Cyclicity

Introduction

The foraging behaviours of gastropods and other marine intertidal grazers have observed to be fairly consistent with most emerging nocturnally to feed on algae. This nocturnal behaviour is thought to have developed to avoid predation, competition of food resources, and desiccation from the suns heat. It is unknown whether nocturnal behaviour is triggered by environmental factors such as light, or if the emergence of these intertidal grazers is due to an internal circadian rhythm.

Circadian rhythms, more commonly referred to by humans as the 'body clock', have been observed in a range of animals from unicellular organisms to cyanobacteria, plants, bees and humans. For most organisms, circadian rhythms are driven by a rough 24-hour cycle which govern the organisms daily activities, as well as physiological and biological functions. It has been observed that circadian rhythms can be influenced by environmental factors and can even induce a change in the circadian rhythm if altered.

Scutus antipodes is a marine gastropod which is fairly abundant in a range of marine habitats from rocky shores to coral reef crests. Usually hidden under rocks or in crevices during the day, S. antipodes emerges during the night low tide to graze on algae. The animals abundance and nocturnal behaviour makes it a perfect candidate for the study of circadian rhythms and light.

The aim of this paper is to determine whether the cue to emerge to feed at night for S. antipodes is light based (phototactic) or due to of an inherent circadian rhythm. It is hypothesised that S. antipodes will express the same behavioural pattern regardless of differences in light, thus confirming that circadian rhythms are solely responsible for feeding patterns.

Methods

The study was carried out on Heron Island in the Great Barrier Reef (Queensland, Australia). Six Scutus antipodes specimens were collected from under rocks on the reef crest at low tide. Once caught, two specimens were placed in each treatment: light, dark, or control. To test treatments, organisms were placed in separate tanks and provided with a piece of PVC pipe to replicate a hiding place found in their natural habitat. The light and dark treatment were subjected to 24 hours of continual light and dark respectively while the control treatment was left in normal light conditions. Organisms were observed at 3 hour intervals for a 24 hour period and recorded 'Hidden' if they were under the PVC pipe or 'Active' if they were observed elsewhere in the tank. Upon completion of the experiment, specimens were returned to their natural habitat on the reef crest.

Results

The results of the study were inconclusive as none of the specimens moved their position throughout the observation period. Light specimen 1 remained 'active' for the duration while Light specimen 2 remained 'hidden'. Both Dark specimens and both control specimens remained 'active' throughout. It should be noted that all specimens were placed in their respective tanks next to, but not underneath the 'hide-outs' provided to acclimatise, suggesting that specimens which were hidden had to first find the 'hide-out'.

Discussion

It was expected that altering the length of light would have a minimal effect on the behaviour of S. antipodes due to the internal circadian clock under laboratory conditions. This hypothesis was unable to be proven due to insufficient results. The fact that one light specimen was the only animal which sought out a hiding place may suggest a level of phototaxis, however this happened prior to the study period and cannot be counted in the results. In addition, the complete disregard of a light source by both control specimens and the other light specimen contradicts any 'phototactic' movements made by the first light specimen. On the other hand, both dark specimens remained active the entire time, possibly enabling the interpretation that the absence of light induces nocturnal behaviour.

It is possible that a lack of movement seen by test subjects was due to stress, starvation, short periods of acclimation, or the lack of a sufficient cue. Due to the nature of the study site, specimens had been held for up to 2 days in tanks with high human presence which may have caused extreme stress. Starvation due to insufficient food supply while being held may also have effected the movement during the study period. A lack of proper acclimation time in treatments may have led to increased stress during study period. The most likely cause of lack of movement is the absence of sufficient stimuli, namely the lack of food.

It is likely that without food in the vicinity, specimens had little reason to move from their respective positions.If lack of food was the cause of no movement in specimens, it would suggest that S. antipodes requires more than the presence/ absence of light or the urges of circadian rhythms to induce nocturnal behaviour.

In order to accurately test the effect of light on circadian rhythms, a number of improvements need to be made. Ideally, specimens should be collected at once and placed in treatments for a number of days to reduce stress and provide ample time for acclimatisation. In addition, the study period should run for more than one 24 hour period, possibly with more frequent observation intervals. Finally, if possible, a higher number of replicates for each treatment should be used to increase confidence in results. Although the result of this study provide little insight into the relationship between light and the circadian rhythms of S. antipodes, the study helped identify a number of areas for future improvements.