What's eating mussells?

[wildliferulesok]

hiya All! First post..Does anyone have any idea what is eating the large freshwater swan mussells on a nearby river. I say river, it's the outpouring from a millpond weir which overflows when it's been raining. Sometimes the river is high. The mussells are in a deep layer of silt - no human footprints or broken down vegetation. I saw there was a similar thread in 2006 which blamed humans but I agree with the replies that this is not the answer.

There is absolutely no damage to the majority of the shells. They have been opened and completely cleaned out. There are no bite marks, scratches or any other damage. They are spread out over quite a wide area every day so I think that whatever is taking them is nocturnal. I would dearly love to think it's an otter - or mink but mink are ''not supposed to eat mussells but otters might'' - but as said, they haven't 'read the book'!!
Any ideas - please!!