Modiolus mussels do occur worldwide.  I am not aware of their being
raised for food like Mytilus, though perhaps some species in some
countries may be.  Here in New England, the big mussel Modiolus
modiolus is common, as well as the ribbed mussel Geukensia demissa,
but no one eats either one of them, even though one Modiolus would
provide 3 or 4 times as much "meat" as one Mytilus.  The reason -
they don't taste good.  I have never actually tasted either one, but
they do not enjoy the reputation for palatability that the Mytilus
mussels do.
Paul M.
Rhode Island