As I do not pretend to great expertise on the desmids, I sent photographs of the specimens I collected during my visit to Loughrigg (see “A visit to Loughrigg Fell”) to Dave John who, in turn, passed them to David Williamson, to confirm their identities. David Williamson co-authored the most comprehensive work on British desmids currently available, so I’m pleased to have his views on these specimens. To be honest, I was a little disappointed that I found so few desmids at a location from which so many had been recorded in the past. But then I am not a desmid expert, and may not have been looking in the best places.
Desmids from the margins of Lily Tarn, Loughrigg Fell, Cumbria, May 2015. a. Netrium digitus var. latum; b. Closterium dianae; c. Closterium dianae var. minus; d. Closterium directum (e. shows an entire cell of C. directum, photographed at lower magnification). Scale bar: 25 micrometres (= 1/40th of a millimetre).
I also found several cells of Eremosphaera viridis in squeezings from submerged Sphagnum at the edge of Lily Tarn. At first, I thought that this was a colony of small cells but it is, in fact, a single large cell containing numerous small chloroplasts around the edge, giving it a very distinctive appearance. Like the desmids, it is a member of the Chlorophyta, or green algae, but it belongs to a different order, the Chlorellales rather than the Zygnemetales. That means that they are as different to one another as a rat is to a human. By contrast, Euglena mutabilis, which we met in the previous post, is as different from a desmid as a human is from a slug.
I can recommend the desmids to anyone interested in microscopy. They are, in many ways, much more amenable to amateur study than the diatoms. Desmids are generally about an order of magnitude larger than diatoms, which means that you can study them with a medium-power objective, rather than an expensive oil-immersion objective. There is, in addition, a good English-language guide available whereas much of the key literature on diatoms is in German. There are also plenty of sources of information available online. The only drawback with desmids is that their habitats are less widespread. Alternatively, I could put a positive spin onto this and remind you that a fascination with desmids will take you to some of our most spectacular landscapes.
Eremosphaera viridis from submerged Sphagnum at the margin of Lily Tarn, Loughrigg Fell, Cumbria, May 2015. Scale bar: 25 micrometres (= 1/40th of a millimetre).
Brook, A.J. & Williamson, D.B. (2010): A Monograph on some British Desmids. Ray Society, London.