There's two ways you can tell if a crab is a black-fingered mud crab just by looking at its claws. One is to look for dark colored tips on the ends, or "fingers," of its claws, and the other characteristic to look for is a "tooth" on the larger of the crab's two claws. They use this larger claw to hunt their prey.
Common prey for the mud crab is the marsh periwinkle snail. Black-fingered mud crabs help maintain healthy snail populations, and this benefits marsh ecosystems because if snail populations get too high, they could destroy the plant life in a wetland.
Keep an eye out for these cute crustaceans next time you are near a marsh!