Life under Arctic sea ice - reproduction strategies of two sympagic (ice-associated) amphipod species, Gammarus wilkitzkii and Apherusa glacialis

The reproductive strategies of two gammaridean amphipod species, Gammarus w ilkitzkii and Apherusa glacialis, that permanently inhabit the Arctic sea i ce were investigated. G. wilkitzkii reaches sexual maturity at an age of tw o years and produces 128 +/- 54 eggs fem.(-1) yr(-1). Mating takes place du ring fall and winter, and the development of the large eggs (0.60 to 0.80 m m diam.) lasts 6 to 7 months. The sex ratio of G. wilkitzkii was dominated by males in a proportion of 1.5:1. In vivo studies showed that juveniles ar e released in batches from the brood pouches of the females during April an d May. A. glacialis reaches sexual maturity at the age of 1 year and produc es 555 +/- 151 eggs fem.(-1) yr(-1). The eggs are between 0.18 and 0.23 mm in diameter, and are the smallest known for gammaridean amphipods. Eggs are kept in packages of two to eight in the brood pouches of females. The sex ratio of A. glacialis was dominated by females in a proportion of 3:1. The high fecundity of both amphipod species, the release of juveniles in batche s over a period of time, a high proportion of females (A. glacialis), and a n elongated life-span with multiple spawnings (G. wilkitzkii) are discussed as possible adaptations to the specific and highly variable conditions und er Arctic sea ice.