Species Description: Valves are narrowly lanceolate with acute apices (Fig. 1, SEM), and lacking the usual sigmoid shape of Pleurosigma species. The raphe is straight, sometimes becoming slightly excentric near the apices; in the center it is somewhat undulate, and the ends bend slightly to the same side (Fig. 2, valve center, SEM). At the apical terminus (Fig. 3, SEM), the raphe is hooked and is accompanied by a calcar (an accessory fissure; arrowhead in Fig. 3) and a small apical pore (Fig. 3, black arrow). On the valve interior, the raphe endings are straight and subtended on both sides by short curved siliceous bars (Fig. 4, SEM). The valves seen are 212-251µm in length and 14.9-18.1µm in width. Transverse striae are 25-26 in 10µm; diagonal striae are 21-22 in 10µm, crossing at a 60-62 degree angle. Excepting size range and the apical pore, these characteristics correspond well with the isolectotype material examined by Sar et al. (2012). Hustedt (1955) notes that the cells he found in North Carolina corresponded to Grunow’s P. intermedium var. mauritania, distinguished by “more slender valves’ (such as those in the IRL), but he suggested that these would be better regarded as formae rather than a variety. Living cells are highly motile and have vermiform chloroplasts (Fig. 5, living cell, phase contrast). The species has a worldwide distribution in warmer waters; it was fairly common throughout the IRL system, though Navarro (1982) found it rarely.

Hustedt F. 1955. Marine littoral diatoms of Beaufort, North Carolina. Bull Duke Univ Mar Sta 6: 1-67.

Navarro JN. 1982. Marine diatoms associated with mangrove prop roots in the Indian River, Florida, U.S.A. Biblio Phycol 61: 1-151.

Sar EA, Hinz F, Sterrenburg FAS, Lavigne AS, Lofeudo S, Sunesen I. 2012. Species of Pleurosigma (Pleurosigmataceae) with lanceolate or slightly sigmoid valve outlines: analysis of type material. Diat Res 27: 237-253.

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.