Mother-young cohabitation in Phronimella elongata and Phronima spp. (Amphipoda, Hyperiidea, Phronimidae)

Reproductive characteristics of poorly studied hyperiid amphipods, especially Phronimella elongata, are described. Among nine species of hand-collected phronimids, females of Phronimella elongata, Phronima dunbari and Phronima sedentaria cohabited with their young in a 'barrel' derived from gelatinous zooplankton. Five growth stages of young were identified in Phronimella elongata: young of stages I-IV were found with the mother in the barrel, but stage-V young clinged to the mother retaining no barrel. The cohabiting young in Phronimella elongata and Phronima sedentaria always consisted of single growth stage. In a single female of Phronima dunbari, however, two different stages of young coexisted in a barrel. In Phronimella elongata, the mother size significantly correlated with the number of brooded eggs or cohabiting young regardless of growth stages of young. Considerable variation in the number of eggs or young per female among phronimid species may attribute to the differences in the volumes of the brood pouches.