Reorganization of the nervous system during metamorphosis of a hydrozoan planula

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.