Suppose you see something in freshwater that certainly appears to be living. How can you begin to determine what it is? Here is a key (not quite perfect) that you might use to help determine the kingdom to which it belongs.
1. Is it green or does it have green parts?
Yes – go to 2
No – go to 3
2. Could be a plant or a protist, or blue-green bacteria. Make sure that the green is really part of the organism, though. An animal might have eaten something green, for example.
Single-celled? go to 6
Multicellular? Plantae. Look for cell walls, internal structure. In the compound microscope you might be able to see chloroplasts.
3. Could be a moneran (bacteria), protist, fungus, or animal.
Single-celled – go to 4
Multicellular (Look for complex or branching structure, appendages) – go to 5
4. Could be a moneran or a protist. Can you see any detail inside the cell?
Yes – Protista. You should be able to see at least a nucleus and/or contractile vacuole, and a definite shape. Movement should be present, using cilia, flagella, or amoeboid motion. Cilia or flagella may be difficult to see.
No – Monera. Should be quite small. May be shaped like short dashes (rods), small dots (cocci), or curved or spiral shaped. The largest them that is commonly found in freshwater is called Spirillum volutans. It is spiral shaped, and can be nearly a millimeter long. Except for Spirillum, it is very difficult to see Monerans except in a compound microscope with special lighting.
5. Animalia or Fungi. Is it moving?
Yes – Animalia. Movement can be by cilia, flagella, or complex, involving parts that contract. Structure should be complex. Feeding activity may be obvious.
No – Fungus. Should be branched, colorless filaments. May have some kind of fruiting body (mushrooms are a fungus, don’t forget). Usually attached to some piece of decaying matter – may form a fuzzy coating on or around an object. In water, some bacterial infections of fish and other animals may be mistaken for a fungus.
6. Most likely Protista. If it consists of long, unbranched greenish filaments with no apparent structure inside, it is blue-green bacteria (sometimes mistakenly called blue-green algae), a Moneran.
Most green protists are flagellates, that is, they move rapidly with a spiralling motion. Unless you get them to stop, you can’t really see the flagella. Watch out for colonial protists, though, such as Volvox, which forms a spinning ball of green cells. Don’t be fooled into thinking it is a plant.
Remember, the more you observe the organism, the more sure you can be. Many living things have stages that make them resemble members of another kingdom.