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In nature, the environment is not composed of biotic factors alone; temperature, climate, pressure, and other non-living variables all play vital roles in making a certain region habitable for organisms. For the sharks and especially the model of the study, Rhizoprionodon terraenovae, there are two main abiotic factors that influence the population density within a certain body of water: temperature and oxygen concentration (Parsons&Hoffmayer, 2005).
The population of Atlantic sharpnose shark, Rhizoprionodon terraenovae, fluctuates regularly with the seasons. Parsons and Hoffmayer found in their study (2005) that the sharks would always enter the freshwater regions of North Central Gulf of Mexico within certain range of temperatures and depart in a similar fashion. When water temperature approached 20-22 o C in spring, R. terraenovae was found migrating into the region. However, when temperatures shift down in the fall from 24-20 o C, the Atlantic sharpnose sharks were found departing.
When studying the changing flux of sharks entering and exiting the bay, an interesting relationship was found: with increasing temperatures, fewer male sharks were found throughout the region, making summer the month where the fewest number of sharks are counted (Parsons&Hoffmayer, 2005). However, the previous temperature ranges indicated that increases in temperature was favorable. However, Parsons and Hoffmayer found that increasing temperatures resulted in decreased saturation of oxygen throughout the body of water. Thus despite favorable temperatures, the lack of essential oxygen made this habitat less inhabitable.
Thus, sharks can be generalized to thrive within limited temperature ranges – warm enough for the cold-blooded shark to survive yet not too warm such that oxygen saturation is too low.
However the thermal niche-fecundity hypothesis lacks direct evidence of benefits S. canicula gain by remaining in the shallows during the day; from what was observed, the behavior of female catsharks better support the social factors hypothesis. Sims et al noted, in 2001, that after copulation, male catsharks often have blood on the claspers they use to hold females. This indicates that reproduction is an energetically demanding activity (Parker, 2006); thus, for females, the lowered fitness and vulnerability associated with copulation forces them to take preventative measures against the frequent attempts male catsharks make to copulate (Parker, 2006). The loss of fitness is so great that despite the fact that female S. canicula have the same diet as males, they choose to reside in narrow tunnels found in the warm shallows away from abundant amounts of prey in order to restrict male access and limit their movement (Sims et al, 2001). Thus social factors hypothesis best explains why females rebuff male advances: in order to minimize their loss of fitness due to copulation, female S. canicula not only rebuff males by taking refuge in inaccessible habitats but also only emerge to feed in the deeper waters during periods of predominant male inactivity (Wearmouth&Sims, 2008).
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