MIGLIETTA, M.P.; CUNNINGHAM, C.W; Duke University; Duke University: The role of the planktonic stage in hydrozoans: ecological interpretation of the medusa loss

The Hydrozoa (phylum Cnidaria) have a complex life cycle with a planktonic, sexually reproducing stage (the medusa) and a benthic, clonal one (the polyp). The medusa, being planktonic, has been considered as the vagile stage. There is a trend in hydrozoan evolution toward the loss or reduction of the medusa stage trough an acceleration of gonad development (heterochronic event), which results in reproduction by a reduced medusa (released from the colony with gonads already mature) or a fixed sporosacs (extremely reduced and modified medusa that never detaches from the colony). The Hydractiniidae are a family of hydrozoans which species show all the possible reproductive strategies (reproducing by medusae, reduced medusa or fixed sporosacs). The phylogeny of the Hydractiniidae built using 6 genes, is here used to infer how many times the medusa has been lost within the family. From the phylogeny can be inferred that the genus Podocoryna (with medusae) is monophyletic and well nested in the tree, indicating that the medusa stage has been either lost many times independently within the family or lost and regained once. The distribution and speciation patterns of the genus Podocoryna (with medusae) and the sister genus Clava (lacking medusae) are compared in order to infer whether having the medusa results in higher gene flow and therefore less speciation. Ultimately the life cycle of the genera Podocoryna and Turritopsis (both having medusa stage) are compared in order to investigate heterchronic patterns at the medusa level and address questions on the role of the planktonic stage in Hydrozoans and on the ecological forces driving its loss.