Darwinian Snails - Lab Report Example

Method: The experiment The open-ended experiment conducted on Darwinian snails and crabs was done using SimBio: an interactive software used in biology education that includes virtual labs. Using SimBio, I clicked on many snails in order to observe the coastline that has snails with thicker shells; it was evident that the east coast had the majority. The main reason for this is that, this is the site where the European green crab; a snail predator, reached after migrating from Europe in the early 19th century. To carry out the experiment I used four tanks that were filled with water, with one additional tank that would act as the control set. The four tanks were filled with water and snails. The crabs to be used in the experiment had been starved for 24 – 48 hours to increase their motivation to feed. Trial 1: In the first tank, I added 2 blue snails with shell thickness of 3.0 and 9.0, one orange snail with shell thickness 10.0 and five yellow-green snails with 3 having a shell thickness of 5.0, and 2 having a shell thickness of 6.0. In tank 2, added 1 crab banded, four blue snails with shell thickness 4.0, 5.0, 7.0, and 9.0, two orange snails with shell thickness 9.0 and 9.0, two yellow-green snails with shell thickness 4.0 and 6.0. For tank 3, I excluded a crab and added the following snails four blue with shell thickness of 11.0, 12.0, 13.0, and 13.0, one dark red snail with shell thickness of 13.0, one red-orange with shell thickness of 11.0, and two yellow-orange snails with shell thickness of 10.0 and 10.0. In tank 4, It included 1 crab banded with added 2 blue snails both having a shell thickness of 12.0, three yellow-orange snails all having a shell thickness of 10.0 and three red-orange snails with shell thickness of 11.0, 11.0 and 12.0. The snails in tank 1&2 were picked from the West, and those used in tank 3&4 came from the East and each tank contained the same number of snails. The setup is highlighted figure 1 below. Figure 1: Experiment setup before running trial 1 Trial 2: For trial two, the setup was as follows. In tank 1 I added two blue snails with shell thickness of 6.0 and 7.0, five yellow-green snails with 2 having shell thickness of 3.0, another 2 with thickness 5.0 and 1 with shell thickness of 4.0, and one 0range-yellow snail with shell thickness of 10.0. Tank two also had same number of snails that comprised of three blue with shell thickness of 4.0, 5.0, and 8.0, one orange with shell thickness of 9.0 and four yellow-green with shell thickness of 3.0, 4.0, 4.0, and 5.0, I also included one crab in the tank. In tank 3, I added two yellow-orange snails with shell thickness of 9.0 and10.0, two red-orange snails with shell thickness of 11.0 and 12.0, and four blue snails with shell thickness of 10.0, 13.0, 14.0 and 14.0. For tank 4, I added 2 dark red snails with shell thickness of 13.0 and 13.0, 2 red-orange snails both with shell thickness of 11.0, and 4 yellow-orange snails all with shell thickness: 10.0, I also included one crab. The setup is highlighted in figure 2 below. Figure 2: Experiment setup before running trial 2 The Results Trial 1 results For trial 1 I stopped time at 407. The change can be seen in figure 3 below. Figure 3: Experiment results after running trial 1 The results showed that in tank 1 the snails had reproduced in large quantities with yellow green snails being evidently more than the rest. In tank 2, the blue snails were missing. The two orange and two yellow-green snails had not reproduced and remained the same after the experiment. In tank 3, there were no snails left and the tank was empty. In tank 4, only the blue snails had reproduced all the other snails remained the same. Trial 2 results For trial 2, I stopped the time at 410. The results were as figure 4 below Figure 4: Experiment results after running trial 2 In tank 1, the snails had reproduced with the results showing blue snails to have multiplied slightly more than the rest. In tank 2, only two snails remained and these were two orange snails, which initially was only 1 therefore it had doubled in numbers. In Tank 3, the snails had reproduced with the orange snails appearing in large numbers. In tank 4, there were no snails left. Discussion Natural selection is a situation that occurs when nature selects those organisms that are well adapted to the environment for its reproductive mechanisms. It is evident that smaller snails are more susceptible to crabs, evident with blue snails in tank 2 for both trials. It was found that snails that were not damaged were found alive and had no shell chips. Snail death means a successful predation, as seen in tank 4 for trial 2. According to Seeley (114), “intense natural selection caused a rapid morphological transition in a living marine snail”. Natural selection by crabs is suggested to promote evolution of thicker shelled snails. Chemical defenses and behavioral avoidance mechanisms have no much effect on predation in marine environments as compared to morphological changes. The morphological changes such as shell wall thickness, shell length, shell height and the aperture area. Shell thickness contributes significantly to the environment that the snails’ live. Shell thickness of the intertidal gastropods varies in comparison to the shores, which are intertidal and this will show the differential predation in relation to crabs. The varying shell thickness of the gastropods tends to be in parallel gradient to predation intensity and wave exposure. Another major factor that contributes to shell thickness is the predator-induced defenses, which usually suggest intertidal plasticity. Phenotypic plasticity refers to a situation where the predator will induce the formation of structures in the prey. Inducible defenses are found in both terrestrial and aquatic organisms and appear to be a common defensive strategy of both sessile and mobile species. This is more common in temperate rocky intertidal communities (Bertness, 80). Thick-shelled gastropods are found mainly on protected shores in this environment, the crab predators are in abundant. The fact that crabs are more at the sites that are sheltered is due to the fact the defensive morphology of the shell is more concentrated. Abstract individuals of Littorina littorea in rocky intertidal pools crawled to pool sites where they were less visible (into rock crevices; under rocks and microalgae fronds) when either crushed co specifics or juice from crushed co specifics was added to the pools (Hadlock, 159). Trussel adds that there has been an evolutionary shift in reaction norm intercept; southern snails regardless of treatment consistently produced thicker shells thus predator induced increases in shell thickness (440). In contrast, sites that are exposed to the action of waves are usually with shells that are thinner. This is because the morphology of the shell will increase resistance to crushing predation. Turbulence when it is extreme inhibits crabs from shores that are exposed to waves. Shell morphology on the wave exposed shores usually reflects that natural selection is imposed by hydrodynamic stress. According to Trussel natural selection is imposed by hydrodynamic environment which is often invoked to explain the correlation between morphology and wave exposure in intertidal organisms. Among the snail predators that were used for the experiment is the European green crab, Carcinus maenas and Pisa armata Littorina obtusata and Littorina coronata are the intertidal snails that were used for the experiment. Of the crabs, P.armata was the most effective predator, while L.coronata was a better survivor because of its rigid shell. In snails, the shells provide refuge from successful predation because they prevent total damage to the species. REFERENCES Bertness, D. M. Crab predation, waterborne cues and inducible defenses. Providence: Brown University department, Rhode publishers. pp 80. 1999 Hadlock, M., N. The Biological Bulletin, Department of Biology Osborn memorial laboratories, Yale press.159 (2).1980 Seeley, Robin, Hadlock. "Intense natural selection caused a rapid morphological transition in a living marine snail." Proc. Natl. Acad. Sci. (1986): 6897 - 6901. Document. Trussell, Geoffrey, C. "Phenotypic Plasticity in an intertidal snail: the role of a common crab predator." Evolution (1996): 448 - 454. Document. Read More