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Figure 1. Ditylum brightwellii, a narrow diameter cell in girdle view, modified from Gran & Angst (1935).

Figure 2. Ditylum brightwellii, intermediate diameter living cell in girdle view; slight plasmolyzed with cytoplasmic strands still in contact with rimoportula (interference contrast).

Figure 3. Ditylum brightwellii. Girdle view of the valve, showing marginal ridge perforated by elongate slits (TEM).

Figure 4. Ditylum brightwellii, flattened cell showing girdle band structure (phase contrast).

Figure 5. Ditylum brightwellii, central portion of valve interior, double slit rimoportula with radiating areolae (SEM).

Figures (6) Ditylum brightwellii, valve view of ‘normal’ cell. (7) Ditylum brightwellii, valve view of biangular cell (brightfield). (8) Ditylum brightwellii, valve view of quadrangular cell.

Figure 9. Ditylum brightwellii, fully formed resting spore, within parent (upper) and free-floating (lower) cell. Modified from Gran & Angst (1935).

Figure 10. Ditylum brightwellii, living resting spore in parent cell from Sebastian Inlet, FL (phase contrast).

Figure 11. Ditylum brightwellii, large diameter living resting spore in parent cell (brightfield).

Figure 12. Ditylum brightwellii, resting spore primary valve in girdle view. Note lack of areolae and presence of spinules (SEM).

Figure 13. Ditylum brightwellii life cycle. Sexual reproduction (auxospore formation) delimited by green rectangle. Modified from Koester et al. 2007.

Species Name: Ditylum brightwellii (West) Grunow ex Van Heurck, 1880
Common Name: Diatom
Synonymy: Triceratium brightwellii West
Ditylum trigonum Bailey
Ditylum inequale Bailey

    Kingdom Phylum/Division Class: Order: Family: Genus:
    Protista Bacillariophyta Mediophyceae - - Ditylum

    Species Description

    Ditylum brightwellii cells are primarily triangular, though often with rounded corners, giving the cells a cylindrical appearance (Figures 1 & 2). The pervalvar axis can be 2-10 times the valve diameter, thus living cells are usually seen in girdle view. Cells are solitary, but short chains can be found in rapidly growing populations where wave and current activity is minimal. Each cell has a centrally located long hollow tube that is the external part of the rimoportula (Figures 3 & 5). The rimoportula is unusual: on the valve interior it is closed at the center, giving the appearance of a bilabiate slit (Figure 5).

    The valve shape can be bi-, tri-, or quadrangular (Figures 6, 7 & 8). The proximal cause of different valve shapes is unknown. The less common biangular and quadrangular shapes have been given taxonomic status, which is probably unnecessary. At the junction of valve and mantles is a raised siliceous ridge, often perforated by vertical slits (Figure 3). Alternatively, multiple slits may separate into a series of slotted short spines with pointed apices. The valve surface is perforated by elliptical areolae radiating from the central rimoportula tube (Figure 5 & 6). The areolae on the mantle are vertically arranged (Figure 3), and more numerous than on the valve. Li and Volcani (1985) have studied the formation of the silicon cell wall during cell division.

    Girdle bands (copulae) are numerous, scale-like, weakly siliceous, and arranged in several columns around the circumference of the cell (Figure 4). The copulae are perforated by numerous pores. Living cells have multiple rounded chloroplasts, which may be concentrated around the peripheral nucleus (Figure 1 & 2). Further characteristics of the genus are found in Round et al. (1990). The diameter of cells as reported in the literature is 25-100 µm and 80-130µm in the pervalvar axis [exclusive of the spines]. There are about 10 areolae in 10 µm on the valve, and about 18 areolae in 10µm on the mantle.

    Ditylum brightwellii has been used as a model for genetic studies (Ryneason et al. 2006; 2009) relative to metapopulation structure and environmental influence on population genetic variability. There are about 150 nucleotide sequences of this species available in Genbank


    Habitat & Regional Occurence

    Ditylum brightwellii has a worldwide distribution in boreal to tropical coastal and estuarine environments, apparently absent only from polar oceans.

    Indian River Lagoon Distribution

    Although uncommon in the Indian River Lagoon (IRL), it is found throughout the system, including the Banana River and Mosquito Lagoon. Because it appears to be more abundant in the vicinity of the inlets, it may be a temporarily introduced species to the lagoon. The maximum abundance, only a few thousand cells per liter, occurs in cooler months, which is consistent with its greater success in temperate regions.



    In some geographic areas, the asexual process of resting spore formation is a regular feature of the life cycle. The relationship of cell size to spore formation has been examined by Hargraves (1982). Both large and small diameter cells can form resting spores. The spores are solitary within the parent cell (Figures 9, 10 & 11) and are morphologically unlike the vegetative cells, though the external hollow spines of the rimoportulae are present (Figures 9-12). Girdle bands are not present, nor are areolae, which are replaced by numerous tiny spinules (Figure 12). Chloroplasts are maintained in the spore, though they are concentrated so that details of the cell structure are difficult to discern (Figures 9, 10 & 11). Resting spores are normally formed when cells enter stationary or senescent growth phase (in cultures), suggesting that nutrient depletion plays a role in spore induction. Occurrence of resting spores is rare in the IRL system.

    Sexual reproduction (auxospore formation) in Ditylum brightwellii has been examined by Koester et al. (2007).  These authors found that clonal cultures were homothallic (i.e. haploid eggs and sperm were produced by the same clone) (Figure 13). Moreover, an auxospore was not mandatory for restoration of maximum cell size, as it is for many diatoms (Round et al. 1990), since vegetative enlargement was also observed in some clones (Koester et al. 2007). Sexual reproduction was size dependent, with the largest cell sizes failing to produce gametes, and the smallest cells producing more spermatogonangia than oogonia.


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    Gran, HH & EC Angst. 1935. Plankton diatoms of Puget Sound. Publications of the Puget Sound Biological Station 7: 417-516.

    Hargraves, PE. 1982. Resting spore formation in the marine diatom Ditylum brightwellii. In: Mann, DG [Ed.]. Proceedings of the VII. International Diatom Symposium. 33-46.

    Koester, JA, Brawley, SH, Karp-Boss, L & DG Mann. 2007. Sexual reproduction in the marine centric diatom Ditylum brightwellii (Bacillariophyta). Euro. J. Phycol. 42: 351-366.

    Li, C-W & BE Volcani. 1985. Studies on the biochemistry and fine structure of silica shell formation in diatoms. VIII. Morphogenesis of the cell wall in a centric diatom, Ditylum brightwellii. Protoplasma 124: 10-29.

    Round, FE, Crawford, RM & DG Mann. 1990. The Diatoms - Biology and morphology of the genera. Cambridge University Press, Cambridge. 760 pp.

    Rynearson, TA, Newton, JA & EV Armbrust. 2006. Spring bloom development, genetic variation, and population succession in the planktonic diatom Ditylum brightwellii. Limnol. Oceanog. 51: 1249-1261.

    Rynearson, TA, Lin, EO & EV Armbrust. 2009. Metapopulation structure in the planktonic diatom Ditylum brightwellii (Bacillariophyta). Protist 160: 111-121.

Unless otherwise noted, all images and text by PE Hargraves
Editing and page maintenance by LH Sweat:
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Page last updated: 25 September 2011


Female gametangia.


In sexual reproduction, the cell that produces sperm cells.


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


Little spines.


Cells that are a result of asexual division.


The external portion of a rimoportula.


A regularly repeated perforation through the cell wall.


In the genus Ditylum, a form of rimoportula that has two slits on opposite sides.


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

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