Medusa loss in leptolid Hydrozoa (Cnidaria), hydroid rafting, and abbreviated life-cycles among their remote-island faunae: an interim review
Paul F.S. Cornelius

Despite apparent dispersive advantage, the medusa stage has been lost from the life cycle in many evolutionary lines of medusozoan Cnidaria. About 70 such independent losses have been identified. Most are in the roughly 400-500 recognised genera of hydroids and hydromedusae. The frequency and incidence of medusa loss are analyzed, especially among the thecate and athecate hydroids and their medusae. The names Leptothecata nom. nov. and Anthoathecata nom. nov. are introduced for these two composite taxa. Advantages of hydromedusa release are compared with those of other dispersive methods. Rafting of the hydroid stage may account for unusually wide species distributions in the group and for the occurrence of just a few dozen ubiquitous species repeatedly in remote localities such as oceanic islands, recognised as a "species club". Nearly all so-far identified have reduced dispersive stages advantageous to remote-island survival. Possible rapid evolution of medusa loss in such precarious localities is discussed, but the habit may simply be pre-adaptive to such existence. The term "costaphily" is introduced to describe the tendency of a planula to settle within hours to a few days, on adjacent coastlines, rather than extremely close to a parent or other member of its species, which is distinguished by the existing and now subsidiary term philopatry.

Keywords: Medusa release, leptolid, evolution, life cycle, oceanic island faunae, rafting, philopatry, costaphily, cosmopolitanism, species club, hydroid, dispersal.
Contents of this volume Sci. Mar. 56(2-3) : 245-261 Back PDF
Please, send comments to
ICM (CSIC) - Passeig Marítim de la Barceloneta, 37-49. E-08003 Barcelona (Spain)