The Really Rare Diatom Show

Having set out the limitations of my exercise to define nationally-scarce or rare diatoms, I have drastically reduced my list of candidates from 377 species down to eight.   I suspect that gathering some more data (see point 1 in my previous post) will mean that I can reinstate a few more species to the list, but that will have to wait for another day.

Six of the ten species on my list belong the genus Gomphonema or near relatives.   One of these is Didymosphenia geminata (see “A journey to the headwaters of the River Coquet”); of the others, the most intriguing is Gomphonema tergestinum, a species that occurred in 81 hectads but which seems to be particularly common in north-west England and south-west Scotland, for reasons that I do not fully understand. This needs further investigation but it could be another species that has a distinct biogeography that is not explained solely in terms of a particular chemical environment.   All six of these species qualify as “nationally scarce” rather than “rare” and do remember that my analyses are, at this stage, very preliminary.


Nationally scarce Gomphonema species? a. Gomphonema clavatum; b. Gomphonema insigne; c. Gomphonema ventricosum; d. Gomphosphenia (Gomphonema) grovei; e. Gomphonema turgestinum. Scale bars: 10 micrometres (1/100th of a millimetre). All images from or

One species that may qualify as genuinely rare is Tetracyclus emarginatus, for which we have just two records.   The genus itself is rare, and known mostly from the fossil record but is also sufficiently distinctive that it would not be misidentified or overlooked by analysts.   Another representative of the genus, T. rupestris, has been recorded from Britain but does not feature at all in our database.   When it has been recorded, it is from rock surfaces and damp mosses rather than submerged in streams, so it could have been overlooked in my preliminary analysis. A third representative of the genus, T. lacustris is, as the name suggests, likely to be under-represented in a database composed of samples from rivers, so I will reserve judgement on the rarity of this species though I suspect that it is another candidate for the red list.


Tetracyclus emarginatus. Scale bar: 10 micrometres (1/100th of a millimetre). Image from (photographer: Bernie ní Chatháin).

Another candidate is Cymbellonitzschia diluviana. Though I have tried not to comment on the distribution of species found in lakes, I will make an exception for this species because the habitat is quite well understood, thanks to the work of David Jewson and colleagues at the University of Ulster.   They found it to be most abundant on sand grains exposed to wave action in the littoral zone of Lough Neagh and a few other loughs and lochs with hard water and high pH.   As this combination of characteristics is relatively rare in the UK, it is reasonable to assume that C. diluviana will also be very limited in its distribution.


Cymbellonitzschia diluviana (left: valve view; right: girdle view of two recently-divided cells. Scale bar: 10 micrometres (1/100th of a millimetre). Image from (photographer: David Mann).

Finally, Chris Carter has suggested Entomoneis ornata as a candidate for the diatom “red list”, pointing out that it has not only rare in this country, but is also already on the Red List of Plants of Germany and is also considered to be rare in The Netherlands. Cells of Entomoneis are characteristically twisted around the apical axis, which tests even Chris’ photographic skills, and the genus is more common in brackish and marine waters than in freshwaters. However, it is certainly a species that should be on our preliminary list, and deserves further investigation.

In the next post I’ll consider the pros and cons of a “red list” of British diatoms.


Entomoneis ornata from the Oxford Canal, England, photographed by Chris Carter. Scale bars: 10 micrometres (1/100th of a millimetre)


Carter, C.F. & Belcher, H. (2010). A UK record of Entomoneis ornata (J.W. Bailey) Reimer in Patrick & Reimer 1975. Diatom Research 25: 217-222.

Jewson, D.H. & Lowry, S. (1993). Cymbellonitzschia diluviana Hustedt (Bacillariophyeae): habitat and auxosporulation.   Hydrobiologia 269/270: 87-96.

Ludwig G., Schnittler M. (1996) Rote Liste Gefahrdeter Pflanzen Deutschlands. (12 volumes but available as the list only from