Microscopic animals are fascinating in their variety and are found all around us — in water, in soil, in food and on our bodies. Observing microscopic animals involves knowing where to look for them and some kind of magnification to render them clearly visible. Usually, a microscope is required, although in some cases a powerful hand lens can reveal quite a lot.
One of the best places to look for microscopic animals is an unpolluted freshwater pond. By taking samples from different parts of the pond, the full variety of microscopic inhabitants can be viewed. Microorganisms need a source of food, so rather than open water, the best places to look are at the bottom of the pond among gravel, mud and detritus such as decaying leaves, and among vegetation such as aquatic plants and algae. It is a good idea to take samples from both sunny and shaded areas. A jar or beaker attached to a long stick is useful for reaching awkward places.
A very convenient way of observing microscopic animals at home or in the laboratory is to make a hay infusion. A suitable container, such as a jam jar, is filled with pond water then a small amount of hay or dried grass is added. It is left to stand in a spot with a reasonable amount of daylight and over the following days and weeks, a variety of microorganisms will appear. These can be observed by taking small samples of the water using a pipette and viewing the sample through a microscope. Samples from different locations — near the surface, at the bottom and around the decaying hay — will reveal different organisms.
Among the many types of microscopic organisms that can be found in pond water or hay infusions are protozoa, rotifers, daphnia and tardigrades — also known as water bears. Protozoa are single celled organisms that are usually free-swimming; strictly speaking, they are not classified as true animals, but they are capable of independent movement and are often regarded as such. Rotifers, although mostly similar in size to protozoa, are multicellular animals and have a clearly visible internal structure. Tardigrades are found in almost every environment where water is present, even if only intermittently, and have four pairs of legs. Daphnia, or water fleas, are relatively complex aquatic organisms, with visible internal organs, compound eyes and a circulatory system.
Other good sources of microscopic animals are soil and mosses. These contain many protozoa and rotifers, as well as nematodes, tardigrades and tiny arthropods such as mites, which, although just visible to the naked eye, require magnification to be observed in detail. Even dried up puddles and other seemingly unlikely places can harbor interesting microscopic life. Many microorganisms can go into a dormant state to survive inhospitable conditions, and will become active again when circumstances are more favorable. Often, adding water — rainwater rather than tap water — to a dry sample and leaving it for a few days will reveal a multitude of microscopic animals.
Pond water samples and hay infusions should ideally be kept in an area with plenty of daylight and oxygenated daily by bubbling air into them with a pipette. If the water needs to be topped up, rainwater or pond water should be used. Tap water normally contains traces of chlorine, which may be harmful to microorganisms.
For viewing with a microscope, take a small sample using a pipette and place a drop on a microscope slide. Place a cover slip over it by lowering it on at an angle to avoid trapping air bubbles. Microscopes will have three or four objective lenses giving different magnifications, for example 30, 100, 200 and 400 times. It is best to start with the lowest magnification then zoom in on areas of interest.
Larger microscopic animals, such as mites, can best be viewed at lower magnifications. Protozoa and rotifers can be seen in detail at 100-200 times magnification. Since most of these microorganisms are transparent, internal details can best be seen by adjusting the diaphragm on the microscope to maximize the contrast.