Life in a Drop of Water

      Adrienne Lopez, LSU SOAR Coordinator



There are many forms of life you can see in a single drop of pond water. Examining water from a pond, lake, or ditch can be a great introduction to the classification of organisms and may generate an appreciation of the diversity of life.



Annenberg/CPB Website. Life Science, Session 2: “Classifying Living Things”.


Reid, George K.  Pond Life.  St. Martin Press. New York:  2001.


Classification of Organisms


I.    Domain Archaea = blue-green algae (unicellular, prokaryotic)

II.   Domain Bacteria = bacteria (unicellular, prokaryotic, largest group of organisms)

III.  Domain Eukarya = divided into four kingdoms (unicellular or multicellular, eukaryotic)

A.     Kingdom Protista = a diverse group of organisms that don’t fit well into the other kingdoms. There are plant-like (Volvox, diatoms), fungus-like (slime mold), and animal-like (amoeba, Paramecium) organisms in this group; most are unicellular, some multicellular.

B.     Kingdom Fungi = multicellular organisms with chitinous cell walls that absorb food through root-like hairs. (Examples: mushrooms, bread mold).

C.    Kingdom Plantae = multicellular organisms that make their own food through a process called photosynthesis. Their cells have cell walls made of cellulose, which gives plants structural support, and contain chloroplasts, which give plants their green color. (Examples: grasses, moss, green algae)

D.    Kingdom Animalia = multicellular organisms that cannot make their own food; they ingest plants, other animals, or both. Animal cells lack cell walls; they are enclosed by cell membranes. This diverse group includes organisms with backbones (vertebrates) and some without (invertebrates). Examples of vertebrates include fish, frogs, snakes, and people. Insects, spiders, coral, oysters, and worms are some examples of invertebrate animals.


collecting your water sample


Locate a body of water that you would like to sample; this can be a pond, lake, ditch, stream, or large puddle. For comparison studies, collecting water from several locations is ideal. You may want your students to bring in their own water samples.

Look for signs of life, such as algae, plants, and animals. Use a scooper (such as a large soup ladle) to collect a sample of water. Make sure to collect plants, especially algae, and get a couple of scoops off the bottom. A good water sample has lots of green (plants) and brown (dirt/substrate). Water can be collected in any type of container – I recycle plastic food tubs. Be sure not to leave a lid on your container for very long! Organisms need oxygen to survive.


Identifying organisms with Scope-On-A-RopE


1. Begin examination of your water sample by looking for large organisms (those which can be seen with the naked eye). These organisms are best viewed with the 1x lens of SOAR. Refer to the “SOAR Key to Macroinvertebrates in Louisiana Freshwater Systems”.


Common “large” organisms include:

·        Crawfish

·        Snails

·        Minnows (Gambusia)

·        Insect larvae (all have legs, unlike worms)

o    Dragonfly nymph = spider-looking, stout body with six long legs

o    Damselfly nymph = long and slender with three feathery gills at end of tail

o    Stonefly and dobsonfly larvae = long and worm-like with many legs and gills

·        Worms

o    Segmented = annelids (bristle worms – small worms with tiny hairs, leeches – flattened worms with a sucker at mouth and tail)

o    Non-segmented round worms = nematodes (round worms that thrash about)

o    Non-segmented flatworms = planarians (have two eye spots, crawl on surfaces of plants and container and generally avoid light)

·        Amphipods - “scuds” (flattened sideways with many legs, swim very fast through the water and usually hang out near algae)

·        Water bugs - water striders, water boatmen, backswimmers, and whirligig beetles

·        Caddisfly larvae - look like worms but have tiny legs and they live in mobile “homes” they build out of debris found in the water

·        Mosquito larvae – worm-like animals with feathery gills that hang upside down from surface of water and swim quickly to bottom when disturbed



2. Take a small sample of water to view with the 200x lens of SOAR. Make sure to get a little “green stuff” in your sample. Use a deep well slide or make a Petri dish slide.


Common “tiny” organisms include:

·           Stalked protozoans – tiny bell-shaped protists that attach to plant matter and filter feed using cilia around their body opening

·           Paramecium - oval-shaped protozoan that moves quickly in a zig-zag pattern

·           Hydra - cylindrical animal, usually attached to substrate, with long tentacles

·           Rotifers - small, fast organisms with a ring of cilia around their mouth

·           Catenulas - small, clear flatworms with many segments

·           Ostracods (seed shrimp) – they move very fast, look like a seed with two halves

·           Water fleas (Daphnia) – look like little fleas with clear body, eye spot, external gills

·           Copepods – small, one-eyed crustaceans that swim with jerky movements; females have two egg sacks that hang off the body on either side of its tail

·           Water mites – tiny water spiders

·           Diatoms – small, green, plant-like protists that can be a variety of shapes, move very slowly or not at all

·           Bacteria – look just like tiny, moving dots with 200x lens, or they can be in long strands or in a corkscrew shape