nahantexperimentdiscussion

Nahant Study: Discussion

According to the ANOVA analysis of the data, there was no significant difference between the mean sizes of the crabs found in the low intertidal versus the mid intertidal. However, there were several important things learned from this experiment. Examining Figure 5 shows that while there may not have been a statistically significant difference between the carapace sizes, several larger individuals with carapace widths of over 20 mm were found in the mid intertidal zone whereas the low intertidal zone only contained crabs that were smaller than 20 mm in carapace width. This may suggest juvenile crabs are found in the lower intertidal whereas the adults move up into the mid intertidal zone. However, there were also more individuals found in the mid intertidal than in the low intertidal zone and many of these were smaller than 20 mm in carapace width.

The size distribution histograms (Figures 6 & 7) show the range of the crabs found in the low and mid intertidal. The low intertidal shows a normal size distribution versus the fairly even distribution found in the mid intertidal. In addition, the mid intertidal had several individuals that were larger than 19 mm in width. The differences in size distribution may be due to the time of year data was taken. As seen in a study on Long Island Sound the size distribution of Hemigrapsus populations was different between June and January. In that study smaller crabs were primarily found in the upper intertidal during June whereas in January smaller individuals were found in the low intertidal (Kraemer).

One of the interesting observations made during my study was the general substrate H. sanguineus settled upon. The individuals settled mostly on the cobble areas of the mid intertidal which consisted of broken rock and shell pieces. However, there seemed to be a tendency for individuals to prefer cobble areas that had larger rocks than cobble areas with large amounts of Ascophyllum and Fucus. This seemed to be fairly consistant throughout both the low and mid intertidal zones. Many of the juveniles seemed to hide under these rocks perhaps for protection against predators (primarily gulls and other birds) and also perhaps to protect against dessication. Hiding among the cobble under an alga may provide protection from dessication, but it may also be an easier area for predatory birds to pick through and find the juvenile crabs. Unfortunatly, this observation was not fully realized until late in the experiment and an improper amount of data was collected to test that idea.

Overall, more data will have to be collected to further understand the settlement of juvenile and adult Hemigrapsus sanguineus in the intertidal coast of New England. There were significant problems encountered with data collection due to the difficulty of finding and capturing individuals. Many crabs quickly ran into hiding when I approached. This resulted in a reduction of randomization in the experiment because I had to capture any individual that I saw just to be able to collect some data on the individuals. To further this study, the transect should be followed much more closely than it was and more care would be taken to isolate a quadrat so that individuals cannot escape. More data should also be taken from the high intertidal zone. This zone was largely overlooked due to time constraints and preliminary searchs that revealed no individuals living in that area.