SEXUAL REPRODUCTION AND AUXOSPORE FORMATION OF THE MARINE MONORAPHID DIATOM COCCONEIS PELLUCIDA

Abstract

Sexual reproduction and auxospore formation of Cocconeis pellucida have been observed using a laboratory culture. This diatom was monoecious. Auxospores occurred at 18°C and 14°C, but not at 5°C. One gamete is formed in a gametangium, without cytokinesis. Each gametangium secretes gelatinous material near where it contacts the other gametangium. Sexual reproduction occurs via behavioural anisogamy and the single auxospore expands spherically. Sexual reproduction and auxospore formation corresponds to Type IIB1 of Geitler's system. The perizonium consists of transverse bands, which exhibit a marked dorsal angle on the dorsal side of the auxospore. The open ends of the transverse bands converge towards the centre of the ventral side of the auxospore, instead of forming a suture as in other pennate diatoms. The auxospore forms first a raphid concave valve and then an araphid convex valve. The initial cell is like a vegetative cell, except for the presence of two plastids. C. pellucida differs from C. placentula in the presence of a raphid valve in the initial cell, the absence of unequal cytokinesis during gametogenesis, delay in the division of the initial cell until after it has left the perizonium, and survival of both daughter cells following division of the initial cell. Cocconeis is unique among pennate diatoms in the formation of a spherical auxospore and die proportional decrease of valve length and width during the life cycle, which are otherwise characteristic of centric diatoms.