Going on a Bear Hunt: Identifying Tardigrades (Water Bears)

Overview

Students will collect tardigrades from lichen or moss cushions and then use a dichotomous key to identify the tardigrades by family.

Objectives

Collect, capture, and observe tardigrades found in local lichen or moss cushions. Use a dichotomous key to identify tardigrades by family.

Lesson Preparation

• Print copies of the Key to Limno-Terrestrial Tardigrade Families and Tardigrade Identification Worksheet in Lesson Materials.
• Hydrate samples approximately 2 hours before beginning the identification lab.
• Prior to the lab students need to be able to use stereomicroscopes and compound microscopes.
• Prior to the lab students should have a basic understanding of dichotomous keys.

Procedure

1. Collect samples of lichen and moss. Samples can be collected by scraping the lichen or moss into a paper lunch sack using a butter knife. Collect a sample with a surface area approximately the size of a credit card. The moss or lichen samples can be stored for many months, or even years, until you are ready to look for the tardigrades. Not every sample will have tardigrades so it is good idea to collect multiple samples. More information about collecting tardigrades can be found in the Nov. 25th journal entry: https://www.polartrec.com/expeditions/tough-tardigrades/journals/2016-11-25
2. Place enough of the sample to just cover the bottom of a small plastic cup (2 oz. cup). Approximately 2 hours before the lab begins hydrate the sample by placing enough spring water (distilled will also work) to completely submerge the sample.
3. Use a disposable plastic transfer pipet to transfer liquid from the bottom of the plastic cup into a small petri dish (35mm) or similar container (the lid may work better than the dish because it has less depth). Most tardigrade species will be easiest to spot with a light source from above against a dark background. The bottom of the petri dish can be painted with black enamel paint. Systematically search through the dish with a stereomicroscope using 20-30x magnification.
4. Tardigrades will need to be transferred to a microscope slide for identification. To observe live tardigrades at higher magnification you can transfer them to a drop of water on a depression slide. To make a permanent microscope slide, transfer tardigrades to a drop of mounting media (Hoyer’s mounting media or polyvinyl alcohol mounting media). Transfer tardigrades to microscope slides using an Irwin loop. A fine tipped pipet may be used in lieu of an Irwin loop.
5. Observe tardigrades at 100-400x with a compound microscope. Use the dichotomous key to identify tardigrades to family.
6. Students will fill out the assessment form with the name of the family of tardigrade they found. They will list the characteristics their tardigrade has that led to their identification.

Extension

Tardigrades have the ability to form a structure called a tun and enter a state called cryptobiosis. While in their tun, tardigrades are extremely resistant to harsh environmental conditions (e.g. pressure, temperature, radiation). Students can design experiments to explore cryptobiosis and resistance to extreme environmental conditions. Tardigrades can be left in a depression slide without a coverslip until they dry out and form a tun. The dried tardigrades can be exposed to extreme conditions (e.g. freezing temperatures or UV radiation). Alternatively, samples of lichen or moss that contain tardigrades can be exposed to extreme conditions prior to rehydration and then survival rates can be compared to control samples.

Resources

• Kinchin, I. (1994). The Biology of Tardgrades. Portland press, London, pp. 186.
• Miller, W.R. (1997). Tardigrades: Bears of the Moss. The Kansas School Naturalist, Vol43(3), pp. 1-16.
• Schram, M. D., & Davison, P. G. (2012). Irwin Loops—A History and Method of Constructing Homemade Loops. Transactions of the Kansas Academy of Science, 115(1/2), 35-40. doi:10.1660/062.115.0106
• Key to Limno-Terrestrial Tardigrade Families in Lesson Materials

Assessment

Tardigrade Identification Worksheet

Author/Credits

Josh Heward (josh.heward [at] gmail.com)