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Class Fragilariophyceae Round in Round et al. (1990)
The Diatoms: Biology and Morphology of the Genera. Cambridge.

Cells elongate and usually symmetrical about at least two planes, one of which is the pervalvar plane. Stria pattern bilateral, orientated about a longitudinal rib (the sternum), as in a feather, but raphe always absent. Costae and septa may be present. Rimoportules usually present. Many genera within the class have many small, discoid plastids lying under the valve face, but some have one, two or a few plastids. Many genera have rows of small spines at the junction of the valve face and mantle which allow sibling cells to form filamentous colonies. However, taxa also form zig-zag or stellate colonies, where cells are joined at the base by mucus secreted from apical pore fields, or clusters radiating from a common pad or stalk.

Genera in Fragilariophyceae


Asterionella Hassall (1850)
Microscopic Examination of the Water. London.
Elongate cells, which are joined at the base to form stellate colonies. The poles are swollen, with the basal pole typically wider (in both valve and girdle views) than the apical pole. Numerous small, plate-like plastids. Valves elongate, more-or-less linear with swollen or capitate poles, the basal (attaching) pole wider than the apical pole. Striae parallel and perpendicular to apical axis. Narrow, linear axial area. Apical pore fields present at both ends. Several girdle bands, each with one or two rows of pores.


Centronella M. Voigt (1902)
Forschungsber. Biol. Stat. Plön. 9: 41


Ctenophora (A. Grunow) D.M. Williams & F.E. Round (1986)
Diat. Res. 1: 330.


Diatoma J.B.M. Bory de St-Vincent (1824)
Dict. Class. Hist. Nat. 5: 461
Cells joined either at the valve face to form ribbon-like colonies, or at one corner to form zig-zag or stellate colonies. Quadrate to oblong in girdle view. Many small discoid or slightly elongate plastids. Valves are elliptical to elongate in valve view. Poles are variable, from rounded to swollen. Striae are fine, transverse at centre, becoming radiate towards poles. Narrow axial area with no central area. Transapical costae present, along with a single rimoportula. Girdle bands have two rows of pores.


Distrionella D.M. Williams (1990)
Archiv für Protistenkunde 138: 171-177.
Rectangular frustules, occasionally with inflated poles. Valve narrowly lanceolate in outline, with distinctly capitate or swollen poles. Occasionally appears slightly heteropolar.


Fragilaria H.C. Lyngbye (1819)
Tent. Hydrophytol. Dan. 182 (1819)
Cells joined at valve face to form ribbon-like colonies. Those taxa (e.g. Fragilaria crotonensis) that are swollen at the centre are linked only there; otherwise linkage occurs across the entire length of the cell. Two plate-like plastids. Valves linear to linear-lanceolate (occasionally elliptical) with rounded, rostrate or capitate ends. Sometimes a slight swelling at centre of valve. Almost parallel striae separated by a narrow linear sternum. Valve face bordered by row of short spines (hard to see using LM). Central area variable, but often reaches to edge of valve on one or both sides.


Fragilariforma (J. Ralfs) D.M. Williams & F.E. Round (1988)
Diat. Res. 3: 265
Rectangular frustles, which form linear or zig-zag colonies. Numerous small, discoid plastids. Elliptical, lanceolate or linear valves (sometimes with a central constriction) with tapering, rostrate to capitate apices. Fine striae separated by a very narrow axial area (sometimes difficult to see under LM). No central area. Short spines at margin of valve face and mantle. 4-6 open copulae, each with a single row of areolae.


Hannaea Patrick in Patrick & Reimer (1966)
The Diatoms of the United States vol. 1, p. 131.


Martyana Round in Round et al. (1990)
The Diatoms. Biology and Morphology of the Genera. Cambridge University Press, Cambridge.
Cells are small, oblong to wedge-shaped in girdle view and ovate in valve view. Small specimens may appear to be more-or-less isopolar in outline. Valves lack marginal spines, so cells do not form linear colonies. Two plate-like chloroplasts, one under each valve. Coarse, slit-like areolae are oriented parallel to the main axis. Narrow, approximately linear axial area with no central area.


Meridion Ag. (1824)
Syst. Alg. Xiv: 2
Cells rectangular or wedge-shaped in girdle view, joined by the valve face to form straight or fan-shaped colonies. Many discoid chloroplasts. In valve view, the outline is either wedge-shaped or linear with rounded, sub-capitate or capitate poles. Thickened costae cross the valve at intervals. Small spines occur along the margin of the valve face. Striae are fine (barely-resolvable under LM) with a very narrow, linear axial area and no central area. There is a single rimoportula towards the head pole and a poorly-defined pore field towards the foot pole.


