Life history of Japanese Stypocaulon durum (Sphacelariales, Phaeophyceae)

Abstract

Phenology, morphology, life history and responses to different temperature and photoperiod conditions were studied in Japanese Stypocaulon durum (Ruprecht) Okamura. Erect thalli of the species were collected year-round, but the mature thalli forming either unilocular sporangia or two different types of plurilocular structures (evidently gametangia) on separate thalli were found only in winter. In culture, an isomorphic life history is suggested for the species, alternating between a sporophyte forming unilocular sporangia and gametophytes forming plurilocular macro-(female) and micro-(male) gametangia. Contents of unilocular sporangia were not released, but germinated in situ, developing into erect thalli forming plurilocular gametangia. Macrogametangia released aplanogametes (oospores), but male gametangia appeared to be non-functional, although flagellated cells were once formed in the loculi. This is the first report of plurilocular gametangia in the species. Although the species grew well and matured under considerably lower temperature conditions than European Stypocaulon scoparium (L.) Sauvageau, its temperature requirements showed similarity to northwestern Atlantic Stypocaulon species. This supports the notion that northwestern Atlantic Stypocaulon is conspecific with S. durum.

A revised classification of the Sphacelariales (Phaeophyceae) inferred from apsbC andrbcL based phylogeny

“…According to Prud'homme van Reine (1991Reine ( , 1993 and Kawai & Prud'homme van Reine (1998) the genera Stypocaulon and Halopteris can be separated on the basis of differences in reproduction and branching pattern. The present results, however, clearly indicate that the genus Stypocaulon cannot be maintained and should be merged with the genus Halopteris.…”