For what it's worth, years ago I worked for a summer out on Martha's Vineyard
(not my favorite place - too much money there. Gimme Block Island any day.)
We often collected mussels because first, they were the tastiest shellfish,
and second, because they were free, easy to collect and required no license.
They often had pea crabs in them. We just ate them along with the mussels. If
you find some who lives on MV (probably because it is their favorite place)
perhaps they'd be able to scare you up some pea crabs.