I suggest invertebrates feeding on algae is a good field of study for amateur microscopists, even those of us with little experience. As you read 'MICSCAPE' I assume you already have a microscope or are about to acquire one. With a 10:1 objective to search a slide, (do get a mechanical stage if you can afford it, it makes searching much easier) a 40:1 for detailed examination, a 10:1 eyepiece, a few slides and cover glasses, a live sample from a pond etc., boggy places and moss squeezings are best for desmids, and you are in business.

Figure 1. Amoeba eating Closterium.
 
 

Take notes of your observations, make sketches of anything you do not recognise to look up later with measurements if possible (how this done will be shown later). It is not too difficult to record your observations with a camera (see earlier article on photomicrography). Although my main interest is in the desmids I take note of other algae and its consumers in a sample.

In 1986 I found amoebae feeding on desmids, I sent some photomicrographs to Alan Brook Professor Emeritus at the University of Buckingham, he made similar observations of a different amoeba and desmid species, for details see Brook & Ells (1987). A good example how the amateur can get involved. Figs 1 & 2 are from the same period.

Figure 2. Amoeba with ingested desmids
 
 

You may need to study a sample at intervals over a day or two in order to see what is being ingested, digested and excreted. The method I use is :- Put a drop of the sample on a slide; spread a thin smear of petroleum jelly (Vaseline ™) on a spare slide, then drag the edge of a cover glass along this slide to pick up a continuous strip of the jelly, repeat with the other three sides of the cover glass, then lower the cover glass over the sample drop, see Fig. 3 right.

You will need to experiment to get the right size drop of water containing the specimens and the right amount of jelly on the cover glass. If you use oil immersion objectives you will find it easier with the cover glass sealed than with the cover glass floating on a film of water.

After the first examination the slide should be kept in a cool place, a north facing window sill is ideal, protected from dust with a watch glass or something similar. You will find algae and protozoa etc. will keep alive for a week or more using this method. Or you may find you have bacterial soup or fungal attack renders the sample useless.

I have also kept samples in jam jars (nothing fancy) for several months dipping into them from time to time. In one such sample collected from a tarn near the Drunken Duck Inn in the English Lake District in the company of a group of sober desmid enthusiast on a foray from the Freshwater Biological Association. The small tarn is not named on my Ordnance Survey map.

Each mount I examined from the tarn had a few small rods of compressed debris I knew to be faecal pellets of a small crustacean or possibly insect larvae. Macerating the pellets on one of these mounts by applying pressure and a twisting motion to the cover glass, I identified large numbers of empty semi cells of the desmid; Staurodesmus triangularis (Lagerh) Teiling 1967, and two semi cells of Closterium costatum Corda ex Ralfs 1848.

In the jar containing the tarn sample were two freshwater shrimps of the genus Gammarus, I removed and examined these under a low power (35:1) stereo microscope and found one had a pellet in its anal cavity. After fixing, I removed the pellet, placed it on a slide and macerated it as before. From this one pellet I found no less than 80 empty Staurodesmus semi cells, one Euastrum sp. and one Staurastrum. Note all the ingested Staurodesmus cells were broken at the isthmus - this agrees with the laboratory experiments carried out by Coesel (1997). However the desmids ingested by the amoeba were excreted with the cell wall intact, the protoplasm apparently being taken up by the digestive process through the pores in the cell walls.

The shrimps having been in the jar six months had little choice of diet. I could not find any living Staurodesmus at this time, possibly they had all been eaten by the shrimps. Other desmids were still thriving, mostly Closterium spp. and a few of that handsome desmid Euastrum verrocusum alatum.

