I am fairly sure that I am not a popular person after my latest choice of slide for the “ring test”, the regular calibration exercise that UK and Irish diatomists perform. I had noticed a few taxa that we had not seen in previous ring tests in a sample I collected during my visit to the Shetland Islands back in May 2019 (see “Hyperepiphytes in the Shetland Islands”) but, on closer examination, the sample proved to be both highly diverse and very challenging. The seven experienced analysts who provide the benchmark analyses for the ring test found, between them, over 150 different species: some we could name with confidence, but others we could match to no published description. Amongst those was the species of Achnanthidium photographed below. It might be Achnanthidium digitatum or possibly A. ertzii but, then again, it does not quite match the characteristics of either of these so, once again, we have left it unnamed (you can find the original descriptions of both these species in the reference list).
According to Algaebase there are 116 species of Achnanthidium that are currently accepted but descriptions of these are scattered through the literature so it is really hard to be confident that you have found a new species during a routine survey. This is particularly the case when we only have light microscopical analyses with which to work, as the small size of Achnanthidium species means that you really need a scanning electron microscope to see the fine details clearly. This, however, assumes that the pool of unnamed Achnanthidium species is finite and that the 116 species on Algaebase is a significant proportion of the total number of Achnanthidium species. A recent study by Eveline Pinseel and colleagues based on samples from Arctic regions offers hints that there is still plenty of diversity within the genus that cannot be linked to named species
This may, however, be a naïve assumption. My colleague Maria Kahlert, who works in Sweden, comments that she is quite happy looking at samples that I send her from polluted sites in the UK as she can name most of the species (Achnanthidium and otherwise) from her own experience. It is the samples from pristine habitats that fox her because so many of the forms are different to anything she has encountered in Sweden. We have, in other words, a neat reversal of the opening line of Anna Karenina (“All happy families are alike, each unhappy family is unhappy in its own way”), with very high beta and gamma diversity of diatoms (probably other microalgae too) as a characteristic of regions with low population density (see “Baffled by the benthos (2)”). We often miss this in our enthusiasm to fit all that we see down the microscope to published descriptions, but when we take time to look hard, that diversity – and those differences between sites – start to mount up.
The unknown Achnanthidium species from Petta Water, Mainland, Shetland Islands (pictured at the top of the post). Scale bar: 10 micrometres (= 1/100th of a millimetre). Photographs: Lydia King
Let’s think of this as an ecological experiment to understand the diversity of Achnanthidium, following the capture-mark-capture approach. Capture-mark-recapture is a technique used by ecologists to assess the size of a population. As it is rarely possible to count all individuals, a portion of the population is collected, marked (a dab of paint on a snail’s back, for example) and released. Some time later, the population is sampled again, and the proportion of those that bear the mark in this second sample is used as an indicator of the proportion of the population captured by the original sample. Though devised for population biology, some have used the same principles to understand diversity in other contexts too so might it work as a means of understanding the yet-to-be discovered diversity of diatoms?
What we have in the scattered taxonomic literature is a record of all the Achnanthidium species that have been “captured” (i.e. observed) and “marked” (i.e. described) by taxonomists. Suppose we now go some locations not previously visited by taxonomists, take some new samples and see 1) how many different forms of Achanthidium we can see and b) how many of these are “recaptured” (i.e. forms that align with previously described species). Or, thinking about the problem in a different way, the number of named species could be compared with the number of distinct “operational taxonomic units” (“OTUs”) detected by metabarcoding. More relevantly, how many extra OTUs are added when more lakes and streams are added to the dataset? There are well-established methods for deriving “rarefaction curves” that might be useful in understanding regional diversity of diatoms, and modifications of “capture-mark-recapture” have been used to understand taxonomic diversity in palaeobiolgoical contexts, so why not in contemporary ecology too?
The Shetland Islands would make an ideal test ground for such a study as they are geologically-diverse habitats providing the types of conditions where Achnanthidium species thrive (low population density and agricultural intensity. The diatoms of the region were studied about 40 years ago by my late mentor John Carter and although one of his samples yielded the type material for Achnanthidium caledonicum there have been so many developments in Achnanthidum taxonomy subsequently that this archipelago represents a tabula rasa for a modern taxonomist. Its many remote lochs and streams offer the setting for a natural experiment which sets out, to put it bluntly, to quantify our ignorance.
Achnanthidium caledonicum from Loch Osgaig, Highland Region, Scotland. Originally described as Achnanthes microcephala f. scotica Carter & Bailey-Watts 1981 (Scale bar: 10 micrometres (= 100th of a millimetre). Photographs: Lydia King.
Carter J. R., Bailey-Watts A. E. (1981). A taxonomic study of diatoms from standing freshwaters in Shetland. Nova Hedwigia. 33: 513-630.
Pinseel, E., Vanormelingen, P., Hamilton, P. B., Vyverman, W., Van de Vijver, B., & Kopalova, K. (2017). Molecular and morphological characterization of the Achnanthidium minutissimum complex (Bacillariophyta) in Petuniabukta (Spitsbergen, High Arctic) including the description of A. digitatum sp. nov. European Journal of Phycology 52: 264-280. https://doi.org/10.1080/09670262.2017.1283540
Van der Vijver, B., Jarlman, A., Lange-Bertalot, H., Mertens, A., de Haan, M. & Ector, L. (2011). Four new European Achnanthidium species (Bacillariophyceae). Algological Studies 136/137: 193-210.
Liow, L.H. & Nichols, J.D. (2010). Estimating Rates and Probabilities of Origination and Extinction Using Taxonomic Occurrence Data: Capture-Mark-Recapture (CMR) Approaches. The Paleontological Society Papers 16: 81-94).
This week’s other highlights:
Wrote this whilst listening to: Sheku Kanneh-Mason’s recording of Elgar’s Cello Concerto. Taking me back to his performance at the proms on a warm evening last summer.
Cultural highlight: Sam Mendes’ film 1917 which, coincidentally, uses the River Tees (as featured sporadically in this blog) as one of its locations
Currently reading: I have just finished Good Economics for Hard Times by Abhijit V. Banerjee and Esther Duflo, which I mentioned a couple of weeks ago. It left me with the feeling that, had both Boris Johnson and Jerermy Corbyn read it and taken on its messages, the election campaign and the UK political landscape might have been very different.
Culinary highlight: OK Diner on the southbound side of the A1 near Grantham. Felt like we were walking into the opening scene from Pulp Fiction (the one where Tim Roth jumps up onto a table and attempts to rob all the customers). Escaped with wallet intact.