My statement in the previous post that “Tabellaria flocculosa is by far the most common species” [in the genus Tabellaria] is true except for one important point: Tabellaria flocculosa is not a species, in the biological sense. We have good evidence from the UK, and my colleague Maria Kahlert has similar evidence from Sweden, that it is actually a complex of several species that no-one has yet taken time to dissect out and establish in their own right.
The picture at the top of the post shows strains from eight different locations in Scotland collected for our library of molecular barcodes, with the black boxes delimiting three groups, based on differences in their rbcL gene. All fit the description of T. flocculosa but the scale of the differences is such that they are highly likely to be different species. However, it is hard to see morphological characteristics that set any of these groups apart. It is possible that some features will be revealed by a more thorough study using scanning electron microscopy but that will not be particularly useful for routine identification using the light microscope. Watch this space …
However, even if Tabellaria flocculosa is not a “species” in the strict biological sense, it is a “species” in a broader linguistic sense, insofar as it is a widely-used term that enables freshwater biologists to exchange information about the organisms that live in lakes and streams. Biologists have a formal way of understanding species, with each and every Tabellaria cell being related to a physical “type specimen”. However, philosophers and linguistics have a more abstract way of using the word “type” in relation to organisms, deriving from the work of Plato. He argued that each and every ox was different but there must be something that unites all of these individual oxen. This universal property could not necessarily be perceived directly with any human sense but must be understood with the mind. The lay person does not need to link a beech tree back to a type specimen: they have the “type” lodged in their memory, enabling unambiguous identification, even from a distance.
Not only is Tabellaria flocculosa recognisable as a distinct entity (using characteristics listed in the previous post), but it also conveys some useful information about the habitat. If T. flocculosa is abundant in a sample (i.e. more than 10% of all cells), then you can be fairly sure that the habitat you are examining has relatively soft water (probably slightly acidic) and relatively low concentrations of inorganic nutrients (see graphs at the end of the post). All this despite us knowing that it is not really a species in the formal biological sense at all. There have been some attempts to split this species in the past, but none have stuck, at least in part, I suspect, to the proposed divisions not being sufficiently convincing – in either morphological or ecological terms – to usurp the longstanding Platonic “type” of T. flocculosa. There are some hints in the work of both Brenda Knudson (see previous post) and John Koppen (see reference list) that there are morphologically distinct planktonic and benthic forms but, again, no-one has produced a convincing rationale for splitting these into separate species.
Linguists would argue that the words we use to describe the world around us are a consensus of usage, not an absolute, and also that meaning of words can change over time. That’s certainly true for diatoms. When I first started out, “Cocconeis placentula” was interpreted in much broader terms than it is now, and the same can be said for many other diatom species. The same is true for genera: we all thought we understood what characterised the genus “Fragilaria”; now, we have a much more constrained definition, with many of its former constituents shifted to other genera. So it is perfectly possible that, in 30 years time, we’ll have a different view of what “Tabellaria flocculosa” means.
The graphs showing the distribution of Tabellaria flocculosa in relation to chemical variables are based on interrogation of a database of 6500 river samples collected as part of DARES project (Kelly et al., 2008, in reference list). Vertical lines on the pH and reactive phosphorus graphs show UK environmental standards for conditions necessary to support good ecological status: blue = high status; green = good status, orange = moderate status and red = poor status. Standards differ between water body types and thresholds for lowland high alkalinity rivers have been plotted here. These indicate the maximum thresholds for particular ecological status classes for each variable and tighter standards will apply in many waters.
Phosphorus standards are based on the Environment Agency’s standard measure, which is unfiltered molybdate reactive P. This approximates to “soluble reactive P” or “orthophosphate-P” in most circumstances, but the reagents will react with P attached to particles that would have been removed by membrane filtration. The current UK phosphorus standards are site specific, using altitude and alkalinity as predictor variables. This means that a range of thresholds applies, depending upon the geological preferences of the species in question. The plots here show boundaries based on the average alkalinity (50 mg L-1 CaCO3) and altitude (75 m) in the whole dataset.
There are no UK standards for nitrate-N; thresholds in this report are based on values derived using the same principles as those used to derive the P standards and give an indication of the tolerance of the species to elevated nitrogen concentrations (see “This is not a nitrate standard”). However, they have no regulatory significance.
The photographs of Tabellaria flocculosa are from cultures isolated by Shinya Sato and David Mann during the UK diatom metabarcoding project (Kelly et al., 2020, in reference list)
Kelly, M.G., Juggins, S., Guthrie, R., Pritchard, S., Jamieson, B.J., Rippey, B, Hirst, H & Yallop, M.L. (2008). Assessment of ecological status in UK rivers using diatoms. Freshwater Biology 53: 403-422.
Kelly, M.G., Juggins, S., Mann, D.G., Sato, S., Glover, R., Boonham, N., Sapp, M., Lewis, E., Hany, U., Kille, P., Jones, T. & Walsh, K. (2020). Development of a novel metric for evaluating diatom assemblages in rivers using DNA metabarcoding. Ecological Indicators 118: 106725.
Koppen, J.D. (1978). Distribution and aspects of the eclogy of the genus Tabellaria Ehr. (Bacillariophyceae) in the Northcentral United States. American Midland Naturalist 99: 383-397.
Wrote this whilst listening to: Brilliant folk-blues guitarist Gwenifer Raymond, another artist who performed at Green Man 2019, but whose set I missed. And Bruce Springsteen’s Girls in their Summer Clothes, because it caught the mood of the weekend so well.
Cultural highlights: The Israeli TV show Fauda now available on Netflix, about life on the West Bank.
Currently reading: still on American Wife by Curtis Sittenfeld.
Culinary highlight: An impromptu French bistro evening at home, with coq au vin followed by tarte au citron.