Hydroclathrus clathratus is a common species on the rocky intertidal shores of the Island of São Miguel, Azores, mainly in spring and summer. Fertile saccate thalli appear in spring and disappear from late summer onward. Two types of life history were observed in culture: a direct-type and a heteromorphic, monophasic life history. In the direct type, reproductive cells from erect plants collected in the field developed directly into new erect thalli with plurilocular sporangia. In the heteromorphic life-history pattern, plurispores from the plurilocular sporangia of the erect thalli developed into filamentous prostrate microthalli. These developed unilocular and plurilocular sporangia in both shortday and long-day conditions at 15-22 ºC. Unispores released from the unilocular sporangia developed into saccate thalli, whereas plurispores gave rise to filamentous prostrate microthalli.