Cannibalism in the snow crab, Chionoecetes opilio (O. Fabricius) (Brachyura: Majidae), and its potential importance to recruitment

Abstract

Density-dependent cannibalism, either on early benthic instars or on late prerecruits, has been hypothesized to regulate recruitment in the snow crab Chionoecetes opilio (O. Fabricius). However, there is little direct empirical or experimental evidence to support these hypotheses. We examined the stomach contents of 1218 C. opilio of 12–135 mm carapace width (CW) for evidence of cannibalism. Crabs were collected by beam trawl in 1993 and 1994 in the estuary and northwest gulf of the Saint Lawrence River (Canada). Complementary laboratory experiments served to constitute a library of digested crab body parts for reference in the identification of stomach contents, and to evaluate the possibility and size-specificity of cannibalism on C. opilio instar I (settlers: 3 mm CW). The natural diet of C. opilio changed with increasing body size, with smallest crabs feeding mainly on amphipods and ophiuroids, while the largest crabs fed mainly on annelids, crustacean decapods and fish. Overall, conspecifics other than exuviae occurred in the stomachs of 7.2% of wild caught C. opilio and were probably derived through predation rather than scavenged. Non-exuvial conspecifics were found most frequently in the stomachs of adult males (sperm-producing, chelae differentiated), with occurrence declining significantly from 22% to 6% of adult males by 10 mm CW size class over the range of 50–120 mm CW. Conspecific prey ranged in size from 3.9–48.8 mm CW, but most were immature crabs belonging to molt instars V (≈ 15 mm CW), VI (≈ 20 mm CW) and VII (≈ 28 mm CW). In the laboratory, instar I crabs were cannibalized by males of 8–50 mm CW, but not by males of 51–130 mm CW. Based on laboratory and field results, there are relative lower and upper size thresholds for vulnerability of intermolt crabs to cannibalism, which vary with predator size. We conclude that intraspecific predation may reduce cohort strength mainly over the first 4 years following settlement.