Bermuda Experiment Results

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Research Organism: P. transversus

Research Experiment

Results

One significant correlation was found in the data between the width of the crevice opening and the presence or absence of an individual crab.  Figure 1 shows this significance and can be found below.  For more results and figures click here.

Figure 1: Logistic Fit of Crab Presence by Hole Width
	The figure to the left shows Pachygrapsus presence against the width of the crevice opening.  The right side indicates the presence (1) or absence (0).  The left side indicates the probability of finding a crab in the crevice.  To properly interpret this figure, it is best to picture a line dividing it horizontally to show presence or absence.  A logistic curve was fit to the data using JMP.  Since the p-value was so low, the data suggests that there is a positive correlation between the width of the crevice and the presence of a crab.
Chi-square value= 8.85, p=0.0029

Discussion: The figure above shows that there was some significance between the presence or absence of a crab and the width opening of the rock crevice.  This seems to suggest that crabs prefer certain sized openings over others for a habitat.  There could be several reasons for this, but the most important is probably due to predation.  When a predator approaches, the individuals will want to select a hole that is both large enough to fit their body size and is tight enough so that it cannot be picked out.  If a hole is chosen with an opening that is too large in width, the crab may be extracted by the predator.  In addition, a tighter fit in a crevice would allow for more chance of grabbing hold of the rock surfaces inside.  Doing so may further prevent the crab from being removed from the hole. Other than this one significant figure, the distribution of P. transversus is fairly random.  There does not appear to be any other significant trends in the rest of the data set, but this could be due to several factors.  First, it was impossible to approach the hiding places without the crabs seeing me as a predator.  I observed them running for cover as soon as I came within several meters of them.  This skewed the data because even if they did return to the same crevice every time, my presence was causing them to find the nearest hole.  They did have to find a hole that was large enough for them and there was some competition for holes as I noticed several dart into a crevice only to be chased out by another inhabitant.  Another problem with this study was that some of the holes were not holes, but instead tunnels that had multiple openings.  I tried to find which ones were like this and only measure one hole, but there could have been some deep tunnels that the crabs were hiding in that I did not see.  There did seem to be some homing in this species since some holes contained crab fecal matter.  This may indicate that certain individual crabs inhabit and defend one particular crevice. What I have learned:  There are several conclusions that I can make from this experiment.  First, the distribution of P. transverus is very random and widely dispersed.  Although there may be some competition between individuals during times of stress (such as active predation) they generally have plenty of places to hide.  My presence acted as a active stress situation which caused them to scramble into crevices regardless of preference.  Due to the relatively low competition for space in the area I was working, the individual crabs hid in the nearest crevice available.