Photosynthetic planulae and planktonic hydroids: contrasting strategies ofpropagule survival

Settlement delays can be important to prevent propagule waste when proper s ettling substrates are not immediately available. Under laboratory conditio ns, the planulae of Clytia viridicans underwent two alternative development al patterns. Some settled on the bottom, forming a hydranth-gonotheca compl ex that produced up to four medusae and later either degenerated or gave ri se to a hydroid colony. Other planulae settled right below the air-water in terface, forming floating colonies that eventually fell to the bottom and s ettled. Halecium nanum released planulae with a rich population of symbioti c zooxanthellae that survived into a rearing jar for three months. After a long period of apparent quiescence (possibly fuelled by photosynthetic acti vities of zooxanthellae) the planulae produced new colonies. Both photosynt hetic planulae and settlement at the interface air-water allow a delay in t he passage from a planktonic to a fully functional benthic life.