Factors that Affect Distribution of Two Species of Killifish in Northeast Florida Marshes

Fundulus heteroclitus and the closely related F. grondis are mainly distributed along the Atlantic coast of the U.S from Maine to Northeast Florida and from the East coast of Florida throughout the Gulf of Mexico, respectively. Both are resident salt marsh fishes whose range is thought to overlap in Northeast Florida, making them an ideal study system to examine resource partitioning between two closely related species. The objective of this study was to examine the effects of temperature and elevation on potential habitat partitioning of these two species. It is hypothesized that the northern species, F. heteroclitus, would have a lower thermal tolerance than F. grandis and would be found in lower marsh elevations, which are thought to be slightly cooler. Fundulus heteroclitus larval and juvenile distribution was examined and elevation was found to be significant (p the distribution with smaller fish utilizing higher elevation areas. Temperature was not found to differ between elevation sites, thus could not account for elevational differences. To further determine the role of temperature in Fundulus distribution, both species were hatched in the laboratory, and larvae were used for critical thermal maxima trials. F. heteroclitus from one population bad significantly higher mean loss of equilibrium (LOE) temperatures (pF. grandis populations. Due to population differences results could not be pooled by species. Critical thermal maximum temperatures show that both species can tolerate roughly the same extreme high temperatures and, based on temperature alone, should be able to live in the same habitat. Temperature tolerance of both species was higher than the actual measured field temperatures and is therefore not a likely factor in determining both species\u27 range. A combination of other abiotic factors and biotic interactions such as competition may play a greater role in determining the observed range of each species than previously thought