CHELA LOSS IN THE SHORE CRAB CARCINUS-MAENAS (CRUSTACEA, BRACHYURA) AND ITS EFFECT ON MATING SUCCESS

Field and experimental studies were conducted to determine the inciden ce of chela loss and its effect on mating success in a population of t he shore crab Carcinus maenas (Linnaeus, 1758) inhabiting the Menai St raits, North Wales. The study was performed between 1989 and 1993. Mal e crabs showed a higher degree of chela loss (12.5%) than females (7.9 %). In males, frequencies ranged from similar to 10% at sizes < 50 mm CW (carapace width) up to similar to 30% in the largest crabs of 70 to 80 mm CW. The percentage of females with missing chelae appears to be unrelated to size. The most common type of chela loss in the populati on studied was of a missing crusher chela of right-handed crabs. Red c rabs, which are assumed to be in prolonged intermoult, had a much high er degree of chela loss (20.5%) than the green, early intermoult crabs (9.7%). The proportion of red crabs with chela losses increased with size, possibly reflecting an increase in intermoult duration with size . In green crabs, there was no such increase. The proportion of male c rabs with missing chelae found in mating pairs in the field was much l ower than that found in the adult unpaired population, suggesting that the loss of a chela constitutes a handicap to a male crab when trying to mate. Also, by studying the relative frequencies of different cate gories of chela loss, it is suggested that the loss of a crusher chela exerts a more deleterious effect than the loss of a cutter. Experimen ts were performed in the laboratory where pairs in pre-copula were con fronted with an additional single male in various combinations of size s and patten of chela loss: These showed that the loss of a chela cons titutes a handicap for a male crab when either competing for or defend ing a paired pre-moult female. This handicap was estimated to be equiv alent to a reduction in size of 7 to 8 mm CW relative to the size of t he competitor.