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Genus 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.

No type originally designated. Williams & Round (1988) comment that Fragilaria pectinalis may be synonymous with F. capcuina Desmazière. The genus Fragilaria and its allies are difficult to distinguish purely on the basis of characters visible under the light microscope and, as a result, have been the subject of considerable controversy in recent years. Traditionally, Fragilaria was distinguished from Synedra due to its tendency to form long filaments whilst Hannaea is characteristically ?banana?-shaped. There are, however, enough common characteristics to lead some workers (e.g. Lange-Bertalot, 1980) to propose that these genera should be combined. By contrast, Williams and Round (1986, 1987) proposed a more circumscribed concept of Fragilaria, partly on the basis of characters visible only by electron microscopy. Several new genera were erected or resurrected to include species that were previously recognised as belonging to Fragilaria. Most of these genera coincide with subgenera and ?sippenkomplexes' of Krammer and Lange-Bertalot (1991a). There is still, however, some confusion about the true limits of Fragilaria and Synedra, with several taxa described here as varieties of F. capucina also having synonyms within Synedra. The reality is that the true taxonomic position of these taxa probably depends upon characteristics not easily seen by light microscopy of cleaned material.

Species in Fragilaria


Fragilaria bidens Heiberg (1863)

Valve view phase contrast

Linear valves tapering to rostrate or slightly capitate ends. There is a slight narrowing of the valve towards the central area which extends to the valve margins and is conspicuously inflated. Striae are parallel throughout, with a narrow axial area.


Fragilaria capucina Desmazière (1825)

Valve view Brightfield

Narrowly linear to linear-lanceolate valves, with cuneate or slightly rostrate ends. Usually with well-defined central area reaching to the valve margins. If central area is one-sided, this is not accompanied by unilateral swelling of valve. When viewed under SEM, interlocking teeth are found on both striae and interstriae.


Fragilaria capucina var. amphicephala (Grunow) Lange-Bertalot (1991)

valve view brightfield

Narrow linear to linear-lanceolate valves usually with capitate ends. Coarse striae, with an obviously "alternate" arrangement along the valve.


Fragilaria capucina var. gracilis (Øestrup) Hustedt (1950)

Valve view phase contrast R. Brathay, Cumbria

As for var. capucina but valve narrower and with finer striae. Central area sometimes obvious, but can be barely visible or absent.


Fragilaria capucina var. mesolepta (Rabenhorst) Rabenhorst (1864)

Valve view Brightfield

Valve linear to linear lanceolate, constricted at the central area, although the central area is not swollen, as is the case for Fragilaria bidens.


Fragilaria crotonensis Kitton (1869)
Science Gossip, 5: 110, fig. 81.

Valve view Brightfield

Frustules swollen at centre, and sometimes at the poles. Attached at central portion to form ribbon-like colonies. Valve outline is linear, with a distinct central swelling, lanceolate in shape, and extending for approximately half the total valve length. Striae are parallel and there is a narrow axial area and a rectangular central area, extending to the margins of the valve.


Fragilaria perminuta Lange-Bertalot (2000)

Valve view phase contrast

Similar to Fragilaria vaucheriae, with a distinctive one-sided central area. Distinguished by more delicate valves, a narrowly lanceolate outline with rostrate to sub-capitate poles and finer striation.


Fragilaria tenera W. Sm. (1856)
Syn. British Diat., vol. 2, p. 98.

Valve view Brightfield

Very narrow, linear valves tapering to capitate ends. No obvious central area.


Fragilaria vaucheriae (Kütz.) Peters. (1938)

Valve view Brightfield

Approximately rectangular frustules typically linked at the valve face to form chains. Occasionally found singly. Valve linear to linear-lanceolate in outline, narrowing towards rostrate poles. Striae are parallel, sometimes becoming slightly radiate towards the poles. There is a narrow axial area and a central area that typically extends to a slight swelling on one side of the valve only.


Fragilaria vaucheriae var. capitellata Ross (1947)

drawing material from Cardingham Stream, Cornwall.

As for nominate variety, but with distinctly capitate poles and finer striae.


Fragilaria spp. H.C. Lyngbye (1819)
Tent. Hydrophytol. Dan. 182 (1819)

Valve view Brightfield

Comments to: Diatom Key Development Team