Bring 'em Back Alive - Coverslip Traps

Created by Lorraine Olendzenski, St Lawrence University

Many organisms, including bacteria, ciliates and diatoms will colonize bare surfaces in an aqueous environment. By providing a substrate one can encourage the formation of these biofilms. Coverslips are a suitable substrate for colonization, and can easily be observed under the microscope.

Materials Needed

  • 4 inch thick-walled rubber tubing
  • Razor blade
  • 8 coverslips
  • String or monofilament fishing line
  • Fishing weights
  • Covered jar or Ziploc bag


  1. Take a 4 inch piece of thick walled rubber tubing and slice four slits across the tubing, taking care not to cut completely through the wall of the tubing. The slits will be perpendicular to the long axis of the tubing.
  2. You will need eight coverslips. Place two coverslips, back to back, into each of the four slits so that the tubing holds them in place.
  3. Tie string or monofilament fishing line through the tube and attach fishing weights to keep the device from floating.
  4. Suspend the tray by tying to a stick, rock or post in a suitable location such as a pond, estuary or marine environment. Be sure that the string or monofilament is long enough so that the trap is not exposed at low tide.
  5. After at least 24-36 hours, the traps can be removed. Place the whole device (tubing and coverslips) into a covered jar or Ziploc bag containing water from the environment the traps were in. Be sure the coverslips are submerged; do not let them dry out at any time.
  6. To observe the organisms on the coverslips, gently remove a pair of coverslips and separate them. Place one of them onto a microscope slide so that the surface that was on the inside of the pair is facing up. If the coverslip starts to dry out, place a drop of water between the coverslip and microscope slide. Observe first under low power and them move up to higher power. Make drawings of the different species you may see and the numbers of each in a given area of the coverslip.

Helpful Tips

  • The longer the traps stay in the water, the more they will be colonized. Diatoms and attaching (sessile) ciliates are the most common protists that will colonize the glass surface. Experiments can be set up to sample and compare organisms from different environments. To study the patterns of colonization from a single environment, traps can be removed and observed at different times over the course of many days or weeks.


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