Introduction
Hermit crabs (Eupagarus bernhardus), are carnivorous crabs, that will scavenge carrion as well as hunt. They mainly look for meals at night, to give them a chance to see their surroundings and taste the water to determine the best direction in which to go hunting (Young 1994). They are often found in large numbers, especially in mid- to lower- shore rock pools and intertidal zones (Gibson et al 2001). This study examines whether the flow of water affects the Hermit Crab’s ability to detect a food source.
The working hypothesis is that Hermit Crabs are more likely to detect a food source if they are downstream from the food, and will therefore move against the flow of water to reach it.
Method
Two quadrats were placed in an area of flowing water on the shore, one had bait in the centre, and one was left empty to be a control. The same was done in a rock pool. The crab’s entering the quadrat, and the direction from which they arrived (against flow, with flow, side), was recorded over a 15 minute period. This was done 5 times in different random locations on the shore. Once all the data was collected, Kruskal-Wallis tests were completed to get a Chi-square value and a p value.
Results
The mean results are summarised in Figure 1.
Figure 1. Mean Direction of Movement of Hermit Crab’s to the bait.
As you can see from Figure 1, a much greater number of Hermit Crab’s moved against the flow of water to reach the bait, and very little arrived with the flow of water. In the rock pool with bait, crabs appear to arrive from all sides with no great variation.
The results of the Kruskal-Wallis test are summarised in Table 1.
Table 1. Summary showing Chi-squared value and p value of Kruskal-Wallis test.
From these results (Table 1), we can be confident in accepting our working hypothesis that the Hermit Crabs detect a food source from downstream, and move against the flow of water to reach the food source. In the non-baited quadrats and in the rock pool, Hermit Crabs are equally likely to approach the food from any direction.
Discussion
Our working hypothesis was accepted as the results demonstrated that crabs are more likely to move against the flow of water to reach a food source (Miller et al 2005). The crab’s head and appendages are covered in chemical receptors which pick up the scent of food. As the scent travels past them, the chemical receptors detect it and therefore the crab follows the scent towards the food. This is why the crab moved against the flow of water towards the bait.
There are a number of things that could affect the crab’s ability to detect the food source such as the size of the rock pool they are in. If the rock pool is very large, it will take longer for the scent of the food to disperse and be detected, and it will then take longer for the crab to respond and move towards the food.
The rate of flow of the water could have affected the crabs ability and time to detect the food. The rate of flow could have been measured and the length of time it takes for the crab to respond recorded. This would have given us an estimate of how long it takes a food scent to reach the crab. If the water was flowing slowly, it may have taken longer for the crab’s to detect it and respond, and they may not have reached the bait in the 15 minute time limit.
Another factor which could have been affecting the experiment was temperature. As it was a cold day the scent of the food would have taken slightly longer to disperse. If this experiment was then repeated on a warm sunny day the heat could have affected the time in which the scent of food travelled and therefore more hermit crabs may appear quicker and even from different directions, possibly changing our results.
While randomly placing our quadrats, the area in which they landed was not cleared of hermit crabs and therefore crabs were already inside some of the quadrats which could have slightly affected our results.
References
Gibson, R., Hextall, B., Rogers, A. 2001. A Photographic Guide to the Sea & Shore Life of Britain & North-West Europe.
Miller, S., Harley, J. 2005 Zoology
Young, A. 1994 Marine Wildlife of Atlantic Europe
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