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 Sübwasserflora 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 Sübwasserflora, 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 Sübwasserflora 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 Sübwasserflora, 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. http://www.rhone-alpes.developpement-durable.gouv.fr.
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.