Behavioural experiment: Response to light stimuli

We observed that R. queenslandia has a strong response to light. This was evident from their choice of habitat in the reef flat and their behaviour in aquarium conditions (Mathers an Bennett, 1993). In the sandy reef area, R. queenslandia is only identifiable by its buccal tentacles. When placed in aquaria, they used their tentacles to follow water flow and escape from tanks several times. We wanted to test the receptive capabilities of R. queenslandia to light. The photoreceptors in the tentacles of R. queenslandia are simple, only having to distinguish between light and dark. We did not know how sensitive these receptors were to light or what spectrum of light they were sensitive to.

Despite these observations, it is also possible that the photoreceptors in R. queenslandia’s tentacles were also used in feeding. If the tentacles were used for feeding then we expected that they would have at least two different types of photoreceptors. This would enable them to distinguish between colours and potentially find food sources. A third possible use for photoreceptors was for predator avoidance.

To examine the photoreceptive capabilities of R. queenslandia’s tentacles we used three different approaches. Firstly, we looked at its reaction to a strong light stimulus and whether it used its buccal tentacles to seek out shelter or just swam around until it found shelter. Secondly we tested the spectrum that the tentacles could sense light in by exposing them to strong lights with different coloured filters. Thirdly we removed tentacles from the worm and exposed them to strong light stimuli to observe their individual response to light stimuli.  

Methods:

First Experiment:

Eight organisms were placed in the centre of a flat white container. One half of the container was covered with lightproof card and the time taken for R. queenslandia to move into the dark half was recorded. Each worm was measured three times

Second experiment:

Worms were placed into a white bucket and left to acclimatise for 5 minutes. Then they were exposed to a strong light with different filters (red, orange, green and blue) and their response was recorded. The response by the worm was scores as either: no response; a slight response; or a strong response.

Third experiment:

Several individual tentacles were removed from R queenslandia  and placed into petri dishes under dissection microscopes. The tentacles were left to acclimatise for 5 minutes then exposed to the light from the dissection microscope. Responses were scored as either a response or no response

Results:

We found that R. queenslandia had a strong response to light stimulus but took 30 seconds to move into a shaded area (figure 1). The buucal tentacles of the individuals measured were not able to see i the short wavelength spectrum (red or orange). Three individuals showed a weak response to green light and all individuals exhibited a strong response to blue light. When tentacles were removed from the worm, approximately 90% of them still demonstrated a strong response to light stimulus.

       Figure 2  Response of eight R. queenslandia to a strong light stimulus under four different filters.

Discussion:

We found that R. queenslandia has a rapid response to light stimuli. All of the worms used their tentacles to seek out the area of the container that was shaded and hide there. However this response was not as rapid as anticipated. Because of this slower response time, it is unlikely that the buucal tentacles are used in predator avoidance. The rapid withdrawal of tentacles towards the mouth when they are touch is a much more effective predator defence mechanism. However our third experiment demonstrated that it was the tentacles that are responding to light and seeking out shelter.

R. queenslandia also have a ring of photoreceptors around their mouth/pharynx which can also sense light. However this experiment demonstrates that the buccal tentacles are actively involved in seeking new crevices to shelter in and the worms do not just swim around until they find a new place to hide. We also found that they are only able to see long wave lengths of light. Our experiment found that they are only able to ‘see’ wavelengths in the blue spectrum (and slightly in the green). This indicates that they only use blue light to sense their environment.

This makes sense biologically as blue light is able to penetrate the deepest into water. Even in the shallow depths that R. queenslandia occurs in, the red and yellow spectrum of light will fade out quickly. This makes it ineffective in seeking out areas that are out of direct sunlight. We also observed that although there was a rapid response to light stimuli, the response did not trigger and instantaneous recoil of tentacles. Instead the tentacles seemed to be used to actively search and scan the area immediately surrounding the worm. Despite being used to search the area, the photoreceptors seem to contribute very little to feeding. If the photoreceptors were involved in feeding, we expected that they would be able to see in at least two spectra of light. We found that the majority of individuals could only see blue light with some displaying a limited ability to see green light.