non-lethal shell damage in whelks, most probably (whelk)fishery. The finding of high shell damage in a sample of whelks fished in the Wadden Sea in 1926 (Cadée, 1995b), when they were still abundant and heavily fished upon, supports this conclusion.