Reorganization of the nervous system during metamorphosis of a hydrozoan planula

Abstract. Laser scanning confocal microscopy is used to reveal the changes that occur in the RFamide-positive nerve net as a free-swimming, solid hydrozoan planula larva is transformed into a sessile, hollow, young polyp. Seven stages of development in Pennaria tiarella are described: planula competent to metamorphose, attaching planula, disc, pawn, crown, developing polyp, and developed primary polyp. The RFamide-positive nervous system undergoes dramatic reorganization during metamorphosis: (1) larval neurons degenerate; (2) new neurons differentiate and reform a nerve net; and (3) the overall distribution pattern of the nervous system changes. This study confirms earlier observations on RFamide-positive neurons of Hydractinia which also show the loss of these cells after the onset of metamorphosis.