Introduction to some aquatic protozoa in Essex
Another very common heliozoan is Actinophrys sol and although much smaller than its cousin
Actinosphaerinm is still spectacular to observe and also far easier to find. Actinophrys usually
grows to about 50 urn in diameter compared to the 250 - lOOOmm of the Actinosphaerium. Both are
found in similar environments and both feed in the same way by catching prey with their axopodia
and then forming vacuoles to digest the meal, which can consist of protozoa, algae, flagellates and
rotifers. When Actinophrys has been feeding particularly well it can be covered with a mass of
vacuoles and looks like a frothy ball floating in the water. Observations can be carried out with
bright field lighting but phase contrast is to be preferred if a clearer view of the cell and axopodia
are needed and also for showing detail in photography. Actinophrys can be found in great numbers
during the spring and autumn when there are large numbers of green flagellates swimming around.
The photograph below in Figure 8 shows a well-fed Actinophrys so! taken with differential interference
contrast (DIC) lighting.Thc photograph was taken using a x40 Nomarski objective and shows a
large number of vacuoles and the large central nucleus.
Figure 8. Actinoprhys sol under differential interference contrast lighting © Steve Durr
Bacteria
The role of bacteria in the ecology of the freshwater habitat must not be forgotten. They help to
clean up any decaying plants and animals that have died and also play a major role in the chemistry
of the water. They are also one of the prime sources of nutrition for many species of protozoa.
Bacteria come in all shapes and sizes from rods, spirals, spheres and many other wonderful and
weird shapes. Most of the bacteria that are found in water are harmless to humans; however, certain
species arc known to be pathogenic and arc responsible for the death of humans in poor, less-
developed countries. Here in England we have managed to contain most of the water- borne diseases
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Essex Naturalist (New Series) 18 (2001)