To most people, amoebas are the blobby bacteria that cause dysentery. They're your standard, Mark I microscopic pondlife. This Entry will aim to show that they are vastly more complex than that - and, indeed, are no more like bacteria than humans are like mushrooms.
Origins of the Name
The term 'amoeba' was originally used loosely to describe any single-celled organism that crawls around by growing pseudopods1. Since these creatures often look very similar, they are still referred to as 'amoeboid'2, thus allowing usually pedantic biologists to justify continuing to call them amoebas. 'Amoebas' is sometimes written as 'amebas', 'amoebae' or (just to confuse you) 'proteus animalculae'.
However, modern biology increasingly classifies organisms according to their descent. This is known as cladistics, and any group of organisms descended from a single common ancestor is a clade. Although all amoeboids look very similar, studies of their genes show that they are not all closely related. Instead, the amoeboids are spread across several groups of organisms. In each of these clades, some species have stayed blobby ('retained their primitive characteristics'), whereas others have developed adaptations that make them non-amoeboid such as flagella (whip-like tails used for movement).
One of these clades is the genus Amoeba, which contains the few species that can strictly be called amoebas. This Entry will use the wider usage, where amoeba can be any similar-looking critter.
Some amoebas may have more than one nucleus - indeed, hundreds of nuclei may be found in a single individual of the genus Chaos. Nor are their genes necessarily simple - several species have DNA over 100 times the length of that found in humans.
All amoeboids are protists. Indeed, the name comes from Proteus, the Greek god who could change his shape. So what is a protist?
One of the great biological discoveries of the 20th Century was just how vastly complex microscopic life is. This revelation has continued into the 21st Century.
There are five great divisions - kingdoms - of living things3. Animals and plants are pretty easy for organisms of the genus Homo to identify (though there are a few red herrings such as sea cucumbers and corals). Most educated people (which naturally includes anyone reading h2g2) are aware that fungi are quite different from plants, even if they're a little sketchy on exactly how and why. But what of the other two kingdoms? What are the differences between protists and monera (also known as prokaryotes)?
Monera, or monerans, are single-celled organisms whose genetic material (genes and chromosomes) is loose in each cell - in other words, they do not have a cell nucleus. Bacteria are monerans, and thus just about as far removed from amoebas as it's possible to be in the tree of life - hence the comment in the opening paragraph of this Entry. Protists, then, are 'anything else'. As such, they are not currently regarded as a valid biological group (despite what may still be taught in schools about the five kingdoms of life). Instead, they are further sub-divided into many different groups.
The largest of these groups to concern us is the supergroup Amoebozoa4. In fact, the amoebozoa are believed to have diverged from the animals before the fungi did but after the plants - microfossils more than 550 million years old are known, and this is supported by gene sequencing.
However, within that supergroup, not all the species restrict themselves to moving via pseudopods5. Quite a few other methods of locomotion have been developed, such as the use of flagella - and these have been developed separately several times within the group. So, confusingly, not all the members of Amoebozoa are amoeboid.
So let us move down through the levels of the phylum Tubilinea (amoebozoa which have undivided pseudopods) and order Tubulinida to the family Amoebidae (which use one pseudopod at a time to move) and finally to the genus Amoeba. By the strictest possible definition, these are the 'true amoebas'. You can get an idea of where amoebas sit by browsing the Tree of Life.
One of the defining characteristics of amoebas is that they move around by extending tentacles known as pseudopods. Their movement is not random; they follow certain chemicals, moving to where the concentration of nutrients is highest and that of toxins lowest. Contrary to popular opinion, they have a recognisable front and back, with the nucleus usually lagging behind the cytoplasm6. They can be found in any slightly damp evironment, from ponds and seas to soils or even the human mouth.
Amoebas feed by encapsulating their prey, in a process called phagocytosis (taking in water in the same way is pinocytosis). The cytoplasm flows around the prey in the same way as extending a pseudopod. Once the prey is totally surrounded, it can be absorbed into the organism or rejected, according to the amoeba's preference.
Although the common image of ameobas is as solitary creatures, some species (such as lab favourite Dictyostelium discoideum) are capable of coming together to form clusters when times are hard. These are called 'fruiting bodies', by analogy with fungi that cluster in order to spread their spores. Amoebas cluster by secreting a chemical called cAMP which is attractive to other amoebas. Social amoebas often move together and are therefore called 'slime moulds'.
They are also known (on rare occasions) to show altruistic behaviour. When a slime mould finds a suitable spot, it will grow a stalk, with sticky globs of cells at the top that can attach to passing animals that might carry them to a more congenial spot. The individual cells that form the plant-like stalk - as much as 20% of the slime - die in the process. Their genes, however, survive in the individuals who escape in the globs.
The largest slime mould ever discovered was 40 feet across.
Less social amoebas can survive hard times by producing a cyst. This involves forming a ball and secreting a layer of chitin7 into their cell membranes. This allows them to survive harsh conditions - stomach acids, arid sand or lack of nutrients - by going into a form of suspended animation. When times improve, they can emerge into their active (trophozoite) form.
Amoebas, like most single-celled life forms, reproduce by cloning themselves. The DNA duplicates, the nucleus divides and then finally the cytoplasm moves until two separate but genetically identical daughter cells are formed.
How Amoebas Can Kill You
There are many species of amoeba that are harmful to humans (pathogenic). Arguably the most famous among these is Entamoeba histolytica, which causes amoebic dysentery. Naegleria fowleri is less well known but more spectacular, and is sometimes known as 'the brain-eating amoeba'. Using its flagellum8 it can travel through the human nervous system (entering through the nose), before resuming its amoeboid form and digesting the nerves.
Legionella is a bacterial infection that can reside within amoebas, thus using their cysting mechanism to survive droughts.
Various amoeba will also be delighted to give you anything from hepatitis to meningitis (the Roman baths at Bath are closed to swimmers for this reason). Acanthamoeba keratitis specialises in infecting eyes and is commonly transmitted by rinsing contact lenses in tapwater instead of solution. This leads to another word to add to your vocabulary: enucleation, the removal of the eye.
It has even been suggested that arthritis may be caused by a small amoeba called Endolimax nana.
How You Can See An Amoeba
Although common, amoebas are among the hardest to find of microscopic life. Despite being among the largest - commonly growing up to 3mm long (and with a few marine species growing to over a centimetre) - they are far from the most obvious.
One tip for catching them is to lower a jam-jar upside-down into a slow, muddy stream. When it is near the bottom, it can be tilted to allow the air to escape - hopefuly this will also suck in some of the loose soil from the bed, and with it the bottom-dwelling amoebas. You will, of course, still require a microscope to see them, and a sharp eye to recognise them among the cells of decaying plant matter. Magnification of at least ×400 is recommended to see them in any detail. Reducing the lighting levels in the microscope can encourage the amoebas out of their dormant state.
Alternatively, you could buy your own research submersible and try diving in the Bahamas or the Arabian sea, where you may be able to find 3cm-long versions.