And another one …

I encountered yet another instance of a species that had dropped out of the collective consciousness of biologists last week, with some help from colleagues Lydia King and Luc Ector.   I co-ordinate an exercise whereby all those of us who analyse diatoms regularly for ecological assessments in the UK and Ireland calibrate themselves against one another. The last sample that we used for this exercise came from a small stream in County Tyrone, Northern Ireland, and it contained a mess of Fragilaria species that proved to be rather challenging. Some we could name quite easily, others took a little more work.   Amongst these Fragilaria species was a nice population of Fragilaria vaucheriae, with its distinct one-sided central area, linear-lanceolate outlines and coarse striae. This was close in all respects to the type description, and presented us with few challenges.


Fragilaria vaucheriae from Trillick Tributary, Carran Bridge, County Tyrone, Northern Ireland, May 2014. Scale bar: 10 micrometres (1/100th of a millimetre). Photographs: Lydia King.

However, there was also a number of valves in the sample which were similar in many respects to the description of F. vaucheriae in the standard floras, but was slightly narrower and which had a more lanceolate outline.   Confusingly, a very similar diatom is illustrated in the 1991 edition of the bwasserflora von Mitteleuropa as one of a number of illustrations of “Fragilaria capucina var. vaucheriae”.   Our sample, however, had two distinct populations: the broader valves which correspond to “true” F. vaucheriae and the narrower ones that many of us, using the bwasserflora, have “lumped” into F. vaucheriae simply because there were no more plausible alternatives on offer.

Luc Ector eventually pointed us towards descriptions and illustrations of another species, Fragilaria pectinalis, which does look very similar to Lydia’s photographs.   Our only slight concern is that the stria density in our population is slightly lower than in the description of the type population of F. pectinalis.   For this reason, we are referring to our population as “Fragilaria cf. pectinalis”, until we are more certain.


Fragilaria cf. pectinalis from Trillick Tributary, Carran Bridge, County Tyrone, Northern Ireland, May 2014. Scale bar: 10 micrometres (1/100th of a millimetre). Photographs: Lydia King.

Fragilaria pectinalis is an interesting case study in the rise and fall of species names.   This species is the type species for the genus Fragilaria, meaning that it was the first species in the genus to be formally described (by Lyngbye in 1819).   It was known from even earlier, albeit under a different name, Conferva pectinalis, although the earliest illustrations are very schematic and hard to interpret.   What is interesting is how it disappeared from Floras and identification guides throughout the 20th century. It is not in Hustedt’s first edition of the bwasserflora von Mitteleuropa, nor is it in West and Fritsch’s Treatise on British Freshwater Algae (1927), Patrick and Reimer’s Diatoms of the United States (1966) or the second edition of the bwasserflora, as I have already mentioned. I suspect that it has generally been lumped into Fragilaria vaucheriae for most of this time. It is only thanks to the detective work of Akihiro Tuji and David Williams that has managed to bring the name back into circulation.

The next step, I guess, is to get it into the standard identification literature again.   My experience is that busy ecologists often do not have the time to trawl through the vast and rather dispersed primary literature every time they encounter a form that they do not recognise.   Hence they exhibit a tendency to squeeze forms into the closest category in the books that they do have.   Luc Ector has started the wheels rolling by including it into his recent publication but perhaps we’ll also see it in the next edition of Diatomeen im Süßwasser-Benthos?  


Bey, M.-Y. & Ector, L. (2013). Atlas des diatomées des cours d’eau de la région Rhône-Alpes. Direction régionale de l’Environnement, de l’Aménagement et du Logement Rhône-Alpes.

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

Tuji, A. & Williams, D.M. (2006b). Typification of Conferva pectinalis O. F. Müll. (Bacillariophyceae) and the identity of the type of an alleged synonym, Fragilaria capucina Desm.   Taxon 55: 193-199.

Tuji, A. & Williams, D.M. (2008). Examination of types in the Fragilaria pectinalis – capitellata species complex.   Pp. 125-139. In: Proceedings of the Nineteenth International Diatom Symposium 2006 (edited by Y. Likhoshway).   Biopress, Bristol.


Food for thought in the River Ehen

Last week’s trip to the River Ehen involved close scrutiny of the hydrographs and weather forecast in the days preceding the trip, followed by some crude extrapolations that predicted that the river would be low enough for us to enter. Nevertheless, there were some anxious moments when we first arrived and gazed into the fast-flowing water.   It was an occasion when I was glad of my veterinary gloves which afford some protection as I have to plunge my arms into river almost to my shoulder in order to pluck stones from the bottom.

The top surface of many of the stones had conspicuous chocolaty-brown patches which are a sign that the diatoms are growing vigorously.   We had been surprised that the amount of algae in the river had been quite low compared to our previous measurements in the autumn and had wondered whether the mild weather had allowed the natural grazers in the river to persist for longer than was the case last year.   Now, with a distinctly autumnal chill in the air, the diatoms no longer seem to be held in check which lends credence to this theory.   If true, it is a good demonstration of just how important “top down” processes are in controlling the quantities of algae that we see in our streams and rivers.


Chocolate-brown patches of diatoms on the upper surfaces of cobbles on the bed of the River Ehen (Mill) in November 2014.

Under the microscope, these brown films resolved into a mass of diatoms, along with a few filaments of Bulbochaete (see “The River Ehen in August”). There were two species of Gomphonema species which live at the end of long stalks: G. acuminatum (see “The River Ehen in February”) and G. truncatum, plus other, smaller Gomphonema species.   There were also a lot of long, needle-like cells of Fragilaria tenera, many of which were joined at their bases to form hedgehog-like bundles of cells. My guess is that these start life attached to either the rocks or other algae but that they easily detach, especially when manhandled onto a microscope slide, and appear to be free-living.


Common diatoms from the River Ehen (Mill), November 2014: a. Gomphonema truncatum valve view; b. G. truncatum girdle view; c. Gomphonema acuminatum valve view; d. G. acuminatum girdle view; e. Fragilaria tenera valve view; f. F. cf. tenera girdle view.   Scale bar: 10 micrometres (1/100th of a millimetre).

Ten or fifteen years ago I would not have referred to this diatom as “Fragilaria” but, instead, would probably have called it Synedra tenera.   Older identification guides (such as West and Fritsch’s British Freshwater Algae, from 1927) differentiate between Synedra and Fragilaria based on whether or not the cells form ribbon-like colonies (Fragilaria) or are solitary or form radiating clusters (Synedra). From the 1980s onwards this simple dichotomy was unpicked just enough to demonstrate that it was too simplistic, but not enough to enable our esteemed taxonomists to put all the pieces back into a coherent whole (a story for another day …).   However, the evidence at present leans to the narrow needle-like species that once belonged to Synedra actually sharing more characteristics with true Fragilaria.   I call this species Fragilaria tenera with mild trepidation, knowing that there is at least one other needle-like Fragilaria present in the upper Ehen and, moreover, that a paper has recently been published (though I have not yet read it) in a rather obscure German taxonomic journal that dissects this complex into yet more fragments. Research on Fragilaria has, alas, generated rather more heat than light over the past couple of decades.


A colony of Fragilaria tenera from the River Ehen, image produced using Helicon Focus stacking software. Individual cells are approximately 50-60 micrometres long.


Lange-Bertalot, H. & Ulrich, S. (2014). Contributions to the taxonomy of needle-shaped Fragilaria and Ulnaria species 1. Lauterbornia 78: