PHYLUM AMOEBOZOA
Phylum Amoebozoa
Introduction
The amoebozoa are a part of the large group of protozoans referred to as the unikonts, with a
single flagellum. The reason that the amaeobas fall into the group is that there are some species
that use amoeboid movement during part of the life cycle and flagellar locomotion in another.
Amoebozoa are the protozoans that use large fleshy pseudopods, lobopods. It was once thought
that all protists that used pseudopods were related to each other and were placed in the
subphylum Sarcodina in the phylum Sarcomastigophora. With protozoan phylogeny now
emphasizing the importance molecular techniques and mitochondrial and membrane
complexity and structure, pseudopods are now found scattered throughout the various
protozoan taxa and are no longer considered a monopyletic trait. What’s the difference
between a protist and a protozoan?
Amoeba sp.
Fig. 1. Major internal features of an
amoeba. © BIODIDAC
Amoebas are commonly studied in almost any biology lab, and no doubt you’ve seen them
before. Like other protozoans, amoebas can be found in almost every moist environment where
they usually glide across the substrate. Their food varies depending on the species, and may
include bacteria, small unicellular algae, smaller protozoans, and even the occasional very small
metazoan not fast enough to get away from them.
Amoebas are large, up to 1 mm, and are visible to the naked eye. If you place the culture on a
black background you should be able to see the whitish-gray amoebas on the surface of the
container. Gently fish one out of the culture dish and place it on a slide. Be sure to add an extra
bit of depth for your wet mount to accommodate the size of these protists. Amoebas don’t like
being moved and their first reaction will be to ball up and become inactive. Check to see that
you’ve caught one and if you have, set the slide aside for a few minutes to let the specimen
adjust to its new home. Then, using low light, make your observations. Amoebas don’t like
bright light.
The cytoplasm in these protists changes from a stiff gel, the ectoplasm or plasmagel, to a more
fluid endoplasm or plasmasol. Changes between the gel to sol state, and back, form the basis
for amoeboid movement by the pseudopods. Locate a growing pseudopod and observe its
structure and cytoplasmic flow. The tip of the pseudopod is clear, free of the granular
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appearance created by organelles and food vacuoles in the cytoplasm. This clear region is the
hyaline cap. The fluid endoplasm spreads back from the hyaline cap and changes to the stiffer
ectoplasm. Watch the cytoplasmic movements at the advancing tip, the middle and retracting
end of the amoeba. Amoebas can suddenly change direction and reverse the cytoplasmic flow.
Do amoebas have an anterior and posterior end?
In addition to the pseudopods you should be able to see the water expulsion vesicle and food
vacuoles. The nucleus is easier to see in the stained preparations and appears as a flattened,
spherical disk. Amoebas feed by phagocytosis and surround their food with pseudopod
extensions forming a cup that surrounds the meal. Depending on our supplier we may also have
vitally stained specimens. Non-toxic dyes have been added to the culture medium in an attempt
to highlight cell organelles and structures. I’ve said “in attempt” because this material is often
variable in quality. When it works it’s great, when it doesn’t there is no real difference between
the two different types of specimens.
Difflugia
Fig. 2. Major external
features of Difflugia. ©
BIODIDAC
Unlike the naked protoplasm of an amoeba, Difflugia protects its cell body with a shell, or test,
composed of materials found in the surrounding substrate. In some species the tests are made
from uniformly sized sand grains and organized in ornate patterns on the surface of the protists.
In others the test may be formed from different sized sand grains, diatom shells, sponge
spicules, or just about anything that will serve its purpose. The test is usually shaped like an
elongated bell or flask with a central pylome through which the lobose pseudopods extend. To
see the pylome you’ll need to move the focal field through the specimen. Difflugia species are
usually between 40 and 240 µm in length.
The inorganic test makes it almost impossible to see the internal cellular structures, even in
slides. In prepared slides the preparation of the material has usually resulted in retraction of the
cytoplasm inside the test. The most prominent feature, if it’s visible, is the nucleus. Take a close
look at the test and by passing the focal field of your microscope through the specimen you
should be able to locate the pylome. If you can’t find a sample of Difflugia with an extended
pseudopod take a look inside Digital Zoology.
Arcella
Arcella, between 30 and 100 µm in diameter, is another species of freshwater protists that uses
pseudopods for locomotion and feeding. It differs from the naked amoebas by surrounding the
central cell body by a shell, or test, formed of chitin. The test is hat-, or dome-shaped and
concave on the underside where the pylome provides an opening through which the
pseudopods can extend. Normally Arcella is attached to the substrate by this surface and the
extended pseudopods are used to feed on green algae and small protists. Prepare a wet mount of
the specimens and allow them to settle and rest for a while before making your observations. To
see the pylome you’ll need to move the focal field through the specimen. The test makes it
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difficult to observe the internal structures although water expulsion vesicles may be seen
around the periphery of the test in live specimens and paired nuclei may be visible in the stained
slides.
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