Studies on the association of Metopa glacialis (Amphipoda, Crustacea) and Musculus discors (Mollusca, Mytilidae)

Associations between amphipods and mollusks are known from several examples, but the possible life-history strategies these amphipods utilize are often unknown. We present here the autecology of the stenothoid amphipod Metopa glacialis, an associate of the mytilid bivalve Musculus discors. Samples collected from northeast Spitsbergen of 279 M. discors show a prevalence of M. glacialis in 75%, with a variation between 1 and 41 amphipods within one mussel, the mean being 3.7. The associated amphipods have been analysed for several life-history traits such as size distributions, brood sizes and cohort compositions. Analyses of fatty acid composition of the associated species indicate different food sources for the amphipods and mollusks. Based on our data we propose two possible life-history strategies for M. glacialis: 3 or more seasons of single broods and parental care over more than 1 year after hatching, or multiple broods in one season and extended parental care. To our knowledge multiple broods in one season and extended parental care have never been reported in arctic amphipods.