Identification by association?

A few months ago, I wrote briefly about the problems of naming and identifying very small diatoms (see “Picture this?”).   It is a problem that has stayed with me over the last few months, particularly as I oversee a regular calibration test for UK diatom analysts.   The most recent sample that we used for this exercise contained a population of the diatom formerly known as “Eolimna minima”, the subject of that post.   Using the paper by Carlos Wetzel and colleagues, we provisionally re-named this “Sellaphora atomoides”.   Looking back into my records, I noticed that we had also recorded “Eolimna minima” from an earlier slide used in the ring test.   These had a slightly less elliptical outline, and might well be “Sellaphora nigri” using the criteria that Wetzel and colleagues set out.   There are slight but significant differences in valve width, and S. nigri also has denser striation (though this is hard to determine with the light microscope).   These populations came from two streams with very different characteristics, so there is perhaps no surprise that there are two different species?


A population of “Eolimna minma” / Sellaphora cf. atomoides from unnamed Welsh stream used in UK/Ireland ring test (slide #39)  (photographs: Lydia King).

The differences in ecology are what concern me here.   Wetzel and colleagues focus on taxonomy in their paper but make a few comments on ecology too.  They write: “The general acceptance is that S. atomoides … is usually found in aerial habitats (or more “pristine” conditions) while the presence of Sellaphora nigri … is more related to human-impacted conditions of eutrophication, pesticides, heavy metal pollution and organically polluted environments”.  This statement is worrying because it suggests that the ecological divide between these two species is clear-cut.   Having spent 30 pages carefully dissecting a confusing muddle of species, it strikes me as counterproductive to repeat categorical statements made by earlier scientists who they had just demonstrated to have a limited grasp of the situation.

The risk is that a combination of slight differences in morphology coupled with (apparently) clear differences in ecology leads to the correct name being assigned based on the analyst’s interpretation of the habitat, rather than the characteristics of the organism.   This is not speculation on my part, as I have seen it happen during workshops.   On two occasions, the analysts involved were highly experienced.  Nonetheless, the justification for using a particular name, in each case, was that the other diatoms present suggested a certain set of conditions, which coincided with the stated preferences for one species, rather than with those for a morphologically-similar species.

I have no problem with environmental preferences being supporting information in the designation of a species – these can suggest physiological and other properties with a genetic basis that separate a species from closely-related forms.  However, I have great concerns about these preferences being part of the identification process for an analysis that is concerned, ultimately, with determining the condition of the environment.  It is circular reasoning but, nonetheless, I fear, widespread, especially for small taxa where we may need to discern characteristics that are close to limits of the resolution of the light microscope.

Gomphonema exilissimum is a case in point.  It is widely-regarded as a good indicator of low nutrients (implying good conditions) yet there have been papers recently that have pointed out that our traditional understanding based on the morphology of this this species and close relatives is not as straightforward as we once thought.   Yet, the key in a widely-used guide to freshwater diatoms (written with ecological assessment in mind) contains the phrase “In oligotrophen, elektrolytarmen, meist schwach sauren Habitaten” (“in oligotrophic, electrolyte-poor, mostly weakly-acid habitats”) amongst the characters that distinguish it from close relatives.  The temptation to base an identification wholly or partly on an inference from the other diatoms present is great.

Including an important environmental preference in a key designed for use by people concerned with ecological assessment brings the credibility of the discipline into question.   Either a species can be clearly differentiated on the basis of morphology alone, or it has no place in evaluations that underpin enforcement of legislation.   That, however, takes us into dangerous territory: there is evidence that the limits of species determined by traditional microscopy do not always accord with other sources of evidence, in particular DNA sequence data.   These uncertainties, in turn, contribute to the vague descriptions and poor illustrations which litter identification guides, leaving the analyst (working under time pressure) to look for alternative sources of corroboration.  I suspect that many of us are guilty of “identification by association” at times.   We just don’t like to admit it.


Hofmann, G., Werum, M. & Lange-Bertalot, H. (2011).  Diatomeen im Süßwasser-Benthos von Mitteleuropa.  A.R.G. Gantner Verlag K.G., Rugell.  [the source of the key mentioned above]

Wetzel, C., Ector, L., Van de Vijver, B., Compère, P. & Mann, D.G. (2015). Morphology, typification and critical analysis of some ecologically important small naviculoid species (Bacillariophyta).  Fottea, Olomouc 15: 203-234.

Two papers that highlight challenges facing the identification of the Gomphonema parvulum complex (to which G. exilissimum belongs) are:

Kermarrec, L., Bouchez, A., Rimet, F. & Humbert, J.-F. (2013).  First evidence of the existence of semi-cryptic species and of a phylogeographic structure in the Gomphonema parvulum (Kützing) Kützing complex (Bacillariophyta).   Protist 164: 686-705.

Rose, D.T. & Cox, E.J. (2014).  What constitutes Gomphonema parvulum? Long-term culture studies show that some varieties of G. parvulum belong with other Gomphonema species.  Plant Ecology and Evolution 147: 366-373.


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