• auxospore, Rhoicosphenia;
  • morphogenesis, diatom;
  • perizonium, diatom;
  • Rhoicosphenia;
  • sexual reproduction, diatom


Reproduction in Rhoicosphenia curvata (Kütz.) Grun. is isogamous. The two auxospores formed expand parallel to the apical axes of the gametangial cells. Expansion is bipolar and leads to the formation of a slightly curved, tapering cell, in which the initial valves are laid down. The perizonium consists of transverse and longitudinal bands. The transverse series, of 35 or so bands, is laid down centrifugally as the auxospore expands and can be classified into three groups on the basis of band morphology. All except the central band are open hoops, orientated so that their ends lie in the midline of the less convex, ventral side of the auxospore. The bands have fimbriate margins on one or both sides, and overlap one another from center to either pole. The longitudinal series includes 5 bands—a wide central band, with two on either side: again, the bands overlap one another from the center outwards. The initial epivalve of the new generation forms beneath the dorsal side of the auxospore, on the opposite side from the longitudinal perizonial series. Comparisons are made with other genera and the relevance of auxospore studies to an understanding of diatom morphogenesis is discussed.