Life history of Onisimus caricus (Amphipoda: Lysianassoidea) in a high Arctic fjord

Knowing the life history of individual species is essential for understanding the functionality of the ecosystems in which they occur. In this study we present the life-history traits of the benthic scavenging amphipod Onisimus caricus Hansen, 1887, observed in Adventfjorden (78 degrees 14' N, 15 degrees 37' E) in Svalbard. The sampling campaign was carried out on a monthly basis throughout a full year (September 2006 to August 2007) using baited traps. The results show that O. caricus is a typical adverse-selective (A-selective) species with a 5 yr life cycle. During the first 2 yr O. caricus individuals do not appear to develop any visible external sexual characters, but these are detectable in 3 yr old immature specimens. Males start to mature in the fourth year and may be iteroparous, depending on whether they mature before the mating season in the fourth year and survive to the mating season in the fifth year. Females are semelparous, maturing in their fifth year. Reproduction takes place in January to February, and juveniles are released in July to August. Unlike many other Arctic scavenging amphipods that are dependant upon the spring bloom for releasing the brood, the release of juveniles is synchronized with peak mortality of zooplankton late in the melting season, which results in a large quantity of dead organic material on which juveniles can feed.