Abstract
Background and aims – During phytoplankton surveys of a freshwater pond in Northeastern France, Fragilaria subconstricta (Bacillariophyceae) was observed. This rare diatom was present in large numbers in several plankton samples from Lachaussée Pond in the French Grand Est region.
 Methods – The morphology of Fragilaria subconstricta was examined in detail using light and scanning electron microscopy. Its identity is discussed and it is compared to the morphologically most similar species, especially Fragilaria capucina, F. mesolepta Rabenhorst, F. neointermedia and F. tenuistriata.
 Key results – Fragilaria subconstricta produces ribbon-like colonies and has narrow linear-lanceolate valves with slightly rostrate ends. The central area is rectangular, usually small or even absent. Spatulate linking spines are present all along the valve margins and a single rimoportula is located near one apex at the valve face-mantle junction. An amended diagnosis and new synonyms are proposed for this species. Additionally, the type material of Fragilaria mesolepta and a population of the very rare species Fragilaria capucina have been investigated and illustrated.
 Conclusions – Fragilaria subconstricta and F. tenuistriata are synonyms. Fragilaria capucina s. str. is a rare species in Europe, and the majority of records for this species should be considered part of the Fragilaria capucina “Sippenkomplex” sensu Krammer & Lange-Bertalot (1991).
Highlights
The diatom genus Fragilaria Lyngbye (1819) was established for species forming linear, flat and articulate colonies, with corner joints alternately loosened and not adhering to each other. Hustedt (1930: 132) stated that Fragilaria formed very long ribbon colonies and that Synedra Ehrenb. cells could live individually or forming star-shaped colonies
Further developments in the taxonomy and morphology of the Fragilaria genus definition were necessary to account for species with two rimoportulae such as F. capucina (Tuji & Williams 2006a)
This paper describes a population of Fragilaria subconstricta Østrup that was observed in plankton samples from Lachaussée Pond (Eastern France) in 2016
Summary
The diatom genus Fragilaria Lyngbye (1819) was established for species forming linear, flat and articulate colonies, with corner joints alternately loosened and not adhering to each other. Hustedt (1930: 132) stated that Fragilaria formed very long ribbon colonies and that Synedra Ehrenb. cells could live individually or forming star-shaped colonies. Hustedt (1930: 132) stated that Fragilaria formed very long ribbon colonies and that Synedra Ehrenb. The relationship with the genus Synedra is so narrow that isolated individuals are indistinguishable at all, some Synedra species occur in ribbon-like colonies”. He still separated these two genera (Hustedt 1931: 115– 117). Further developments in the taxonomy and morphology of the Fragilaria genus definition were necessary to account for species with two rimoportulae such as F. capucina (Tuji & Williams 2006a). In this paper a population of the very rare species Fragilaria capucina, collected in a small river in Luxembourg, was analysed, discussed and illustrated using LM and SEM
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