Scrub and rinse the crabs thoroughly. Detach the claws from each crab. Lift off the carapace and discard. Scrape out any roe and discard the gills. Rinse well, halve the crabs with a cleaver and crack the claws with a mallet.
Heat the oil in a wok over high heat and stir-fry the garlic and black pepper until fragrant, about 30 seconds. Add the crabs and stir-fr