Life history and population dynamics of the Arctic sympagic amphipods Onisimus nanseni Sars and O-glacialis Sars (Gammaridea: Lysianassidae)

The two sympagic amphipod species Onisimus nanseni Sars and O. glacialis Sars (Lysianassidae) are important constituents of the macrofaunal community in Arctic sea ice but little, as yet, is known about their autecology. In this paper we provide a description of the life cycles and population dynamics of these two species that were sampled beneath the ice pack surrounding Svalbard/Norway in September 2002 and May 2003. The distribution of Onisimus spp. was patchy and their numbers were low in sea ice throughout the year (similar to 0.8 ind. m(-2) in spring, similar to 1.0 ind. m(-2) in autumn). In May the juveniles accounted for 37% and 22% of the population of O. nanseni and O. glacialis, respectively. The population structure of O. glacialis was relatively constant in spring and autumn, but of O. nanseni a shift towards a more mature structure was observed in September. Ovigerous females were virtually absent in both species during both sampling periods. Length-frequency distribution analysis on spring- and autumn-data showed three cohorts for O. nanseni and four cohorts for O. glacialis and suggests a temporal shift in the reproductive cycles of these two species. Both species appear to be univoltine (one generation per year) and probably iteroparous (several broods per female lifespan). The offspring of O. nanseni is presumably released in late spring and shows secondary sexual characters the first year allowing for reproduction in year 0+. In contrast, O. glacialis leaves the brood pouch in early spring and sexually matures in year 1+. The growth constants were comparable for the two species. The estimated life span was 3.5 years for O. glacialis and 2.5 years for O. nanseni.