A sea crab with a dark purple shell marked with pale blotches, the most fished crab in the world, nearly all of it caught off China. Females full of red roe are sold as 红膏蟹.
梭子 means 'shuttle'. The crab's shell is shaped like the weaving shuttle that slides back and forth on a loom: flat, pointed at the sides, with back legs flattened into paddles.