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ABSTRACT
Marine snails in the family Neritidae are found in the intertidal zone and are subject to extreme temperatures, desiccation, and wave action. Studies have shown that there are several strategies that are used by snails in order to increase their survival rate. One of those strategies is a smaller shell size. The three nerites that were sampled in San Salvador, Bahamas were the bleeding tooth nerite, the four tooth nerite, and the checkered nerite. Transects were run in a high energy environment and a low energy environment in order to sample nerites that were exposed to different amounts of wave energy. The nerites that were randomly sampled were measured along the longest part of their shell to determine sizes and compare among the three species. The nerites on the high energy side were comprised mostly of checkered nerites, the smallest of the three studied, supporting the hypotheses. Since the checkered nerites were already smaller than the four tooth nerites it seems reasonable that they should dominant in numbers on the high energy side. Also, the four tooth nerites were statistically smaller on the high energy side compared to the low energy side, which also supports the hypotheses. This seems reasonable because smaller individuals have a better chance of avoiding dislodgment and can fit into small holes within the rocky shore for extra protection. It should be noted that the checkered nerites were statistically smaller on the low energy side compared to the high energy side, which does not fully support the hypotheses but may be due to the small sample size of the study.
INTRODUCTION
The intertidal shore consists of four zones: white zone, gray zone, black zone, and yellow zone. The white zone only comes into contact with seawater a few times a year during storm tides, so for the majority of the time it is dry. The gray zone is half dry and half wet. The upper part of the gray zone will remain dry for long periods f time while the lower part will become covered with water at some point during the spring tidal cycle. The black zone is completely covered during spring tides and receives a mist during the remainder parts of the year. The yellow zone is the true intertidal zone. This zone contains a steep slope and a seaward margin that often undercuts the ledge and marks the surf line. Here the lower portion of the zone is under water continuously (Kaplan 1988).
Marine snails in the family Neritidae can be found in the gray and yellow zones of the intertidal area. Within the gray zone, The bleeding tooth nerite (Nerita peloronta) will mark the upper zone, which is furthest away from the water. The four tooth nerite (Nerita versicolor) will be located from the middle of the gray zone to the lower part of the gray zone. Finally, the checkered nerite (Nerita tessellata) will be seen in the lower gray zone as well as part of the black zone, which is closest to the water. It should be noted that the four tooth nerite and the checkered nerite can also be located within the yellow zone of the intertidal area (Kaplan 1988).
The three nerites above are usually found in warm weather areas and in the shallow waters of the intertidal zone. The bleeding tooth nerite is the largest nerite reaching up to 3 cm in length. Its shell is yellowish in color with red and black zigzag marks. The underside of the aperture is stained reddish brown around 2-3 teeth structures, which gives the nerite its name. As stated above, the bleeding tooth nerite is found higher above the water line than the other two nerites (Kaplan 1988). The four tooth nerite only reaches 2.5 cm in length. The shell of the four tooth nerite is black and white with additional red markings. It looks very similar to the bleeding tooth nerite from the top but lacks the color on the underside of the aperture. Another striking difference between the four tooth nerite and the bleeding tooth nerite is that the four tooth nerite has 4 prominent teeth, which gives it the name of four tooth. The checkered nerite is the smallest of the three reaching 2 cm in length. Its shell is usually black and white checkered and it appears to have no teeth or very small, modified teeth (Kaplan 1988).
These marine snails have adapted to living in the intertidal zone where the tides rise and fall resulting in areas of the shoreline either becoming covered with water or completely exposed to the air. This constant change from a marine environment to a terrestrial environment can place stresses upon the organisms located within the intertidal zone (Denny and Paine 1998). These stresses can range from extreme temperatures of the air and water and desiccation to wave action. The affects of temperature upon an intertidal organism may have varying outcomes because an animal's "thermal regime" is influenced by the animal's morphology. Animals, which regulate their body temperature through the temperature of their surroundings, will be able to partially use their size to aid in heat regulation (Helmuth and Hofmann 2001).
As a result of the extreme temperature, intertidal organisms, such as the nerites, become subject to desiccation. It appears that body size plays a crucial role in the survival of organisms that are in this type of environment. Smaller organisms will be more susceptible to mortality in this harsh physical environment. It has be noted that larger organisms are more effective in maintaining their body temperature below that of the surrounding rock in the intertidal zone during the time in which they are exposed to air at low tide. Barnacles provide a good example. A study completed by Foster (1971) showed that as the size of the barnacle increase, so did the ability to withstand desiccation (Vermeij 1972).
Nerites also have to contend with wave action as well as the extreme temperatures and desiccation. The forces generated by the waves can first of all result in dislodgment of the organisms from the substrate upon which they are attached. Dislodgment is a direct impact of the waves and other debris that may be carried up into the intertidal zone. The indirect effects include altering food availability, biotic interactions, and foraging efficiency. Studies have shown that wave action is also correlated to changes in community composition, primary productivity, predation intensity, competitive abilities, growth rates, mortality rates, morphologies, and size (Etter 1989).
