By William Ells
Coniferae, Walnut Tree Lane, Loose, Maidstone, Kent.
ME15 9RG. Tel. (01622) 744819
The Director of a local Nature Conservation Trust asked "what are desmids" when I told him I had found some in a small acid bog at Hothfield in Kent. Twenty years ago I was asking the same question. The majority of Naturalists study birds, as is evident by the great number of books on birds published every year. Butterflies and flowering plants are also popular subjects. If you study flies; millipedes; spiders; or anything most people find repulsive you will be thought eccentric, if you study algae your mental condition will be considered unstable.
Desmids are an attractive and unusual group of freshwater algae. They are microscopic flowerless plants without roots; stems; or true leaves. In fact complex single cells, some are the largest single cells in the Plant Kingdom, the largest can just be seen with the unaided eye.
Many of the more complex types are divided into two semi-cells by a distinct isthmus or constriction, once thought to be a pair of joined cells, hence the name desmid derived from 'desmos' Greek for a bond or chain. Desmids belong to a group of algae; the group includes some non desmid filamentous types; that come together, usually when their habitat is drying out, in a process known as conjugation to form resting spores, from which new plants will grow when the right conditions prevail (Fig.1). The more common method of reproduction is by the impossible mathematics of multiplying by dividing, usually at the constriction (Fig.2).
They possess three perfect planes of symmetry and exhibit
an incredible diversity of shapes as illustrated (Fig.3).
In addition to their remarkable variation in shape the cellulose walls of many species are ornamented with granules, spines or wart like protuberances arranged in complex, yet orderly and symmetrical patterns. They are coated in a clear jelly like substance known as mucilage which can be shown by adding Indian ink to the water (Fig.4). Some are joined together to form quite long filaments (Fig.5) which the beginner could confuse with the more common filamentous algae that forms 'blanket weed' on ponds.
Do they have common names? No , because they are not commonly seen ! but don't let the scientific names deter you from studying them. I only know the meaning of a few of them and cannot pronounce many of them.
An amazing 1200 species and varieties have been found in the British Isles. World-wide some 8000 species have been described. Most seem to prefer nutrient poor, acidic waters, the most prolific collecting sites in the British Isles are the Sphagnum bogs and peat pools of Scotland, Wales, & Ireland. David Bellamy (1) refers to 'sheets' of desmids occurring in the peaty pools Thursley Common, Surrey.
Another good reason for not extracting the peat and destroying these valuable habitats.
Some do occur in other parts of the British Isles, sometimes in large numbers but fewer species, including my home County, Kent. Indeed some of the earliest British records (150 years ago) are from near Tunbridge Wells. I have found fifteen species in Hothfield bogs, a dozen in a spring at Louisa Lake in Bedgebury Park, and even in temporary water in a plough furrow at Staplehurst I have collected a few species. Some I have found in garden ponds in Maidstone, probably introduced with fish or aquatic plants, or maybe brought on the feet of visiting birds. In one case exceptionally large numbers of individuals of a single species turned the pond water green (2). Any freshwater, damp moss &c., may hold a few desmid treasures, the desmid hunter should always be on the alert for possible sites and always have some collecting bottles or plastic lunch bags and closures in his pocket.
What use are desmids
Well ! what use are human beings ? Like all organisms they have their place in the complex web of life. Like other plants that contain chlorophyll, desmids live by capturing the sun's energy (photosynthesis). From carbon dioxide dissolved in the water they manufacture sugar and starch as food. They; and the food stored in them; are consumed by Amoeba (3) and various other invertebrates (4) which are in turn eaten by other animals. Desmids are known to absorb large quantities of barium and strontium (5), thus they play a part in the re-cycling of these chemical elements in freshwater.
To study the fascinating world of desmids and other microscopic freshwater life, you will only need the simplest collecting equipment. You WILL require a microscope with a magnification of 100 times and 400 times, this need not cost you more than a decent pair of binoculars. Look out for a good second hand model; Do consult a microscope club or society if in any doubt, they may help directly or put you in touch with someone living in your area. Desmid hunting will make your walks and country outings much more interesting, You may become like myself a desmid 'Twitcher'. GOOD HUNTING.
I wish to thank Alan J. Brook PhD DSc FRSE, Emeritus Professor of Life Sciences, University of Buckingham, for his help & advice in preparing this article .
SUGGESTED READING :-
A general introduction to the flora & fauna in ponds & streams Freshwater life by John Clegg.
A beginners guide to Freshwater Algae by Belcher & Swale. 1976.
An Illustrated guide to River Phytoplankton by Belcher & Swale 1979.
I do not know if the above are still available, but still obtainable is the modestly priced (£7 July 1996) Desmids of the English Lake District by Lind & Brook 1980. FBA pub. No. 42. From the Ferry House, Ambleside, Cumbria LA22 0LP. UK. A good introduction to the Desmids, many of the species described can be found World Wide.
For the serious student:- The Biology of Desmids by A.J.Brook 1981.
(1) Bellamy (1986) BBC Wildlife Vol.4. No.1 page 4.
(2) Ells (1987) A desmid bloom, Microscopy Vol.35. part 8. page 661.
(3) Brook & Ells (1987) The Feeding of Amoebae on Desmids, Microscopy Vol.35. part 7. pages 537-540.
(4) Ells (1990) Some notes on the Desmidiacae. & Invertebrate feeding on Algae. Microscopy Vol.36.part 5. pages 429 -431.
(5) Brook (1987) Desmids - Algae with a taste for the unusual. New Scientist 10/10/1987.
Fig. 1. 2 cells of the genus Tetmemorus have come together to form a zygospore.
Fig. 2. (a) A Closterium in the early stage of division - compare with Fig. 3. (e) which is a complete cell of a different species.
Fig. 2. (b) A Micrasterias that has divided & is growing a new semi-cell. Compare with Fig. 3(a) that is a complete cell of the same species.
Fig. 3. (a) Micrasterias,
Fig. 3. (b) Netrium,
Fig. 3. (c) Staurastrum,
Fig. 3. (d) Cosmarium,
Fig. 3. (e) Closterium.
Fig. 4. A Staurastrum, showing mucilage surrounding it.
Fig. 5 A filamentous desmid: one cell has broken away & can be seen in end view. Note: desmids can be circular, elliptical, triangular or square in 'end' view.
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