Oxyneis Round in Round et al. (1990)
The Diatoms. Biology and Morphology of the Genera. Cambridge University Press, Cambridge.
Cells quadrate or oblong in girdle view, forming either straight or zig-zag chains. Valves either elliptical or panduriform with flat valve face and shallow mantle. Striae irregularly distributed along valve, more-or-less parallel at centre, becoming radiate towards poles. Short spines at junction of valve face and mantle. Apical pore fields present at both poles, and a single rimoportula is present at one pole. Several open copulae, each with a short terminal septum and a row of areolae.


Pseudostaurosira (Grunow) D.M.Williams et Round (1987)
Diat. Res. 2: 276.
Cells are approximately rectangular in girdle view and are joined tightly to form filaments. Each cell contains two parietal chloroplasts, one under each valve face. Valve outline is linear to elliptical. The poles are rounded or sub-rostrate. Some species (not yet recorded from Britain and Ireland) have undulate margins, or are cruciform, with poles that can be rostrate or capitate. The sternum is wide and the striae are, correspondingly, short, with only a few areolae near the valve face. A single spine occurs between each pair of striae. Rimoportulae are absent, and apical pore fields are sometimes present.


Staurosira Ehrenb. (1843)

Cells are approximately rectangular in girdle view and are joined tightly to form straight or (less commonly) zig-zag filaments. Each cell contains two plastids, lying along the girdle. Valve outline is oval, elliptical, cruciform or, occasionally, triangular, with rounded ends. The sternum is variable, but never very broad (as in Pseudostaurosira). One or two spines are found between the striae at the junction of the valve face and mantle. Rimoportulae are absent, and there is a small apical pore field at each pole. There are several open copulae, all lacking pores, per valve.


Staurosirella D.M.Williams et Round (1987)
Diat. Res. 2: 274.
Small cells, rectangular in girdle view and often linked, via marginal spines, to form linear (occasionally zig-zag) filaments. Two parietal plastids, adjacent to the girdle. Valves are linear, elliptical or cruciform in outline, with a wide sternum. Broad, uniseriate striae extend down the valve mantle. Spines are found at the margin of the valve face and mantle between the striae. Apical pore fields are present at both poles, and there is no rimoportula. Eight to ten open copulae.


Synedra Ehrenb. (1830)
Akad. Königl. Akad. Wiss. Berlin, 1830:40
Cells approximately rectangular in girdle view, typically long and thin, attached by mucilage pads at the base to form radiate colonies. There are two long plastids lying against the girdles and overlapping slightly onto the valve face. In unhealthy material, these plastids may split up, giving the appearance of many small discoid plastids. Valve outline is usually linear or linear lanceolate (lanceolate in the case of S. parasitica) with rounded, rostrate, capitate or swollen ends. The striae are perpendicular to the narrow axial area. The central area, if present, is variable in shape and often contains ghost striae. There are no marginal spines. A rimoportula is found close to each apex, along with a well-developed apical pore field. The copulae are few in number and have a single row of areolae.


Tabellaria Ehrenb. (1840)
Ber. Bekanntm. Königl. Preuss. Akad. Wiss. Berlin, 1839: 217.
Cells square or rectangular in girdle view and joined to form zig-zag or partly linear colonies. Cells typically lie in girdle view, with septa clearly visible on the copulae. Short strip-like plastids lie between the septa. Valves are elongate, approximately linear in outline but with swollen poles and central regions. The valve face is flat with irregularly spaced striae that are either parallel or slightly radiate. Small spines occur along the valve margin. An apical pore field is present at both poles, and there is a single rimoportula, found near the centre of the valve. Copulae are either complete, in which case a septum is present at one pole, or are open, in which case no septum is present.


Tabularia D.M. Williams & Round (1986)
Diat. Res. 1: 320
Needle-like cells which are found either alone or in small clusters, but not attached to one another. Two plastids. Valves are narrowly linear or lanceolate in outline, with rounded, rostrate or capitate ends. Under LM the striae appear short, leaving a wide axial area (approximately one third to half the total valve width). There is no central area. Apical pore fields are present at both poles, and a single rimoportula is found close to one pole. At least three girdle bands are associated with each valve.


Tetracyclus Ralfs (1843)
Ann. Mag. Nat. Hist. 12: 105.
Cells are square to oblong in girdle view and form zig-zag chains held together by mucilage pads at the corners. Several discoid chloroplasts Valves are elongate to elliptical in outline, often centrally expanded to give a cruciform outline. Fine, parallel striae are separated by a narrow axial area and there is no central area. Transapical costae are present. There is no apical pore field and 0, 1 or 2 rimoportulae may be present. If rimoportulae are present, these lie to one side of the sternum close to the valve centre. There are no marginal spines. Several open copulae are associated with each valve. These have septa across the closed ends and a single row of pore.

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