Phototaxis Experiment
Aim
To determine if P. bedfordi is negatively phototactic in response to various light colours simulating different wavelengths.
Hypothesis
Flatworms are negatively phototactic and will move away from the light when exposed to light. They will have no response to red light as it is the first wavelength to be absorbed in seawater.
However, they will respond to green light and white light.
Materials & Methods
(i) Specimen collection
4 individuals were collected from Heron Island. 2 were taken from the intertidal reef flat on the South side and 2 individuals were collected from the lagoon of Heron Island on the North side by flipping over boulders. Small plastic jars were used to collect them from each site.
(ii) Tank set up
A large square glass tank measuring 21.5cm X 21.5 cm was used. All 4 sides were wrapped in black plastic garbage bags and a circular hole measuring 2 cm in radius was cut out from the plastic wrap on one side of the tank. The artificial light source used was a bright desk lamp and different coloured plastic filters (red, green) were used for each change of colour. The light source was placed directly in front of the hole made in the black plastic wrap and placed at the opposite end of where the flatworm was (Fig 1).
(iii) Variable – 3 different lightcolours (wavelength)
3 lightcolours were used to simulate different wavelengths to test the sensitivity of P. bedfordi to different light colours.The green and red light filters came from Lee Filters. The green was Oregon Freen 488 and the red filter was primary red 106. The white light was from a desk lamp and the control was simply blocking out all light source with the aid of a black cloth that covered the entire tank.
(iv) Experiment method
1. Each of the 4 individuals were placed at the back end toward the centre line of the tank. A slab of coral rubble was placed in this position for the flatworm to anchor onto.
2. Once the flatwormwas moving over the coral rubble, the stopwatch was used to time how long ittook the flatworm to move towards or away from the light source.
3. Response validation:
- A positive response: Flatworm swims towards the light source and touches the surface of the other end of the tank with movement of its pseudotentacles.
- A negative response: Flatworm swims away from the area of the tank where light source is shining on.
- A non-response: Flatworm is given up to 500 seconds to respond. If it moves in a random motion over the coral rubble, onto the glass tank and the pseudotentacles do not sense the light source at all then it is considered a non- coherent response that will not be taken into consideration as a valid result.
4. The timer was stopped if the flatworm touched the end of the tank OR
showed a negative response away from the light source OR
after 500 seconds of non-coherent response.
5. The flatworm was then placed back into the holding tank to rest and the same experiment was carried out on the 3 other individuals for each light colour.Each experiment was repeated over 2 days.
Results
Video 1. P. bedfordi response to white light
Response to white light showed they were positively phototactic to white light.
There was incoherent response to red light and no response to blue light.
Figure 2. Average time in secs taken by 4 flatworms in response to different light colours.
Table 1. Average time (seconds) recorded for 4 flatworms to respond to various light colours.
Discussion
All 4 flatworms in the experiment responded positively to the presence of light. White light gave the fastest positive reaction time followed by green and finally red light. The results suggest that flatworms are not negatively phototactic to the presence of artificial light as demonstrated in this experiment. Perhaps the strength of the light source also affects their behaviour as strong light directed onto a swimming flatworm inhibits its locomotion causing it to sink to the bottom and move in search of cover (Ruppert et al., 2004).
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