Grasping behavior of male snow crab Chionoecetes opilio (O. Fabricius, 1788) (Decapoda, Majidae)

The grasping behavior of snow crab, Chionoecetes opilio, was observed in 43 males when only male crabs were collectively held in an aquarium. A larger male always grasped a smaller sized male which exhibited submissive behavior similar to a mature female in a mating pair. This grasping behavior was observed in terminal molt larger sized males except for one case observed for one intermolt juvenile male. During the mating season, although a wide size range of morphometrically mature males was present (51.0-131.0 mm in carapace width) in the wild population, mainly larger sized morphometrically mature males (72.1-128.4 mm in carapace width) participated in the mating courtship. A small number of solitary males was observed on the mating ground and their sizes were comparable to the males in mating pairs. Among the males in mating pairs, the larger sized males tend to occupy favorable mating grounds at greater depths, while the smaller sized males are found on less favorable grounds. The grasping behavior observed in the aquarium and in the wild suggests that the male-male grasping behavior appears to be related to the establishment of hierarchy among the mating males. Smaller sized morphometrically mature males appear to be eliminated before the mating pairs are formed and appear on the mating ground. On the mating ground, the second phase of competition among males occurs for favorable mating grounds.