The Impacts of Salinity Tolerance and Predation on the Allopatric Distribution of Two Species of Palaemonid Slu'imp
Both abiotic and biotic factors are crucial components of a species' environment. To examine the importance of both forces in determining the distribution and interactions among closely related species, two experiments were performed with the congeneric grass shrimp Palaemonetes pugio Holthuis and Palaemonetes paludosus (Gibbes). In the first experiment, each species was subjected to 7-day salinity exposures to determine if an overlap existed in their physiological tolerances to this abiotic factor. Significantly higher survival rates of P. pugio were found at the highest level of salinity (10.8 parts per thousand, ppt) than at the lowest level (1.0 ppt), whereas, significantly lower survival rates of P. paludosus were demonstrated at the highest level (15.0 ppt) versus the two lowest levels (2.4 and 1.0 ppt). A range of salinities (~2.0-6.5 ppt) was found at which we observed high survival (>65%) for both species, suggesting an overlap of their fundamental niches. Thus, to take the initial step in determining whether biotic interactions, such as predation and competition, contribute to the allopatric distribution of the two species, individuals of each were held together at a tolerable salinity (2.5 ppt) during the second experiment. A factorial-design was used to determine whether predation by the large-mouth bass, Micropterus salmoides, would have a significant effect upon the survival of P. pugio and P. paludosus and whether these effects would differ between species or in the presence or absence of refuge space. Survival rates for both species were found to be significantly higher in treatments without a predator than in treatments where a predator was present. However, survival rates did not significantly differ between species or between treatments with or without cover. In addition, the interactions between predator and cover, and predator, cover, and species type did not significantly affect the survival of P. pugio or P. paludosus. These latter results are undoubtedly related to the high variability and low number of replicates utilized during the experiment. However, with repeated experiments and further studies into the ecology and interactions of P. pugio and P. paludosus, competition between the two could be examined, perhaps offering an explanation of why they have remained allopatrically distributed in nature. The results of both experiments serve to illustrate the potential importance of both abiotic and biotic factors to species distribution in nature.With honors.