Gomphonema vibrio is part of a complex of species that has only begun to be unravelled in the past few years. In the first edition of the Süsswasserflora von Mitteleuropa in 1930, Hustedt included it as one of three varieties of G. intricatum, along with G. pumilum and G. dichotum. By the time of the second edition (1986), however, Krammer and Lange-Bertalot had subsumed G. intricatum into G. angustum, creating a single species that spanned an enormous range of size (see their Plate 164 if you don’t believe me). A few years later they revised this opinion, and unpicked the G. angustum complex, reinstating several of the taxa that they had originally subsumed and also recognising some more recently described species (many by Erin Reichardt). There may well be more changes to come as this group has not yet been subjected to critical study by molecular geneticists.
One of the other species in this melange is Gomphonema pumilum, a much smaller diatom that is common in both running and standing waters (Hustedt’s comment on the species complex only referred to a preference for “stagnant waters”). We have met it a few times previously (see, for example, “Pleasures in my own backyard”) and I also found it in a 1999 sample from Croft Kettle whilst searching for G. vibrio. However, I then turned to an older slide, based on a sample collected in 1872 and given to me by John Carter (see “Remembering John Carter”). This had some cells of G. pumilum but also some that exceeded the quoted dimensions for G. pumilum (length: 12 – 36 mm; width: 3.5 – 5.5 mm) and which fell within the size range for G. vibrio. I suspect that we are, in fact, dealing with a mixture of the two species and if this is a common situation then it may explain why Hustedt had difficulties unpicking the two species. When I arranged the images of G. vibrio and G. pumilum that I found in this sample in order of diminishing size, there is a continuum between the two forms. We now know that width is a better discriminator than length and, armed with this, we can see a difference between the two species. But that is one of the benefits of hindsight.
Gomphonema pumilum from Croft Kettle, May 1999. a. – e.: valve views; f., g.: girdle views. Scale bar: 10 micrometres (= 100th of a millimetre).
Gomphonema vibrio (h. – k.) and G. pumilum (l. – m. [and n.?]) from “Hell Kettles”, 1872. Scale bar: 10 micrometres (= 100th of a millimetre).
This raises a question about the reliability of the size ranges quoted in the literature A couple of the smaller valves of G. vibrio were less than 7 mm wide. Yet, in other respects, they were more similar to the “true” G. vibrio valves than to those of G. pumilum. The answer will vary from species to species but, as a general rule, we should not be too bothered if the extremes of a population stray a little beyond the values quoted in the literature. These are usually based on the largest and smallest cells found in a thorough scan of one or more populations, but not necessarily on observations of an initial cell (the largest in a population) or of cells at the point immediately before sexual reproduction is initiated (the smallest). We simply don’t have that information for most species so, as a result, should be prepared to accept larger and smaller valves into a species if they are qualitatively similar to, and quantitatively part of a continuum with, the rest of the population. My post “Diatoms and the Space-Time Continuum”, also on Gomphonema, offers some further insights into this story.
Hustedt, F. (1930). Susswasserflora von Mitteleuropa 10: Bacillariophyceae. Gustav Fischer, Jena.
Krammer, K. & Lange-Bertalot, H. (1986). Susswasserflora von Mitteleuropa 2: Bacillariophyceae. 1 Teil: Naviculaceae. Spektrum Akademischer Verlag, Heidelberg.
Krammer, K. & Lange-Bertalot, H. (1991). Susswasserflora von Mitteleuropa 2: Bacillariophyceae. 4 Teil: Achnanthaceae. Kritische Ergänzungen zu Achnanthes s.l., Navicula s.str., Gomphonema. Spektrum Akademischer Verlag, Heidelberg.
Reichardt, E. (1997). Taxonomische revision des Artencomplexes um Gomphonema pumilum (Bacillariophyceae). Nova Hedwigia 65: 99-129.
Reichardt, E. & Lange-Bertalot, H. (1991). Taxonomische revision des Artencomplexes um Gomphonema angustum – G. intricatum – G. vibrio und ähnliche taxa (Bacillariophyceae). Nova Hedwigia 53: 519-544.
In my post on Gomphonema rhombicum, I mentioned that the location on the type slide is given as “Appleby”, which was not very precise. My 1872 slide is labelled “Hell Kettles, Durham”. “Hell Kettles” is the name for the pair of ponds, of which Croft Kettle, which I described in my earlier post, is the larger. However, the location “Durham” is not very illuminating. The closest town to Croft Kettle is Darlington, whilst Durham City is 40 km to the north. “Durham”, in this context, could refer to the county, which covers 2721 square kilometres and habitats from calcareous ponds such as these to moorland pools. A slide label offers very little space to give precise details of location but, in both these cases, a little more information would be useful. The likelihood is that Firth had more detailed notes elsewhere but these have been lost over time, so we are left with these scant words. There is a lesson here for all of us in how we record the meta-data that accompanies our samples.