Species Description: Pleurosigma acutum Norman ex Ralfs is a morphologically highly variable species whose structure has been examined by Sterrenburg (2003), and is closely related to P. angulatum (Queckett) W.Smith. The diatom illustrated here (Fig. 1, entire valve exterior, SEM) corresponds closely to P. “acutum” as shown in Peragallo (1890-1891) but is not the same as Norman’s P. acutum. The two species are rather different in many of their features, and so the IRL specimens are not P. acutum Norman (F. Sterrenburg, personal communication, 9 April 2009). Peragallo’s and PE Hargraves' examples therefore need new names.

The sigmoid valves are about 150µm long and 22-24µm wide (Fig. 1), with 23 transverse striae in 10µm and 18-20 oblique striae in 10µm (Fig. 2, central portion of valve exterior, SEM). The angle of the oblique striae is 75 degrees, and the raphe angle is about 15 degrees, curving to the opposite margins at either apex (Fig. 3, apex of valve exterior, SEM). The areolae are slits on the exterior side (Fig. 2, 3); interiors were not observed. Because of confusion with Norman’s P. acutum, as well as P. angulatum, the global distribution of this species is unknown. It was very rare in the IRL, and a few specimens were seen near the Fort Pierce Inlet.

Sterrenbur, FAS. 2003. Studies on the diatom genera Gyrosigma and Pleurosigma (Bacillariophyceae): Pleurosigma strigosum W. Smith and some presumptive relatives. Micropaleontology 49: 159-169.

Peragallo H. 1890-1891. Monographie du genre Pleurosigma et des genres allies. Le Diatomiste 1: 1-35.

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.