How do swimming crabs use their legs for propulsion?

Swimming crabs, which belong to the family Portunidae within the class Crustacea, possess a specialized adaptation for movement. Unlike many other malacostracans that rely primarily on abdominal pleopods for swimming, swimming crabs utilize their fifth, or hindmost, pair of thoracic legs for propulsion.12

These specific legs are flattened into distinct, paddlelike structures that allow the crab to propel itself through the water. While other malacostracans may use their tail fans for sudden backward escape reactions or abdominal appendages for general swimming, the swimming crab's modified thoracic appendages are the primary mechanism for their characteristic swimming motion.12