Volume 50, Number 1, December 2003:
A Toxicology Primer for Student Inquiry: Biological Smoke Detectors

Text-only version

ISSUE HOME PAGE

ABOUT THIS ISSUE
- about KSN
- about the author
- acknowledgements

IN THIS ISSUE
- disclaimer, objectives
- "biological smoke detectors"
- purpose of invertebrate toxicity testing
- lethal and sublethal effects
- some wormy ideas for toxicity testing
- sublethal chemical effects in lumbriculus
- selecting the chemical(s)
- safety
- exposure methods
- preliminary experiments and concentration range-finding
- final stages of toxicity testing
- typical equipment and supplies
- other organisms, other ideas
- obtaining background information
- references
- glossary of toxicological terms

A Toxicology Primer for Student Inquiry:
Biological Smoke Detectors
by Charles Drewes

EXPOSURE METHODS 

A simple way to expose worms to water-soluble chemicals is by immersion.  Worms are placed in individual containers along with a small volume (about 20-30 ml) of  test solution of known chemical concentration.  The chemical is thus absorbed through the skin (termed contact exposure).  Always use just one worm per container, since a dead, decaying worm may be toxic to others.

For water-insoluble (and non-volatile) chemicals, there is a simple and reliable alternative to exposure by immersion.  This involves placing the worm in direct contact with wet filter paper that has been uniformly pre-treated with the insoluble test chemical.  Pre-treatment is done by placing a dry filter paper disk in the bottom of the glass exposure container.  The disk should fit snugly and flatly at the bottom of the container.  Then, prepare stock solutions as described in section “E” below.  Each solution should contain a known amount of the water-insoluble chemical dissolved in a known volume of suitable solvent, such as ethanol or isopropyl alcohol. 

Using a calibrated, hand-held pipette, transfer just enough of the desired stock solution to completely saturate the filter paper disk.  Allow the solvent to evaporate completely in a fume hood.  This leaves behind a known and nearly uniform residue of the test chemical on the paper (assuming that the test chemical is not volatile).  Next, add a known volume of spring water into the container so that the paper is immersed in shallow water.  Use enough water volume so that the worm could be easily drawn up into a disposable pipette if later transfer is needed.  For example, 5 ml of water is adequate for a 6 cm diameter plastic petri dish.  Next, add a worm.

All toxicity tests should include a control group in which the paper in test containers is initially wetted with an identical volume of solvent (but no chemical in it).  Once the solvent evaporates, water and a worm are added, just as in treated groups.