Checking for cracks in Mussels, is a symmetrical crease/crack around top and base of mussel a normal part of the shell?

Cooking some mussels.

A lot of them seem to have a elliptical crease/crack that follows the ridges on the shell around the top and bottom of the mussel. Does anyone know if this is a crack or is it a feature of the shell?

Some don't have it, some have it more pronounced, some have it more subtly.

I thought it might just be a feature, but it could also be a crack if their shells are weaker at particular points.

Finally, should I cook or discard them?

Thanks!