Amidst the dreary nothingness of the sample that prompted the previous post, I stumbled across the diatom in the photograph above. This image gives a misleading impression as it is a relative large diatom with considerable variation in three dimensions and my first thought was that I was looking at a fragment of vaguely diatom-like structures amidst a unfocussed blur. Careful use of the fine focus control revealed the twisted nature of the structure and I was able to create this semi-focussed image from a “stack” of images of the individual focal planes using Helicon Focus software. The scale bar is 10 micrometres (= 1/100th of a millimetre). As there are relatively few diatoms with a frustule with such a contorted form, it was relatively easy to identify it as Surirella spiralis Kützing 1844.
Surirella spiralis is one of a small number of diatoms whose outline is twisted. There are diatoms that show considerable curvature within a single plane (see Stenopterobia sigmatella in “Reflections from Ennerdale’s Far Side”) but few where this curvature occurs between planes. The only other diatom with this feature that I have written about in this blog are Entomoneis (see “A typical Geordie alga …”) and Cylindrotheca (see “Back to Druridge Bay”). These twisted diatoms, like sigmoid diatoms such as Stenopterobia, typically have motile habits. In my post on Stenopterobia I wondered what advantage a sigmoid outline conferred on a diatom and we really need to ask the same questions when thinking about twisted diatoms. I have the germ of an idea, but want to think it over some more before unleashing it onto the world.
Surirella, Stenopterobia and Entomoneis are all members of an order of diatoms, the Surirellales, that are the subject of a recent paper by Elizabeth Ruck, from the University of Arkansas, and colleagues. They compared morphology and genetic differences amongst members of this order, along with a related order, the Rhopalodiales, two of whose members are Epithemia and Rhopalodia, both of which I have also written about in this blog. Their conclusion is that current generic limits need an extensive shake up with long-established genera that seemed to be based on sensible criteria when viewed with the light microscope split apart and reassembled, based on ultrastructural and genetic characteristcs.
The main changes relevant to a freshwater ecologist are as follows:
- Campylodiscus: some freshwater species retained in Campylodiscus, some moved to Iconella; marine species moved to Coronia. The Fastuosae group of Surirella are now included in Campylodiscus;
- Cymatopleura: now included in Surirella
- Entomoneis: no change
- Epithemia: all species now merged into Rhopalodia;
- Rhopalodia: now includes Epithemia;
- Stenopterobia: now included in Iconella;
- Surirella: now limited to the Pinnatae group of Surirella, plus former Cymatopleura species;
- The genus Iconella has been re-established for a group of former Surirella species (section Robusta) along with some freshwater Campylodiscus species and Stenopterobia. Of particular relevance to this post, Surirella spiralis is now Iconella spiralis (Kützing) Ruck & Nakov in Ruck et al. 2016; and,
- The order Rhopalodiales has been subsumed into Suriellales.
It will be interesting to see whether or not, and how quickly, these names diffuse through the community of scientists who study diatoms. Taxonomy has a dual nature: on the one hand, specialists are driven by a desire to understand how evolutionary forces have shaped and differentiated a group of organisms; on the other hand, taxonomists act as biology’s janitors, sorting and organising information about species so that other biologists can use this for their own purposes. I am the editor and co-translator of a guide to European diatoms that was being finalised just as this paper was published and which, as a result, uses the “old” names. These books often have a ten or twenty year shelf life which will prolong the use of these names, and slow the uptake of new ones. I also know, from many years training people to analyse diatoms, that taxonomic changes, however well justified, sow confusion among beginners. On the other hand, we are entering a new era, when molecular barcoding will be used more widely for routine identification of diatoms and, for this, a correct understanding of the phylogenetic relationships amongst a group of organisms improves the accuracy of the bioinformatics routines that assign names to the diatoms.
For most practical purposes, in other words, Surirella spiralis will remain S. spiralis for some time (and Stenopterobia sigmataella will remain S. sigmatella too), if only because of the innate conservatism of most of the people who work with diatoms. My use of the old name in this post means that the part of my readership who know at least a little about diatoms could place the diatom within a familiar framework, even if Iconella spiralis is the correct name. The term “post-truth” has entered our political vocabulary over recent months; in diatom taxonomy and identification, however, we sometimes have to accommodate “pre-truth” as well.
Ruck, E.C., Nakov, T.., Alverson, A. & Theriot, E.C. (2016). Phylogeny, ecology, morphological evolution, and reclassification of the diatom orders Surirellales and Rhopalodiales. Molecular Phylogenetics and Evolution 103: 155-171.
Ruck, E.C., Nakov, T.., Alverson, A. & Theriot, E.C. (2016). Nomenclatural transfers associated with the phylogenetic reclassification of the Surirellales and Rhopalodiales. Notulae algarum 10: 1-4.