Etter (1989) completed a study involving Nucella lapillus to demonstrate the life history variation across a wave-exposure gradient. He was able to show that the N. lapillus which were exposed to wave action grew more slowly, terminated growth at a smaller size, the larger sized individuals comprised the majority of the mortality, and females produced more offspring per capsule. It was noted that Littorina rudis also utilizes the above strategies to increase survival. Having two different snails use the same techniques to increase their survival shows that these techniques are successful.
It has been suggested that the growth rate of intertidal snails is suppressed by the wave action. The high energy waves reduce the foraging time which in turn reduce the amount of energy the snails receive to put towards growth. As a result smaller snails are found in high energy zones where their size enables them to avoid dislodgment as the waves crash onto the rocky shore (Etter 1989).
Other marine snails, such as the nerites, may utilize the strategies that were stated above to increase their survival in the harsh conditions of the intertidal zone. The hypotheses for the following study are: 1) The population of checkered nerites will be larger on the windward side of the island and the population of the four tooth nerites will be larger on the leeward side of the island. 2) The nerites on the windward side of the island will be smaller in size than those on the leeward side.
METHODS
The study site was located on San Salvador in the Bahamas and was conducted in June of 2002. The low energy transects were taken from North Point and the high energy transects were taken from North Point and Rice Bay. The study organisms included three species of nerites: checkered nerite (Nerita tessellata) four tooth nerite (Nerita versicolor), and bleeding tooth nerite (Nerita peloronta).
Transects were randomly placed in the yellow zone of the intertidal area using a 10m tape measure. The tape measure was either placed on a horizontal surface of the yellow zone or a vertical surface of the yellow zone. At times horizontal and vertical surfaces were mixed in one transect. Along the 10m transect 5 random numbers were generated on which to place the metal quadrant system (0.5m, 4.2m, 6.3m, 6.8m, 8.1m). At each quadrant location on the transect another random number was generated to indicate which box (1,2,3,4) to sample, these random numbers corresponded to the location. (ie: at 0.5m, box 2 was always sampled). A total of 20 transects were sampled. Ten transects were sampled on the low energy side and 10 transects were sampled on the high energy side. Within each transect 5 samples were taken.
Within each randomly sampled box, all of the nerites were pulled off of the substrate. Due to the yellow zone containing both horizontal surfaces as well as vertical surfaces some judgement was made to determine which individuals were to be included in a particular sample. The species was recorded along with the measurement of the shell length using a caliper. Individuals were placed back on the substrate where they were found upon completion of measurements.
The average salinity of the water for each energy zone and the average ambient temperature for each energy zone was recorded to assure similarity of the high energy zone and the low energy zone. This was completed in an attempt to rule out other environmental factors in the study. A refractometer was utilized to measure salinity and a Boxcar temperature logger was utilized for the ambient temperature. After the 20 transects were completed statistics were then calculated using the Statview statistical program.
RESULTS
There was a statistical difference between the number of individuals of two of the species (checkered and four tooth) in the high and low energy zones (n=409; df=2; p-value<0.0001). The checkered nerites numbered in the high 20's on the low energy side compared to just over 100 on the high energy side. The four tooth nerites numbered around 140 on the low energy side compared to around 120 for the high energy side (Figure 1). These population numbers of the two species above were statistically significant using a chi square test showing that the numbers really are different for each species on the low energy side compared to the high energy side.
Figure 1: Chi square analysis of population differences among Nerite populations in the two energy zones (n=409, df=2, p-value<0.0001) populations are different in the high energy zone and the low energy zone.
Figure 2 shows the percentage abundance of each species in the high energy zone and in the low energy zone. In the low energy zone the four tooth nerite made up 84% of the total number sampled and only 49% in the high energy zone. The checkered nerite comprised 16% of those sampled on the low energy side and jumped up to 44% of those sampled on the high energy side. This shows that the two species were nearing a point where a switch of dominance in numbers could be occurring going from the low energy side to the high energy side.
Figure 2: Percentage abundance in the high energy zone and the low energy zone. Four tooth nerite composes most of the low energy samples and the four tooth nerite and checkered are about even in the low energy samples.
After it was determined that the population numbers were significantly different from the low energy side to the high energy side, ANVOA tests were run to determine if the actual size of the individuals were statistically different from the low energy side to the high energy side. An ANOVA was run for the four tooth nerite to compare the measured sizes found in the high energy zone to those found in the low energy zone. The p-value of the test was less than 0.0001 showing that there definitely was a significant size difference among the four tooth nerite individuals within the two different energy zones (Figure 3). The four tooth nerites were statistically smaller in the high energy zone.An ANOVA was also run for the checkered nerite to compare the measured sizes found in the high energy zone to those found in the low energy zone. The p-value of this test was 0.0136 showing that the size differences from the low energy zone to the high energy zone were significantly different (Figure 4). The checkered nerites were statistically smaller in the low energy zone.
Figure 4: Checkered nerite ANOVA. The checkered nerites were significantly smaller in the low energy zone compared to the high energy zone.
It should be noted that the temperature and salinity measurements were comparable for the high energy zone and the low energy zone. Table 1 shows the average measurements that were taken during the study period. The high energy zone averaged 33.3C with 40.6pph for the salinity. The low energy zone average 40.0C with 39.8pph for salinity. For this study, it appears that both energy zones were comparable environments and did not have to be considered factors within the study.
