The next time you are at a lake or pond you can quickly assess the aquatic life by simply picking up submerged rocks from the bottom. Depending on the habitat, if you look closely there should be an abundance of life present on and around the rock you pick up. The slimy layer is not simply 'slime', but a cool and complex biofilm made of algae, diatoms, bacteria, protists, and the slimes they excrete. There will also likely be several types of aquatic insects present on the rocks (a magnifying glass may be helpful but not necessary). Caddisflies, mayflies, stoneflies, midges, damselflies, and dragonflies all have immature nymph or larval stages that develop for up to year more underwater. Some of these species, such as mayflies, may only live above water as flying adults for a few hours before death. Other larger and better known species such as fish and crayfish may also be present but are often harder to observe. Picking-up and examining rocks from a body of water is also a trick many fishermen, especially those fly fishing, use to determine what types of things the fish may be eating. Speaking from experience, knowing a little aquatic biology can make you a much better fisherman! Bugs, slime, and potentially better fishing skills? This is biology at its best!
A common Arizona crayfish caught from East Clear Creek.
I tried my hand at examining some of these bugs and slime on a recent trip to East Clear Creek in near the Arizona Mogollon Rim. To start, I picked up many rocks covered with a thick slimy brown-green biofilm. Gross to some but cool to the biologist! On all of these rocks I found a remarkably low number of mayfly nymphs. Why so few aquatic insects? On closer inspection of the stream bottom there were dozens of crayfish scurrying. In Arizona, crayfish are not native and are a nuisance invasive that consumes aquatic insects and plants. This is bad news for many insect, fish, and amphibian species as they themselves are consumed or their habitat or food source is consumed by the crayfish. Fortunately, catching crayfish along with examining rocks can be a fun activity that can be used to teach aquatic biology to anyone. In addition, though I have never done so, crayfish are trapped, eaten, and enjoyed by people throughout the world.
As with some of the best scientific experiments, there is little to no equipment necessary to examine the things living on or around submerged rocks. A magnifying glass may be nice for looking at insects though. A basic aquatic insect field guide is also great if you want to identify species. If you have a microscope available a sample of the biofilm can be scraped off and stored in a vile to examine later. But for the more exiting task of capturing crayfish you will need a trap or container of some type. The container we used on our recent trip was a gallon milk jug with the top quarter cut off and the handle intact. Holes were also cut in the bottom so water can easily move through the container.
In future posts I plan on giving details on how a more thorough investigation of aquatic habitats can be carried out. I also hope to in the future acquire a trap and maybe have a little crayfish boil of my own.