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Abstract
Life in the intertidal zone is harsh. Many factors affect the species composition and physical morphology of organisms that live there. Among these, the most crucial may by wave energy. To see the effects of wave energy on the speciation and size of three species of nerites on San Salvador Island, the Bahamas, a transect study was performed. It was found that four tooth nerites made up a smaller amount of the population and were smaller in areas that received high wave energy when compared to those that received low wave energy. Checkered nerites, on the other hand, were found to be in greater abundance and larger on the high energy areas of San Salvador. This is likely due to their smaller size, in comparison to four tooth nerites, which presents less surface area and allows them to enter crevices. This provides them greater protection against the energy of the waves.
Introduction
The intertidal zone is one of the harshest environments on the earth. Organisms living within this zone face stress from predation, soaring temperatures, desiccation, high salinity, and other factors, both biotic and abiotic (Dybdahl, 1995; Schoch and Dethier, 1996). Many have held that the zonation among organisms that appears in the intertidal area is due to an established paradigm of physical stress from above (such as temperature, lack of water, and high salinity) and biological factors from below (such as predation and competition) (Yamada and Boulding, 1996). However, one factor that is often not stressed may be the most crucial of them all. This factor is the effect of wave action in determining the physical morphology and speciation of the intertidal zone. Studies have shown that wave action may be more equally or more important than competition and predation at influencing species assemblage and turnover rate (Gaylord, 1999). It may also affect biotic processes by inhibiting the activities of grazers and predators or by opening up areas in a previously crowded region of the intertidal zone (Gaylord, 1999). Waves are able to physicaly remove large numbers of organisms from tidal rocks, thus opening opportunities for other species to become established. It is also conceivable that they have a selectivity function that affects the general physical morphology of intertidal organisms.
The Commonwealth of the Bahamas consists of approximately 700 islands than span over 100,000 square miles. Located in the convergence zone of the Atlantic Ocean and the Caribbean Sea, the Bahamas are bathed by the warm water Gulf Stream Current.
The small island of San Salvador is located to the south and west of the main group of large islands. Only 12 miles long and six miles wide, San Salvador has a large variety of ecosystems contained on it. Saline lakes, ancient coral bluffs, limestone caves, and miles of beach and intertidal area are found on the tiny island.
The intertidal zone in home to a suprisingly large diversity of organisms, considering the harsh conditions. One group of organisms that make there home within this region are the gastropods known as nerites. Belonging to the genus Nerita, these gastropods are consumers of algae and also processors of rock. They are very much responsible for the pitted and jagged appearance of the yellow zone in the intertidal area. On San Salvador, there are three species of nerites. The largest is the Bleeding Tooth Nerite, Nerita peloronta, so named because of a blood-colored mark near its operculum. The next largest is the Four Tooth Nerite, Nerita versicolor, aptly named for the four projections located near its operculum. Finally, the smallest of the San Salvador nerites is the Checkered Nerite, Nerita tessellata. These species all live and are found in shallow water in the yellow zone of the intertidal area. Pictures of the nerites can be seen in Figure 1.
a. b. c.
Figure 1. The Nerites of San Salvador: a. The Four Tooth Nerite (Nerita versicolor),
b. The Checkered Nerite (Nerita tessellata), and c. The Bleeding Tooth Nerite
(Nerita peloronta)
Taking all this into account, a research project was formulated to determine the effect of wave energy on the size and speciation of nerites in the yellow zone of San Salvador Island, the Bahamas. This study was specifically aimed at analyzing the following two hypotheses:
1. The population of checkered nerites will be larger on the high-energy side of the island and the population of four tooth nerites will be larger on the low energy side of the island. From general observation and limited research, it appeared that the four tooth nerite tended to be larger than the checkered nerite. Thus was seen as a disadvantage on the high-energy side of the island due to the fact that the four tooth nerite would present more surface area to the wave and thus be more likely to become dislodged from the rock. On the low energy side, the larger four tooth nerites would have no disadvantage and should have similar if not greater numbers than the checkered nerites due to out competition.
2. The nerites on the high energy side of the island will be smaller than those on the low energy side over all. This goes back to the idea of surface area and dislodgment from the rocks where the nerites are relatively safe.
Materials and Methods
This study took place in June and July of 2002 on the Island of San Salvador, the Bahamas. The three sites chosen for study were: Rice Bay and North Point for the high energy side and Graham's Harbor for the low energy side. All sampling was done within the yellow zone of the intertidal region.
The study was designed around a transect sampling method. A ten meter transect was laid out using a measuring tape at each sample area. Five points along the transect were randomly chosen using a hand held calculator. Each point was then divided into quadrants using a two-foot by two foot measuring square. The square was quardened off into four sections. One section of the square was randomly chosen for sampling by using a hand held calculator. All nerites within the chosen sample square were removed, identified by species and measured. Measurement was performed using a caliper and taking the longest point along the shell of the nerite.
A total of ten transects were performed in each energy zone. The high energy zone contained two sites, of which three transects were performed at North Point and the remaining seven were performed in Rice Bay.
The temperature of the rocks and the salinity of the water were recorded at the three sites during sampling. All data was recorded by hand and analyzed using Microsoft Excel and STATview.
Results
Physical Area Measurements
In order to try to limit the study to one variable being tested, that being wave energy, the physical measurements of each transect area were determined. These measurements included temperature and salinity, two factors that are often sited as having effects on intertidal community formation. It was thought that if these measurements were recorded and similar, it would add validity to any difference seen between nerite populations on the high and low energy sides of the island. The physical measurements are displayed in Table 1.
