Colonization and Migration of Mosquitofish in Bahamian Blueholes


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Why do populations of mosquitofish in freshwater blueholes have remarkably different body sizes?

Blueholes are vertical caves that have formed historically as sea levels have fluctuated during the past hundreds of thousands of years. At present sea levels the caves are filled with water and appear as a round, landlocked pond. In the Bahamas blueholes dot the surface of the islands. On Andros Island, the larges of the Bahamian islands, freshwater collects on top of the saltwater and creates a lense within which freshwater organisms can live. However, the are more or less trapped in the blueholes because they can not move down without passing through anoxic seawater, and can't move outward because the blueholes are landlocked.

Drs. Jerry Downhower and Luther Brown discovered that an species of mosquitofish, Gambusia hubbsi live in this freshwater lense. Remarkably, the body size of mosquitofish in each bluehole is markedly different. Body size is related to life history characters such as age at maturity (males stop growing once mature), and number of offspring (mosquitofish bear live young). Furthermore, the ecogical community is often very different in different blue holes. This then is potentially a perfect animal for studies how life hsitories evolve in different populations. An organisms life history is believed to be one of the most fundamental factors influencing evolution. What factors then influence life history change in mosquitofish populations? The answer to this question undoubtedly involves components of the ecological setting such as food resources, population density, predatory type and density, and the populations demographic history and ancestry.

For part of my doctoral research, I performed protein and DNA analysis on populations from a variety of blueholes in the Bamahas to determine the potential ancestry and hisotrical demographic history and ancestry. I found that populations of mosquitofish in blueholes are highly isolated, not suprisingly, and were founded by very few individuals during their original colonization of blueholes. If migration among population occurs even rarely, it is not enough to influence the gene pool of populations within blueholes. Blueholes are essentially sinks into which mosquitofish rarely migrate.

This is interesting because it suggests that the populations are very much isolated and exposed to significantly different environments, creating an opportunity for natural selection to have a strong effect on their subsequent life histories.

Blueholes in the Bahamas are natural laboratories for studies of life history evolution in mosquitofish and many other organisms that inhabit these fascinating geological structures.

Schug, Malcolm D., Jerry F. Downhower, Luther P. Brown, Dustin B. Sears, Paul A. Fuerst.  1998.  Isolation and genetic diversity of Gambusia hubbsi (mosquitofish) populations in blueholes on Andros Island, Bahamas.  Heredity 80:336-346. 

Schug, Malcolm D.  Invasion of a freshwater archipelago:  Inferences from population genetics of the mosquitofish (Gambusia hubbsi) in blueholes on Andros Island, Commonwealth of the Bahamas.  Ph.D. Dissertation:  The Ohio State University, Columbus, Ohio.  December 1995.