The morphological life history of the marine brown alga, Petalonia fascia (O. F. Müll.) Kuntze, has been investigated in unialgal culture, using defined culture media with different iodide concentrations. The zoospores produced from the plurilocular sporangia of P. fascia blades developed directly into protonemata, plethysmothalli, or Ralfsia-like thalli, depending upon the iodide concentration. Protonemata and plethysmothalli survived in iodide-free media. For development of Ralfsia-like thalli and blades the minimal iodide concentrations required were 50.76 × 102 μg/1 and 50.76 × 101 μg/1, respectively. Iodine appears to be an essential element for growth, morphogenesis, and reproduction of P. fascia.
