A record of Achnanthidium trinode from Wales

My ongoing interest in “rare” diatoms (see “The Really Rare Diatom Show” and associated posts) has precipitated some online discussions with Chris Carter about a diatom that he found in one of his recent samples from Tarran y Esgob, a petrifying stream in the Black Mountains (SO 23926 31499). Neither of us had seen it before and, with a little help from Luc Ector, we managed to identify it as Achnanthidium trinode Ralfs in Pritchard 1861 (synonym: Achnanthes trinodis (W. Smith) Grunow in Van Heurck 1880).   Other algae at this site included the Cyanobacteria Nostoc and Rivularia (or Dichothrix) and the diatom Ellerbeckia.

It has a linear outline with a swollen middle portion and capitate ends. Hofmann et al. (2011) describe the length as 15-28 mm and the breadth as 5 – 6 mm. The striae are very fine (30-35 / 10 mm) and barely discernible with the light microscope.   There is a narrow linear axial area on the raphe valve with a small, rounded to lanceolate central area; the rapheless valve is similar but the central area is larger than on the raphe valve.


Achnanthidium trinode from Tarran y Esgob, Wales, January 2015. a & b: raphe valves; c & d: rapheless valves; e & f: girdle views.   Scale bar: 10 micrometres (1/100th of a millimetre).

Hofmann et al. (2011) comment that it is found occasionally in small numbers in oligotrophic, calcareous lakes in Germany and that it was more common in Bavaria in the 1920s before the onset of cultural eutrophication. They regard it as an indicator of very good ecological quality. I had not seen it in the UK before, but it is listed in Hartley’s checklist of diatoms from the British Isles and also in the checklist of Irish diatoms.   A little Googling also revealed a record of the species in The Diatomaceae of the Hull District by Mills and Philip, published in 1901. There is chalk geology in the areas around Hull so this is, perhaps, not surprising given what we know about it’s habitat preferences.   However, I have only two other records on my databases (over 10,000 samples) and in both of these samples it is present in low numbers. This means that it is probably another candidate for our provisional “red list”.   More so because the habitats where Chris’ samples came from (petrifying springs with tufa-forming mosses) are, themselves, endangered.


Hartley, B. (1986). A check-list of the freshwater, brackish and marine diatoms of the British Isles and adjoining coastal waters. Journal of the Marine Biological Association of the U.K. 66: 531-610.

Hofmann, G., Werum, M. & Lange-Bertalot, H. (2011). Diatomeen im Süsswassser-Benthos von Mitteleuropa. A.R.G. Gantner Verlag K.G., Rugell.

Mills, F.W. & Philip, R.H. (1901). The Diatomaceae of the Hull District. Transactions of the Hull Scientific and Field Naturalists’ Club 1:157-191, plates 12-28.

More about the habitat in:

Graham, J. & Farr, G. (2014). Petrifying springs in Wales. Field Bryology 112: 19-29.

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