Fragilariforma meireana: A new species of araphid Diatom from Belgium.

Diatoms are single celled algae related to Kelp and Water Moulds. They are encased in silica shells with two valves. During reproduction the cells divide in two, each of which retains one valve of the shell, growing a new opposing valve, which is slightly smaller and fits flush within the older valve. This means that the Diatoms grow smaller with each new generation, until they reach a minimum size, when they undergo a phase of sexual reproduction, giving rise to a new generation of full-sized cells.

In a paper published in the journal PhytoKeys on 9 June 2025, Bart Van de Vijver of the Research Department at Meise Botanic Garden, and the Department of Biology at the University of Antwerp, and David Williams of the Department of Life Sciences at the Natural History Museum, describe a new species of Diatom from a pond on the outskirts of the city of Damme, in the province of West Flanders, Belgium.

The new species was discovered in a pond on the outskirts of Damme while carrying out a biological survey of the region. Diatoms were collected by taking water samples and then centrifuging. A number of different Diatoms were recovered, including Eunotia bilunaris, Aulacoseira italica, Fragilaria campyla, Nitzschia archibaldii, Gomphonema utae, and an unknown species of Fragilariforma, which Van de Vijver and Williams name Fragilariforma meireana, in honour of Patrick Meire of the Department of Biology at the University of Antwerp, who formerly led the ECOBE research group and instigated the Damme biodiversity survey.

Fragilariforma meireana. Light microscope images taken from the holotype material (BR-4867, Damme, Belgium) (A)–(N) Valves in valve face view in decreasing length. The white arrows indicate the rimoportulae. The black arrows indicate anomalies in the striation pattern. (O)–(Q) Three frustules showing internal cells. Scale bars are 10 μm. Van de Vijver & Williams (2025).

Members of the genus Fragilariforma are 'araphid', which is to say elongate and symmetrical without a raphe system (slits on the midline from which mucus is secreted to aid motility). Valves of Fragilariforma meireana are 35–100 μm long and 4–5 μm wide, with no central area and parallel striae throughout the entire valve length, becoming very weakly radiate at the apices. 

Fragilariforma meireana, scanning electron microscope (SEM) images taken from the holotype material (BR-4867, Damme, Belgium). (A) SEM external view of an entire valve. (B) and (C) SEM external detail of two valve apices showing the rimoportulae (white arrows) and the apical pore field. Note the rudimentary spines and the irregular striation pattern on (C). (D) SEM internal detail of a valve apex showing the rimoportula (white arrow). (E) SEM internal view of an entire valve. The rimoportulae are indicated with white arrows. (F) SEM view of the broad valvocopula. Scale bars: are 10 μm in (A), (E) and (F), and 1 μm in (B)–(D). Van de Vijver & Williams (2025).

Some specimens of Fragilariforma meireana have an internal cell, this being large and robust, and filling most of the space between the valves. This is surprising, as such internal structures are unusual in bilaterally symmetrical Diatoms (it is more common in centric Diatoms, i.e. Diatoms with radial symmetry), however it has been seen in some other species, including Eunotia soleirolii, Meridion circulare, and Fragilariforma soleirolii. Internal cells such as these are interpreted to represent a resting phase, enabling the Diatoms to survive periods of adverse conditions, such as droughts or cold winters, although why the Damme specimens were forming them.

Fragilariforma meireana, SEM micrographs taken from the holotype material (BR-4867, Damme, Belgium). (A) SEM view of a complete frustule with the internal cell inserted between the valves. (B) SEM view of the resting spore covered by the valvocopula. At the apices, the valvocopula is well visible. Occasionally, very shallow ridge present on the plain side visible as a thin line. (C) SEM external detail of the valve apex with the valve, internal cell and one detached copula. (D) SEM external side view of the internal cell. Note the ridges on the top. (E) SEM external valve face view of the internal cell surrounded by the valvocopula. Scale bars are 10 μm in (A) and (B), and 1 μm in (C)–(E). Van de Vijver & Williams (2025).

The conditions in the pond from which Fragilariforma meireana was collected were not recorded, although most of the other species found there are known to prefer meso- to eutrophic conditions with moderate to higher electrolyte contents, and neutral to alkaline water. The exception to this is Eunotia bilunaris, which prefers oligotrophic, oligosaprobic conditions, although this species is known to be relatively environmentally tolerant.

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