Sounds like Mud Crab. But boy - what a tiny size! All the recipes I've seen for Mud Crab are for the larger adult crabs - nothing for anything that small.
My only advice might be sauce. Way back when I was a little sprout & used to go crabbing with my family, & before there were size limits on Blue-Claw Crabs, while we only kept the large ones, Italian folks kept everything, regardless of size. My dad once asked one of these gentlemen what he did with those teeny-tiny Blue Claws & he said they just tossed them whole into Marinara sauce & let them simmer for a long while, then fished them out & tossed them.
Unless they're soft-shelled & can be eaten whole, I really can't think of any other way to use them.