Guest post by Brandon Waldo, Katherine Moore, Sam Bickley, Christopher Anderson, & Moises Bernal.
Salinity, Urbanization, and Genetic Divergence of Killifish
Coastlines are unique and dynamic environments that are always subject to intense changes. Organisms living in beaches, estuaries, and marshes constantly face fluctuations in water conditions that can alter their habitat, making hardiness and versatility crucial adaptations for survival. As with most aspects of our natural world, coasts and wetlands are quickly modified as a result of human activities. For instance, the urbanization of areas adjacent to coastal ecosystems has increased in recent decades, introducing new challenges that could affect even these resilient organisms.
When watersheds become more urban, the combination of less vegetation and designed stormwater infrastructure results in more freshwater runoff that is delivered to water bodies faster. One of the effects of these changes is that the salinity of tidal creeks and other drainages may fluctuate rapidly. Further, previous studies have shown that changes in salinity fluctuations can lead to changes in species composition, as some fish species avoid urbanized fast-changing areas. In this study, we wanted to confirm the prediction that urbanized sites had higher and faster salinity fluctuations, using data loggers in four tidal creeks along the Alabama and west-Florida coast. This area was well suited for the study, as it has seen a substantial increase in urban development over the past two decades. In addition to this, we wanted to evaluate if differences in coastal development could lead to differences in genetic composition of populations found in urbanized (intense salinity fluctuations) vs. more natural environments (low salinity fluctuations). The species of choice for this was the Gulf killifish (Fundulus grandis), which is one of the few species present in both types of environments. For these analyses we chose to look at the cytochrome oxidase 1 (CO1) and control region (CR) genes, as these have been extensively used in evaluating differences among fish populations.
Using data loggers, our study confirmed the prediction that highly urbanized sites have more intense salinity fluctuations, while sites with natural coastal vegetation show less environmental variance. In addition, the genetic analyses showed differences between fish in opposing urbanization conditions for one of the genetic markers (CR). The lack of difference in the other marker (CO1) could be associated with its slow mutation rate, as it encodes a protein necessary for cellular respiration, and mutations in this marker could lead to issues with metabolism There was also a pattern of isolation due to geographic distance, where samples collected further apart had the highest level of differentiation with CR. With this in mind, this study suggests that both changes in urbanization and geographic isolation can lead to changes in the genetic composition of killifish populations.
One additional finding from our study is that it allowed us to estimate the potential age of the populations of Gulf killifish in the northern Gulf of Mexico. Using computation analyses, we were able to determine that these groups have been in the Gulf for at least 650,000 years before present. This coincides with changes in sea levels that occurred 600,000 to 700,000, which gave rise to the landmass that we know today as the Florida Peninsula. Today, Florida isolates the northern Gulf from populations of the Atlantic coast, and is responsible for the isolation of many marine groups in the Atlantic.
While our study provides valuable data on how coastal species can be affected by urbanization, more extensive research is necessary to illuminate the main drivers of the genetic differences, what are the differences between killifish exposed to intense salinity fluctuation in terms of energy metabolism, and if there is differential survival of individuals at urbanized sites. This will be essential given the substantial increases in coastal development in recent years, as well as the projected chances in precipitation expected with climate change in coming decades.
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Featured Image: Long Bayou by Christopher Anderson.
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