Study of geotaxis behavior in earthworm

 

Investigating geotaxis, the movement of an organism in response to gravity is a classic behavioral biology experiment. Earthworms (Lumbricus terrestris) are subterranean organisms that have evolved complex sensory mechanisms to navigate their environment without the aid of sight. Understanding their geotactic response is crucial for comprehending how they maintain their biological niche. By sensing the pull of gravity, earthworms can effectively burrow deeper into the soil to reach optimal moisture levels, avoid surface-level predators, and escape the lethal effects of UV radiation.

Earthworms typically exhibit positive geotaxis (moving toward gravity) as an adaptation to stay underground, maintain moisture, and avoid predators.

1. Objective

To determine whether earthworms exhibit positive, negative, or neutral geotaxis when placed on an inclined plane.

2. Materials Required

• Organisms: 5–10 healthy earthworms (e.g., Lumbricus terrestris).
• Apparatus: A flat wooden or plastic board (approx. 30cm x 50cm), a protractor, and a stopwatch.
• Environment: Paper towels, dechlorinated water, and a dim light source (earthworms are photonegative).
• Safety: Gloves and a soft brush for handling.

3. Experimental Procedure

Setup

1. Preparation: Cover the board with a damp (not soaking) paper towel. This provides traction and prevents the worm from desiccation.
2. Angle Selection: Use a stack of books or a laboratory stand to tilt the board at a specific angle (e.g., 45°).

Execution

1. Orientation: Place an earthworm in the center of the board, oriented horizontally (perpendicular to the pull of gravity).

[ Starting them horizontally ensures they must actively choose to turn upward or downward.]

2. Observation: Allow the worm to acclimate for 30 seconds.
3. Recording: Once the worm begins moving, track its head direction for 2 to 3 minutes.
4. Repetition: Repeat the trial at least 5 times with different worms to ensure statistical significance.
5. Variable Change: Adjust the incline to different angles (30⁰, 60⁰&  90⁰) to see if the intensity of the response changes with the gravitational gradient.

4. Data Collection Table

Trial	Incline Angle	Initial Direction	Final Direction (Up/Down)	Time Taken (s)
1	450	Horizontal	Downward	45
2	450	Horizontal	Downward	38
3	450	Horizontal	Downward	40

5. Analysis and Interpretation

To quantify the behavior, one can calculate the Mean Response Ratio:

R = n_down - n_up  /  N

Where:

• n_down = Number of worms moving downward.

• n_up = Number of worms moving upward.

• N = Total number of trials.

Result: In most conditions,  the earthworm will turn its prostomium (head) downward and move toward the base of the board. This confirms positive geotaxis.

Fig: Design of experiment to study geotaxis behavior in earthworm

6. Critical Success Factors

• Moisture Control: If the board is too dry, the worm may exhibit "escape behavior," moving randomly and rapidly regardless of gravity.

• Light Interference: Ensure the light source is directly above the board. If light comes from one side, the worm may move away from the light (negative phototaxis), which could be mistaken for a geotactic response.

• Vibrations: Earthworms are sensitive to vibrations; keep the lab environment quiet to avoid startling the specimens.