Aquatic oligocheate worms are inveterate feeders on algae, in water collected from Thursley Common, Surrey on the 21.5.88 and examined on the 18.7.88 there were several oligocheatae that had been feeding on two species of Trachelmonas. One of the worms had ten Trachelmonas which could be clearly seen in its digestive tract, and there appeared to be even more in a dense mass within which individual cells could not be easily distinguished. Trachelmonas is one of several micro-organisms claimed by both the botanist and the zoologist, the smaller of the two species consumed, about 21 x 11 µm. and the one in greatest abundance in the sample, had an ochre coloured lorica with no obvious markings and a distinct collar. The other was larger 36 x 19 µm. with a paler colour lorica, a granular surface and a distinct collar. More than a hundred species have been described in texts on algae and protozoa.

Further observations made in September 1990 in a sample collected from a boggy pool near Blair Atholl, Perthshire, oligocheate worms were found that had been feeding on diatoms and desmids. In one could be seen a diatom of the genus Surriella, and in another three cells of the desmid Tetmemorus granulatus. The worms were approximately 1.25mm. long, their head 219µm broad. The Tetmemorus were approximately 200µm. long by 35µm broad, so the three were nearly half the length of the worm. The cells were unbroken, one empty, one nearly so, the other with shrunken chloroplast.

Later in the same year an aquatic oligocheate worm with a desmid of the genus Closterium in its gut was found in a sample from Sutherland, Scotland. (Fig. 4. right - note the recently divided cells of the desmid Spirotaenia condensata). Both these samples were freshly collected material without the restrictions on diet of the earlier samples. Lund & Lund (1995) show part of an oligocheate worm with a desmid, genus Euastrum in its gut (page 275).

In 1997 during examination of a sample of water from Dartmoor, I found a rotifer that I now know to be Tetrasiphon hydracora, in its stomach could clearly be seen among a green mass a desmid of the genus Micrasterias. I sent a drawing of the rotifer to Mr. Eric Hollowday who identified the animal for me, I quote from his letter :-

' This animal is a real "desmid cruncher" to be sure! Rus Shiel of Australia showed a remarkable video in Poland three years ago showing it crunching and munching its way through one desmid after another with astonishing speed, the trophus seems to be excellently adapted for this purpose, so keep it out of your cultures.'

This rotifer is probably rare, fortunately for the desmids, it is the only time I have seen it, Eric Hollowday a rotifer specialist tells me he has seen it alive only once, from Thursley Common.

I understand that in Japan a species of algae is cultivated to feed the specially bred rotifers of the genus Brachionus which in turn are fed to fish fry for the ornamental fish market.

I have seen in a video by Mike Dingley a Heliozoan with a desmid , genus Cosmarium impaled on one of its spines (axopodia) we do not know if it is ingested or not.

In spite of all these observations there is still plenty of work for the amateur. Are there other rotifers that actually take in alga cells, we know there are some that feed by sucking out the protoplasm? Do protozoa feed on algae they chance upon, or are these primitive animals able to select their prey? Do some species of aquatic worms prefer diatoms to desmids? Why not share anything you discover with us? Send a description of your observations with a photomicrograph if possible to the Micscape Editor for possible publication.

Acknowledgements:-

The author is indebted to Eric Hollowday for identifying the rotifer and for his helpful comments. Thanks to Faith Coates for the excellent sample from a bog on Dartmoor.

References:-

Brook A.J. & Ells W. (1987) The Feeding of Amoebae on Desmids. Microscopy Vol. 35 Part 7. pages 537-540. The Quekett Microscopical Club.

Brinkhurst R.O. (1963 revised 1971) Aquatic Oligocheata. F.B.A. Pub. No. 22.

Coesel P.F.M. (1997) The edibility of Staurastrum chaetoceras and Cosmarium abbreviatum (Desmidiaceae) for Daphnia galeata/hyalina and the role of desmids in the aquatic food chain. Aquatic Ecology 31: pages 73-78. Kluwer Academic Publishers.

Jahn T.L., Bovee E.C. & Jahn F.F. (1949) How to know the Protozoa. Wm. C. Brown Pub. Dubuque Iowa.

Lund H.C. & Lund J.W.G. (1995) Freshwater Algae. Biopress Ltd. Bristol. England.

Editor's note: Comments and feedback via email to Bill Ells are welcomed.
 
 

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