Table 1. Physical Measurements of Transects
Energy Zone Temperature (¡ C)
Salinity (pph)
High
33.3 40.6
Low
40 39.8
Speciation of Nerites in the Low and High Energy Areas
The total number of nerites in each energy zone was recorded. A chi square analysis was run to determine if a statistical difference existed between the population of the nerite species in the high and low energy areas. At a 95% confidence level, the
Chi square analysis determined that there was a statistical difference between both the checkered nerite population and the four tooth nerite population on the high and low energy sides. The p-value was <. 0001 at 2 degrees of freedom. These results are displayed in Figure 2.
Figure 2. Chi square analysis of differences among nerite populations in the two energy
zones (n=4009; df=2; p-value <. 0001)
Size of Nerites in the Low and High Energy Areas
In order to determine if there was a statistical difference between the size of the same species of nerites on the high and low energy sides of the island, an ANOVA was conducted. An ANOVA was performed on both the checkered and four tooth nerites. One was not performed on the bleeding tooth nerite as non were found to be present on the low energy side. The ANOVA for the four tooth nerite found that there was a statistical difference between the size of four tooth nerites on the high and low energy sides. At a 95% confidence level, the p-value for the ANOVA was <. 0001. The mean values of the size showed that the four tooth nerites tended to be larger on the low energy side of the island. The ANOVA for the checkered nerite found that there was a statistical difference between the size of checkered nerites on the high and low energy sides. At a 95% confidence level, the p-value for the ANOVA was .0136. The mean values of the size showed that the checkered nerites tended to be larger on the high energy side of the island. The results of the ANOVAs are displayed in Figure 3.
a.
b.
Figure 3. ANOVA results for size differences between the high and low energy areas for
a. four tooth nerites and b. checkered nerites
Discussion
Physical Measurements
The temperature and salinity were recorded in order to attempt to control the factors that affect speciation in the intertidal zone. From the results, it can be seen that the salinities were nearly the same in the high and low energy areas sampled. However, the temperatures varied, on average, by seven degrees. This may be accounted for by the fact that it was partly cloudy on the day that we sampled in the high energy zone. Generally, the yellow zone has a fairly consistent temperature profile. However, it may also be due to the fact that the higher energy of the waves led to more splash which cooled the high energy yellow zone down.
Speciation and Size of Nerites in the Low and High Energy Areas
It was found that there were statistical differences between the number of the checkered and four tooth nerites in each zone. There were more checkered nerites in the high energy area than there were in the low energy area. The opposite is true of the four tooth nerites. Moreover, there was a far greater ratio of four tooth to checkered nerites in the low energy zone than there was in the high energy zone. The ratio in the low energy zone is almost 5:1 whereas it is nearly 1:1 in the high energy zone. The four tooth nerites were also found to be smaller on the high energy side, whereas the checkered nerites were larger on the high energy side. This last result is opposite of what was expected, according to the second hypothesis in this study. However, it does make sense if one looks at the data a little deeper. The issue here, in both cases, is size. Four tooth nerites are generally larger than checkered nerites. While this may be an advantage in low energy zones, as seen by the 5:1 ratio of four tooth to checkered nerites there, it clearly would not be in a high energy area. A larger shell would present far more surface area for a wave to exert force on. This increases the likelihood of a nerite being ripped off the relative safety of the rocks and thrown into the tumultuous sea. A smaller nerite has less surface area and thus is more resistant to the waves. Smaller size also conveys the advantage of being able to enter into small crevices for protection from the waves. This behavioral adaptation was seen numerous times among the checkered nerites in the high energy zone. While the checkered nerites were actually larger on the high energy side, they were still smaller on average than the four tooth nerites. The size increase was not that large either as they averaged about 10 mm on the low energy side and 11.5 mm on the high energy side. This difference in size may be attributable to the decrease in competition from the four tooth nerites on the high energy side.
Future Directions
In the future, more work encompassing a larger sample area and size could be performed to flesh out the differences in speciation and size of the nerites in high and low energy areas. Moreover, it might be worthwhile to study any differences in the species in vertical and horizontal zones, both of which occur frequently in the intertidal zone. It would also be interesting to sample during different times of the day to see if there is a behavioral response to tidal changes among the nerites.
Acknowledgements
The author would like to thank the members of his research group, Jen Weiskettle, Dusty Gebhard, Corey Ullman, Crystal Hansen, and Dave Stamler. He would also like to thank the following individuals and organizations: Dr. R. Hays Cummins, Rebecca A. Deehr, the Gerace Research Center, and the Bahamian Government.
Works Cited
1. Dybdahl, Mark F. Selection on life-history traits across a wave exposure gradient in the tidepool copepod Tigriopus californicus. Journal of Experimental Marine Biology and Ecology. Vol. 192 (1995) Pgs. 195-210
2. Gaylord, Brian. Detailing agents of physical disturbance: wave-induced velocities and accelerations on a rocky shore. Journal of Experimental Marine Biology and Ecology. Vol. 239 (1999) Pgs. 85-124
3. Schoch, G. Carl and Megan N. Dethier. Scaling up: the statistical linkage between organismal abundance and geomorphology on rocky intertidal shorelines. Journal of Experimental Marine Biology and Ecology. Vol. 201 (1996) Pgs. 37-72
4. Yamada, Syvia Behrens and Elizabeth G. Boulding. The role of highly mobile crab predators in the intertidal zonation of their gastropod prey. Journal of Experimental Marine Biology and Ecology. Vol. 204 (1996) Pgs. 58-83
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