Field and experimental studies were conducted in a population of C. maenas inhabiting the Menai Straits, North Wales. Male crabs showed a higher degree of chela loss than females. The percentage of males with missing chelae appears to be related to size. Field results suggest that the loss of a chela constitutes a handicap to a male crab when trying to mate; laboratory experiments indicate that this occurs when a male crab is either competing for or defending a paired premoult female.