Anyone know anything about the tiny sometimes found in mussels?

We were on holiday in Normandy and Brittany this Summer and came across these in our mussels. It's because they are mussels fresh from the open sea and not from a mussel farm. In mussel farms the water is constantly filtered and nets keep most other life forms out. It's a bit off-putting to find so many little in all the mussels and I didn't enjoy the crunch. So I picked them out after that. You won't come to any harm from eating them.