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24 October 2003

Collecting Nematodes

by Shawn Carlson
Excerpted from "Scientific American's The Amateur Scientist", first published September 1996.

The soil around a pond is host to millions of tiny roundworms called nematodes. Only about one millimeter long, these nearly microscopic organisms are second only to protozoans as the most abundant creatures on the earth. A cubic meter of soil can harbor 12 million of them. A vital part of pond ecology, they can be especially interesting to study. Just make sure you observe strict sanitary practices-ingested, a few species are parasitic. Wear rubber gloves and wash thoroughly after fieldtrips.

To collect nematodes, slip a small piece of rubber surgical tubing over the end of a funnel and clamp the opening shut with a clothespin. Place muddy soil into the funnel, pouring in enough pond water so that some water stands free on the soil surface. The nematodes will sink into the funnel's neck. Wait five minutes before disgorging your booty into a container with a few gentle shakes and a momentary release of the clothespin.

A similar technique enables you to collect insects from most soils. Carpet the bottom of a glass jar with blotting paper. Insert a funnel, neck downward, into the jar's mouth and loosely fill the funnel with collected soil. Place a bright, incandescent desk lamp directly over the soil. To escape the light and heat, the insects will tunnel deeper into the soil until they fall onto the blotting paper. Try conducting an insect and nematode census around a pond at different times of the year.