The Amoebae.
The amoebae are here dealt with in two groups: the naked amoebae (this page), and the testate amoebae (those having tests, or shells), on the next page.
The Naked Amoebae.
These are the classic amoebae of the general biology textbooks, and part of the intention of this collection of photomicrographs and observations is to dispel the notion that an amoeba is a shapeless blob of animated jelly, randomly engulfing objects it happens upon en route to nowhere in particular.
It has been known for a long time that the general direction of an amoeba's movement is a chemotaxic response to concentration gradients of nutrient (and other) substances in the surrounding water, and that most amoebae maintain a fairly definite shape, with a recognizable front and back end, during all of their activities.
They move by extending pseudopodia (Latin, false feet) which are able to absorb food particles and which increase and decrease in size by a process called protoplasmic streaming, which term describes it well. Examination of individual amoebae over a period of time shows a high level of discrimination in regard to food particles taken in. Some are immediately absorbed, and others, apparently similar, are rejected or avoided.
Whilst the textbooks always describe amoebae as common, this is not always apparent to the casual microscopist. Most are quite small, requiring a magnification of x400 or more to see much detail. They move slowly and so do not draw attention to themselves, and can spend long periods after the preparation of the microscope specimen in a dormant, rounded state before they resume normal behaviour.
In addition, the higher light levels required by the high powers of the microscope can inhibit their movements, causing them to round up into balls difficult to recognize as amoebae. The use of a deep blue-green filter in the illumination path, and the progressive reduction of the lighting level by dark adaptation of the eyes can help in this case.
With all this in mind, the observation of amoebae and their behavior is a most rewarding business, and they will turn out to be more common than may have been thought at first.
Click for
diagrams of amoebae (57KB).
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This amoeba is moving in a 5 o'clock direction. The clear zone of cytoplasm in the advancing pseudopod can be clearly seen. The nucleus is visible as a dark spot surrounded by a clear zone at the centre of the body, and a number of food vacuoles and a contractile vacuole are seen in the rear portion. The branched structure to the left is a hypha of the predaceous fungus Zoophagus.
Phase Contrast: x500. |
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This amoeba is moving in a 7 o'clock direction. The clear zone of cytoplasm in the advancing pseudopod can be clearly seen. The nucleus cannot be made out in this picture.
Phase Contrast: x700. |
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This amoeba is moving towards the 2 o'clock position, showing the same clear zones in the advancing pseudopodia as in the picture above. One of the food vacuoles contains a group of four algal cells. An amoeba of this kind does not glide smoothly over the contact surface, but extends pseudopodia to the surface and "tiptoes" over any particles it doesn't consider suitable as food. Desirable objects, on the other hand, are engulfed without pause.
Amoebae are highly selective diners.
Phase Contrast: x600. |
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An amoeba in the act of ingesting an algal cell. The appearance of a mouth siezing an object is an effect of optical sectioning as the amoeba forms a vacuole around the food particle. A passing flagellate is seen to the upper right. This picture is an illustration of what the textbooks call phagocytosis. A similar process, but not involving a food particle, is called pinocytosis, and results in the inclusion of pond water into the amoeba's body.
Phase Contrast. x800. |
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Two amoebae drawn to the same algal cells. The amoeba on the right (with large contractile vacuole) is in the act of ingesting the cells -- the one on the left has already ingested several algae of a similar kind which can be seen in its food vacuoles.
Phase Contrast. x800. |
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Here is an amoeba in full flight (so to speak) extending its pseudopodia towards a group of algal cells which could well become its next meal. Several vacuoles are seen towards the rear of its body. The nucleus cannot be distinguished.
Phase Contrast: x700. |
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Here an amoeba is seen in the act of engulfing a pennate diatom.
Phase Contrast: x800. |
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Pinocytosis is the name given to the process by which the amoeba "imbibes" water from its surroundings. The process is essentially similar to phagocytosis, in which food particles are ingested, but instead, pond water is enclosed in a vesicle or vacuole which becomes part of the cell contents.
Single algal cells and short pieces of algal filament which have been absorbed by phagocytosis can be seen within the amoeba's body.
Phase Contrast: x700. |
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Another example of an amoeba taking in water by pinocytosis. The green colour of the picture is due to the use of a narrow bandpass green filter to maximize contrast.
Phase Contrast: x700. |
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An amoeba engulfing the end of a filament of algae too large to ingest. Many algae produce a mucous secretion which can provide an amoeba with some nourishment. The amoeba is probably in the act of stripping off this secretion before moving on to other pastures.
Phase Contrast. x1000. |
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An amoeba found in sewage sediment from the activated sludge process. There are a large number of apparently empty vacuoles, and the nucleus, enclosed in a clear membrane, is seen near the centre of the body.
The colour of the image is due to the use of a Meopta (anoptral?) phase contrast objective which has yellow-brown coloured phase plates.
Phase Contrast: x600. |
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This amoeba is probably Saccamoeba limax or S. lucens, and is moving towards the left. The term "limax amoeba" applies to this genus, and describes an elongated, cylindrical amoeba which proceeds in a steady forward flow, and maintains a definite, characteristic (monopodial) shape during locomotion. The frequent sudden eruptions of cytoplasm which are typical of many amoebae are seen less in the limax amoebae, occurring mostly during changes of direction. The posteriorly placed contractile vacuole is typical of Saccamoeba.
Phase Contrast: x800. |
