Clinging, still living slippers have been washing up on the beaches of the Wadden Islands for weeks in a row. “Very special, but not worrying,” says professor David Thieltges, who is fascinated by this “beautiful, bizarre sea snail”.
‘Slippers. Hundreds of thousands have been washing up on the beach for weeks,” tweets Richard Kiewiet, retired nature manager who can be found on the beach of Ameland almost every day. ‘Never seen before’.
‘A slipper orgy!’, islander Inge Klinkert responds to X. ‘The beach is full of them. Slippers reproduce in chains. A male slipper attaches to a female slipper, fertilizes her and then turns into a female herself.
It is really special that so many slippers wash up, confirms Thieltges, a marine ecologist affiliated with the NIOZ research institute and the University of Groningen. Not only on Ameland, they are also in the surf on the beaches of Terschelling, Schiermonnikoog and Vlieland.
Where the stranded sea snails come from is anyone’s guess, says the German-born scientist. The slipper is an invasive species. The sea snail arrived with American oysters in the nineteenth century, when experiments were carried out in England.
The slippers were first observed in the Wadden Sea in the 1930s, near the German island of Sylt. This is where the largest colony of slippers in the Wadden Sea is located, but the animal is now found in many places in Europe, provided it is not too deep. The greatest densities are located off the coast of France.
A-typical sea snail
“For a moment there was a bit of fear that the slipper would compete with the oyster, but that doesn’t happen very often,” says Thieltges, who specializes in invasive species. The slipper is an atypical sea snail. The animal does not graze algae on the seabed, like most snail species, but filters water for plankton, just like oysters and mussels.
What makes the species even more “bizarre” is that it is a ‘protandrous hermaphrodite’. This means that the slipper changes gender during its life. “That is why you find long chains on the tidal flats, sometimes containing twelve or more males and females.”
The snail does not have an opening that they can close off, like a periwinkle or a common garden snail. That is why a slipper usually attaches itself to another, larger shellfish. On the Wadden Islands they currently mainly wash up on fairly large, golden brown nipple horns, snails of the Naticidae family.
Because the washed-up snails are relatively heavy and still alive, Thieltges and shellfish expert Loran Kleine Schaars suspect that they do not come from very far away. “The Channel is not that likely. They may rather come from the deeper channels of the Wadden Islands, where empty nipple horns ended up in the North Sea due to sand nourishment. So maybe the beach finds come from there.”
It remains to be seen whether there is a link with climate change, just like with the previous mysterious mass washing up of bryozoans. However, Thieltges does not find the mass death worrying. “It is not a species that has increased dramatically. What we do know is that slippers don’t like cold winters. That has been a drag on growth. In recent years we have had less cold winters and the species has increased in Germany. Maybe there is a connection.”