Geoff and Chris, two of our band of desmid hunters, chose to stay in the FBA’s brand new holiday apartments and, rather than cross the lake to join us on Saturday morning they headed out to Moss Eccles Tarn, in the area between Esthwaite Water and Windermere. One of Dave’s first dips into one of their samples yielded an almost pure monoculture of another filamentous desmid, Spherozosma vertebratum which presented some beguiling abstract patterns on my computer monitor.
Spherozosma vertebratum from Moss Eccles Tarn, September 2017. Scale bar: 25 micrometres (= 1/40th of a millimetre).
Curiously, after our first encounter with Spherozosma vertebratum we did not see it in any of our other dips into the Moss Eccles samples although there were plenty of other desmids on display. The most abundant of these was Staurastrum productum and, usefully, there were examples showing both apical and side views. The three arms are distinctive (and distinguish it from relatives such as S. arachne which have five) and you can also see the knobbly “verrucae” on the spines as well as a broad mucilaginous envelope around the cells.
Staurastrum productum in side (left) and apical (right) views. Images photographed from a computer monitor so apologies for their poor quality. Scale bar: 25 micrometres (= 1/40th of a millimetre).
Another desmid with spines and mucilage was quite common. This was Staurodesmus bulnheimii. Spines slow the rate of sinking so are associated with several genera of predominately planktonic desmids. The star-shaped arrangement of colonies of the diatom Asterionella formosa play a similar role (see “Little bugs have littler bugs upon their backs to bite ‘em”). There were also several cells of a small Cosmarium species, including some that had recently divided and the image shows how one cell has split down the central isthmus and a new semicell is growing back on each of the two daughter cells. Finally, I have included an illustration of Micrasterias radiosa. To the uninitiated this may look little different to M. compereana, illustrated in the previous post, but if you look closely you will see that the incisions between the lobes are much deeper in M. radiosa.
One sample from Moss Eccles Tarn kept me busy for half the morning and this account describes only part of the diversity. Note how the differences between this and the School Knott Tarn sample are not just in the genera and species present but also in the life-forms I found. The School Knott sample was from a Sphagnum squeezing whilst the Moss Eccles sample was from a plankton net. That explains why I saw more spine-bearing desmids in the latter. If I had looked at a plankton sample from School Knott and a Sphagnum squeezing from Moss Eccles, I might have found a different balance of life-forms between the two tarns. But time was running out and I had to move on …
More desmids from Moss Eccles Tarn, September 2017: a. Staurodesmus bulnheimii; b. Cosmarium quadrifarium var. hexastichum; c. Euastrum cf. gemmatum. Scale bar: 25 micrometres (= 1/40th of a millimetre).
Micrasterias radiosa from Moss Eccles Tarn, September 2017. Scale bar: 25 micrometres (= 1/40th of a millimetre).