Species Description: This rare species was originally found in the Salton Sea, California (Sterrenburg et al., 2000). As in the original description, the valve is flat sigmoid and lanceolate, tapering to acute apices (Fig. 1, SEM of fractured valve). On the valve interior, the areolae are mostly bisected distal to the raphe area (Fig. 2, detail of Fig. 1) and the valve center has a small hyaline area surrounded by convex raised bars. The central raphe ends are straight, though the original description found that the raphe ends are undulate on the valve exterior, which was not seen. Only one damaged specimen was seen, slightly larger than the Salton Sea material, at about 123µm long and about 17.5µm wide, with 31-33 transverse striae in 10µ and 32-34 oblique striae in 10µm, with the striae crossing at an angle of approximately 55 degrees. The measurements are approximate because of damage to the valve. The specimen was found in the IRL near Vero Beach.

Sterrenburg FAS, Tiffany MA, Lange CB. 2000. Studies on the genera Gyrosigma and Pleurosigma (Bacillariophyceae). Species from the Salton Sea, California, U.S.A., including Pleurosigma ambrosianum, nov. sp. Proc Acad Nat Sci Philadelphia 150: 305-313.

Annulus: A hyaline ring of slightly thickened silicon on a valve surface.

Areolae: A regularly repeated perforation through the cell wall.

Auxospore: A special cell that restores cell size; normally a result of sexual reproduction.

Auxospore Envelope: A hyaline envelope that surrounds the auxospore cell.

Auxosporulation: The process of auxospore formation.

Capitate: Head-shaped.

Fascia: A plain, unthickened transverse strip.

Foramina: Opening on one side of a complex areola.

Homothallic: Possessing both male and female capability in a single morphologically and physiologically identical cell.

Hyaline Line: An unperforated siliceous strip between two striae.

Interstriae: An unperforated siliceous strip between two striae.

Morphogenesis: A developmental sequence that causes an organism to develop its shape.

Mucilage: Glycoproteins and/or exopolysaccharides excreted by some diatoms.

Ocellus: A group of small pores surrounded by a thickened hyaline rim and raised from the surface of the valve.

Oogamous: Sexual reproduction in which the sperm is small and motile, and the egg is larger and nonmotile.

Oogonia: Female gametangia.

Otaria: Membranous costae that occur opposite each other on a valve, especially the genus Rhizosolenia.

Pili: Long siliceous hairs on a diatom valve.

Poroid: Simple unchambered hole through a diatom valve.

Pseudoseptum: A siliceous projection to the cell interior at the edge of the valve.

Rimoportula: A tubelike opening through the cell wall with an internal flattened tube or lip-like slit; also called labiate process.

Rimoportula Tube: The external portion of a rimoportula.

Rostrate: The blunt tapered end of a valve, shaped like a beak.

Spermatogonangia: In sexual reproduction, the cell that produces sperm cells.

Spinules: Little spines.

Uniseriate: Arranged in a single series of cells.

Vegetative Cells: Cells that are a result of asexual division.