Comparative Study of Postlarval Life-History Schedules in Four Sympatric Species of Cancer (Decapoda: Brachyura: Cancridae)

Abstract

Size-at-instar, growth-per-molt, reproductive schedules, and morphometric allometries were investigated in four sympatric species of Cancer (magister, the Dungeness crab, gracilis, productus, and oregonensis) in Garrison Bay, North Puget Sound. Complementary observations were made on mating systems, mortality, habitat utilization patterns, and feeding. Numerical methods were successfully employed to discriminate instars in size-frequency distributions. Growth pattern, contrary to our expectation, was determinate in the four species. Geographic variation in prereproductive growth rate of C. magister is attributed to environmental factors. It is suggested that an independent stock may inhabit the Strait of Georgia-North Puget Sound area. Observations on mating behavior suggest that these polygynic species have different types of mating systems, leaning towards resource defense in C. oregonensis, female defense in C. gracilis (and perhaps also in C. productus), and explosive breeding assemblages in C. magister. Degree of sexual dimorphism is consistent with this hypothesis. Adult males of C. gracilis, C. productus, and C. oregonensis have proportionally larger chelae than females; no significant dimorphism was detected in C. magister. Male C. gracilis and C. productus show two clear allometric phases in the chela-carapace size relation. Contemporary studies of diversity within decapod guilds have frequently been done with food-resource partitioning as an explicit or implicit hypothesis. In contrast, we stress the importance of habitat, mating systems, and sexual selection as primary mechanisms underlying the diversification of this genus.