Stentor - a large protozoan
On this page we shall look at some of the smallest creatures to be found in ponds. To show their size, all the images taken through a microscope have been marked with a scale bar. One micron (µm) is one millionth of a metre, or one thousandth of a millimetre, hence a scale bar marked 50 µm is 0.05 mm (or 1/20 mm) in length.
Many of these will be unicellular, but do not think that because they possess only one cell they are simple. Far from it, because in that one cell they have to perform all the functions that multi-celled creatures share out amongst their many specialised cells. Additionally, just to make the requirements even more demanding, because they live in fresh water but themselves contain many complex chemicals, they tend to absorb water by osmosis. This means they continually have to expel excess water. This is achieved by collecting the water in a vacuole, commonly called a contractile vacuole or water expelling vacuole. These vacuoles grow quite large and then will collapse when the water is expelled to the exterior through a temporary pore. We use our kidneys to perform this sort of task. Vacuoles are also used for storage and digestion of food, and in the image above of Stentor, a large protozoan, you can see greenish inclusions in vesicles or vacuoles. These are algae the creature has ingested. Stentor feeds rather similarly to rotifers, by using cilia to create water currents that draw in food. It also attaches itself to objects by means of its "foot", seen in the image as the narrow end of the creature. The food is engulfed through an oral cavity. When attached to a solid object and feeding, Stentor takes on a trumpet shape, with a ring of cilia at the "bell" end.
| It is tempting to regard creatures such as Stentor as animals and other creatures such as algae, which generally contain green chloroplasts, as plants, but life is not quite so simple. Consider a creature such as Euglena. It moves around with a characteristic rolling action. It has a red eyespot and possesses a whip like appendage called a flagellum, which it uses for movement. This specimen came from the mud in main Mill Pond. It has characteristics of both animals and plants and the green colouration and red eyespot can be clearly seen. |
Euglena |
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Phacus, another euglenid, also has chlorophyll and a red eyespot. |
So, it seems the distinction between animals and plants is not, well, distinct! For this, and other reasons, these organisms are not classed as either animals or plants. Even an alga such as Spyrogyra, that we might feel quite comfortable classing as a plant, is not regarded as such. No, all these creatures are classed in a separate kingdom as Protists. This does mean that this kingdom has a rather diverse range of members, however.
Spyrogyra This is a filamentous alga. The individual cells are strung end to end forming long hair-like strands. You can see the spiral chloroplasts punctuated with pyrenoids. In the cell centres the nuclei hang like spiders in a web.
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Stentor is a member of a group of protozoa called ciliates because their bodies are covered in small hairs, or cilia. This is a very diverse group in itself. Here are a few of its members. |
Litonotus You can see a large vacuole near the tail, also in front of that, the characteristic double nucleus of this protozoan. There are cilia visible around much of the body. It uses these for swimming. This specimen came from the main Mill Pond. It is a very smooth and steady swimmer.
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Oxytricha, is a common
hypotrich ciliate, a very fast swimmer and very difficult
to photograph. This one was photographed using flash to freeze motion,
as were many of the photos on this page. Even so, the frontal cilia
are blurred. |
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Peritrich ciliates are so called because their cilia are at the top end of their bodies. They commonly live on stalks attahed to larger objects and the stalks may be either rigid or able to be contracted suddenly to avoid danger. Some hitch lifts on larger creatures. Peritrich ciliates are often found, for example, on Cyclops or other copepods.
Vorticella A peritrich ciliate, showing cilia and twisted contractile stalk (myoneme) |
Vorticella viewed from above This shows various vacuoles, but also shows the wave-like motion of the cilia:- metachronal rhythm. |
No discussion of protozoa would be complete without mentioning amoeba. There are in fact many species of amoeba. They usually move quite slowly, extending pseudopodia as they glide over surfaces. They engulf other organisms as they encounter them "spitting out" those that are found unsuitable to eat. Not all amoeba are "naked" - some build cases from grit or other materials. These are termed testate amoebae.
Arcella A very common testate amoeba. The central light area is the hole in the base of the case or test. It extends pseudopodia through this so that it can move rather in the manner of a snail. You can see all around the edge of the amoeba the pseudopodia that it uses to hold onto the inside of the test. The test is shaped like a very flattened dome.
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Unidentified amoeba This one was evidently partial to algae. |
Other organisms related to the amoeba are the heliozoans,
"sun animalcules". I think their shape explains their name
well enough. They have long spines that they use to catch their prey,
which they draw in down their spines. |
Raphidiophrys, a heliozoan
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Desmids are single celled algae that are sometimes
found in the dipping ponds. The most commonly found is Closterium |
Closterium sp The dots in the extreme tips of the cell are crystals of barium sulphate.
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Diatoms also have a pleasing geometry, but unlike desmids they have silica impregnated cell walls. Effectively they are made of glass. They come in many forms, the one on the left is Gyrosigma, found in the main Mill Pond, and below is an epiphytic diatom (grows on the surface of other plants, algae, etc.) called Gomphonema which grows on stalks attached, in this case, to filamentous algae.
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| All the above organisms are Eukaryotes, that is they possess cell nuclei, but there remains one kingdom of important organisms, the Prokaryotes, which do not possess cell nuclei. The most prolific of these are the bacteria, and also very common in ponds are the cyanobacteria or blue-green algae. | |
This is a typical filament of cyanobacteria. They glide mysteriously amongst the mud and debris at the pond bottom and the filament is composed of a string of cells that have no nuclei. Bacteria are even smaller and appear as small rods, wiggly helices or small spheroids under the microscope. These organisms are the very bottom of the food-chain, but make an enormous contribution to the food web of the pond by breaking down waste matter and by being food for the protozoa and larger creatures. |
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This concludes the brief overview of pond life. It is by no means comprehensive and apologies are due to any organisms that may feel neglected or overlooked... there are so many different creatures etc. out there that even a life-time pond dipping and peering through the microscope would not see more than a small fraction of what there is to see.
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