The Life Cycle of the Green Crab Carcinus Maenas at the Northern End of Its Range

Abstract

Along the central coast of Maine most female Carcinus maenas extruded their eggs in spring. The smallest ovigerous female was 34 mm in carapace width. Mating occurred from July to October, and in all mating pairs the male was larger than the female. Megalopae and Stage 1 crabs did not settle until late August, growing only to a mean of 5.5 mm carapace width (Stage 5) before winter. Renewed growth began the following June, and juveniles grew to 13-25 mm carapace width by their second winter. Most mature males molted by the end of July, whereas mature females molted from July to October, while guarded by males waiting to mate with them. On the central coast of Maine, therefore, C. maenas matures when 2-3 years old, breeds 2-3 times, and generation time is a minimum of 3 years. Compared with warmer water populations in southern Britain and Holland, the Maine coast population of C. maenas has later settlement of megalopae, slower growth, delayed maturity, longer generation time, and a longer life span. Such differences may account for the inability of populations to establish themselves in waters much colder than those of the central coast of Maine. The northern limits of the range of the green shore crab Carcinus maenas in the Gulf of Maine have reflected the periodic warming of the surface waters that occurred in the Gulf during the 1930's, 1950's and 1970's. During the warmer years the numbers of C. maenas increased in the northeastern part of the Gulf, but during the intervening colder years their numbers declined again and they then became uncommon north of the coast of central Maine (Welch, 1968, 1979). Do colder than average temperatures slow growth and delay larval recruitment to an extent sufficient to restrict the species from permanent or extensive colonization of the northern Gulf? The life cycle of C. maenas on the North American coast has not been examined in detail. Most accounts of its life cycle have described populations on the coasts of Britain and Holland (Broekhuysen, 1936; Crothers, 1967, 1968, 1970; Klein-Breteler, 1975a, b, c, 1976a, b) where water temperatures average 4-5?C warmer than on the coast of central Maine. The purpose of the present study is to compare the life cycle of C. maenas on the coast of central Maine with that of the populations from warmer waters as a way of assessing the ability of the species to penetrate cold-temperate regions.