VALVE MORPHOGENESIS IN THE MARINE ARAPHID DIATOM GEPHYRIA MEDIA (BACILLARIOPHYCEAE)

Abstract

Valve morphogenesis in the marine araphid diatom Gephyria media W. Arnott was studied using dense populations epiphytic on red algae. Valves begin as a narrow sternum with virgae (pinnate lateral outgrowths). The apical pore fields and rimoportulae at each end of the concave valve are produced early in valve morphogenesis. The rimoportulae of the convex valve are also formed early and the virgae are branched at the apex. The timing of rimoportula formation well after formation of the virgae appears to contradict the hypothesis that the raphe system may have evolved from rimoportulae of araphid diatoms. Formation of the bi- and triseriate striae begins at the sternum with the lateral outgrowth of forked vimines, which then fuse. Completion of valve morphogenesis involves the development of an internal silica sheet. When the sheet is fully extended it forms an internal zigzag pattern of foramina near the valve margin in both kinds of valves, thus producing chambers. The rimoportula lengthens internally and in the final process forms the labiate process upon a stalk. Delicate internal rotae were observed in mature valves. Several features of valve morphogenesis in G. media are similar to those observed in other araphid and raphid species. The apical pore field develops in a manner similar to that of Gomphonema, the chambering as in Pinnularia and Catacombas and the rimoportula as in Diatoma.