Table 1: Physical measurements of temperature and salinity within the high energy zone and the low energy zone.
ENERGY ZONE TEMPERATURE (C) SALINITY (pph)
High 33.3 40.6
Low 40.0 39.8
DISCUSSION
The results of this experiment support the following hypotheses: 1)The population of checkered nerites will be larger on the windward side of the island and the population of the four tooth nerites will be larger on the leeward side of the island. 2) The nerites on the windward side of the island will be smaller in size than those on the leeward side.
It was shown that the populations were significantly different in the low energy zone when compared to the high energy zone. The checkered nerite population made up a larger percentage on the high energy side while the four tooth nerite population made up a larger percentage on the low energy side supporting the first hypothesis. Since the checkered nerites were already smaller than the four tooth nerites then it made sense that the checkered nerites would dominant in numbers on the high energy side.
Even through the four tooth nerites are found on the high energy side they have to compete with the checkered nerites, which may have the advantage of being smaller and are able to out compete the four tooth nerites for resources, such as food or holes. As for the low energy side, the four tooth nerites made up the majority of the individuals sampled on the leeward side of the island. Since four tooth nerites are already larger than the checkered nerites it makes sense that four tooth nerites would dominant in numbers on the low energy side. If the larger body size was more advantageous for the snails on the low energy side where desiccation was more likely then the checkered nerites, being smaller in size may be being out competed for resources by the four tooth nerites.
The high energy zone appears to be more favorable for the smaller individuals.
This may be because smaller individuals have a smaller surface area for the waves to hit. The small surface area may be the reason they can survive better on the high energy side. As the waves come crashing up onto the yellow zone the smaller nerites can resist the pressure that the waves create and therefore remain attached to the rock. Another possible reason for the smaller individual being favored on the high energy side is that smaller nerites are able to enter holes that will protect them from the waves. Many of the holes that are found in the yellow zone are small and the smaller the nerites, the more that can fit into one hole. During the sampling period it was noted on the high energy side that approximately 6 nerites were found in one tiny hole. Being able to fit easily into holes increases the chance of survival and may slightly reduce competition from other species that may be larger.
While the four tooth nerites held up the second hypothesis by being smaller in the high energy zone, the checkered nerites deviated from the hypothesis. The study showed that the checkered nerites were slightly larger on the low energy side of the island. This does not fully support the second hypothesis that smaller nerites would be found on the windward side of the island. There may be several explanations for this result. First, this result could be due to the small sample size done during the short duration of the experiment. If more samples would have been taken in both energy zones the result may have fully supported the hypothesis. Second, the smaller size of the checkered nerites on the low energy side could be due to the amount of competition that is found on that particular side of the island or in that particular location where the samples were retrieved. It may have been the case where competition was high and therefore the checkered nerites were not able to gain access to enough resources, yet size may not have been a prominent factor for survival in this particular situation.
Several errors were encountered during this study. One, the small sample size may have thrown off the results. Out of 409 individuals that were sampled only 16 bleeding tooth nerites were sampled. Due to the low number of bleeding tooth nerites they could not be used in the ANOVA statistical tests. Two, transect locations were not exactly random. Many times the study was limited to areas that were not too physically dangerous to collect samples. This really came into play on the high energy side where the environment was much more rocky, steep, and slippery. Three, the quadrant borders were sometimes too difficult to determine, especially on vertical surfaces. For the vertical sample points a guess had to made to determine which nerites were in the sampling box to be measured. To correct for this error all samples should have been taken from horizontal surfaces within the yellow zone or a better sample devise should be created to accurately sample on the vertical surfaces.
For future research it would be interesting to sample other islands to see if the marine snail populations and shell size would show similar results. Possible islands could include some of the cays in Graham's Harbor. Another study could be completed to compare the vertical surfaces and the horizontal surfaces to see if there are any differences in nerite shell size and population numbers. Also, it would be beneficial to sample at different times of the day to see if any differences occur compared to this study which was completed over the course of several days at random times. Finally, it would be interesting to repeat this study, but include other gastropod species, such as the knobby periwinkle, to see if differences exist between species .
REFERENCES
Denny, Mark W. and Robert T. Paine. 1998. Celestial Mechanics, Sea-Level Changes, and Intertidal Ecology. Biol. Bull. 194: 108-115.
Etter, Ron J. 1989. Life History Variation in the Intertidal Snail Nucella Lapillus Across a Wave-Exposure Gradient. Ecology. V.70 n. 6. P.1857-1876.
Helmuth, Brian S. T. and Gretchen E. Hofmann. 2001. Microhabitats, Thermal Heterogeneity, and Patterns of Physiological Stress in the Rocky Intertidal Zone. Biol. Bull. 201:374-384.
Kaplan, Eugene H. 1988. Peterson Field Guides: Southeastern and Caribbean Seashores. Houghton Mifflin Company. New York.
Vermeij, Geerat J. 1972. Intraspecific Shore-Level Size Gradients in Intertidal Mulluscs. Ecology. V.53. n.4 p.693-700.
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