Pleurosigma decorum
Family: Pleurosigmataceae
Pleurosigma decorum image
Pleurosigma decorum  

Species Description: There has been substantial confusion of this species with P. formosum, as Hendey (1964) discusses in detail. That species has also been listed as occurring in the IRL system. The valves of P. decorum are lanceolate, somewhat narrow, and gently sigmoid with sub-acute apices (Fig.1, live cell with multilobed chloroplasts and transparent central nucleus; phase contrast; Fig. 2, entire valve, SEM). The raphe is median, with the central raphe ends slightly overlapping and undulate on the valve exterior (Fig. 3, enlarged central area, SEM), but not so on the valve interior (Fig. 4, SEM). The raphe gradually becomes excentric near the apices (Fig. 2). There are two curved siliceous bars enclosing the interior central raphe ends (Fig. 4). The areolae are slotted on the exterior (Fig. 3), and mostly oval and bisected on the interior, except in the small laterally expanded central area (Fig. 4). This species apparently has a global distribution, but confusion with P. formosum makes the distinction difficult. In the IRL, P. decorum is fairly common throughout the year. The cells are variable, 145-270µm in length, 23-29µm wide, with 18-22 transverse and 16-21 oblique striae in 10µm. The striae angle is 73-78 degrees, and the raphe angle (see Sterrenburg 2002) is 8-10 degrees. Living cells have multilobed chloroplasts (Fig. 1). Note that, with few exceptions, Pleurosigma cannot be identified to species from living cells, and frequently, from light microscopy alone.

Hendey NI. 1964. An introductory account of the smaller algae of British coastal waters. Part 5, Bacillariophyceae (Diatoms). In: Ministry of Agriculture, Fisheries and Food. Fisheries Investigations Series IV. 317pp. HMSO, London.

Sterrenburg FAS. 2002. Nulla Vestigia Retrorsum. The case of Pleurosigma aequatoriale Cleve. Constancea 83: 1-10.

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.

Pleurosigma decorum image
Pleurosigma decorum  
Pleurosigma decorum image
Pleurosigma decorum  
Pleurosigma decorum image
Pleurosigma decorum  
Pleurosigma decorum image
Pleurosigma decorum