Desmids are an attractive unicellular microscopic form of green algae (Chlorophyta) found only in freshwater. They belong to the class Zygnemaphyceae which is unique among the freshwater algae in that each plant can join with another of the same species in a process known as conjugation to form zygospores. Although zygospores have not been found for all species.
Samples are collected from boggy areas or the margins of Lochs in North West Scotland. Part of the sample is preserved in about 5% formalin the remainder is examined in the living state over a period of several days. I prefer to examine live, not only is the bright often vibrant green attractive, it makes them easier to pick out, particularly the smaller species, from amongst any debris. Also they can often be observed multiplying asexually by division. and more rarely conjugating to form zygospores, they normally do this in adverse conditions usually when the habitat is drying out, forming new plants when climatic conditions are right. Attack by fungi or fauna can also be observed, Brook & Ells (1987). The drawback of examining live material is that drawing or photography is much more difficult owing to movement in the water caused by the fauna or the temporary mount drying out.
All observations were made with a Nikon Skt. trinocular microscope using 20:1 plan achromatic and 40:1 plan fluorite objectives, eyepiece HKW 10:1. Photographs were taken with A Praktica SLR. using a Zeiss projection eyepiece 4:1. Drawings were made using a Biolam camera lucida.
A sample was received from Loch Chealamy, Strathnaver, Sutherland on 28.9.94 and examined the same day, this was found to contain some 50 species of desmid including Closterium nematodes Josh. var. proboscideum W.B.Turner 1892. This is the first time the species had been found in a living sample from the British Isles, the type C.nematodes var. nematodes has not been seen. Brook (personal communication) also examined a sample and confirmed the species and variety.
The cells (Fig.1.) are 8-12 times longer than Broad, moderately curved. The walls have a thickened ring just behind the apices which make this species distinctive and gives rise to the name nematodes, 5-9 broad ridges known as costae can be seen across the breadth of the cell these run from apice to apice, when the two edges of the costae are brought into focus where they meet the cell wall they are seen as pairs of lines, the cells are circular in the centre with the coastae spaced evenly around the cell wall. There are 5-7 pyrenoids in each semi-cell.
Tell & Couté 1993, showed the shape of the costae of Closterium costatum Corda ex Ralfs 1848. in Scanning Electron photomicrographs. Fig.2. showing how the centre of a cell in cross section should look is based on these micrographs.
Cosmarium praegrande Lundell (1871). Several specimens were found in a sample from Rhiconich, Portle Vorchy, Sutherland in October 1993, This large species of Cosmarium is exceedingly rare in Britain. The dimensions of the Rhiconich specimens; Length 120 microns. Breadth 71 microns isthmus 26.5 microns; are larger than those described by W.&.G.S.West (1908). Also larger than those described and figured by Prescott et.al.(1981).
The semi-cells are spheroid, end view
circular, the cell walls are densely covered with prominent
conical granules (or warts) except for a small area at the apex
and finely punctate between the granules. The size of the
granules are as those figured by W.&.G.S.West. (after
Lundell) those figured by Prescott et al. are smaller.
The specimen in the photomicrographs has had a neat hole cut in the wall of one semi-cell, the piece removed can be seen nearby, the hyphae of a microscopic fungi can also be seen, the cell is devoid of any vegetable matter.
Acknowledgements: Thanks to Mr.Alan Joyce of Sutherland for the samples from Chealamy & Rhiconich, to Dr.J.W.G.Lund CBE,DSc,CBiol,FRS. Freshwater Biological Association, England, for figures from the Fritsch Collection of Algal Illustrations. and to Prof.A.J.Brook PhD,DSc,FRSE. of The University of Buckingham for his comments on the sample from Loch Chealamy.
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Brook A.J. & Ells W. (1987) The feeding of Amoeba on Desmids. The Quekett Journal of Microscopy Vol.35. Part 7.
Prescott G.W., Croasdale H.T., Vinyard W.C., &. Bicudo De M. C.E. (1981) A Synopsis of North American Desmids. Part 2. section 3. University of Nebraska Press.
West W.&.G.S. (1908) A Monograph of the British Desmidiaceae. Vol. 3. The Ray Society.
Tell G. & Couté A. Cryptogamie Algol 1983. 14(1) 43-63.
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