Species and deserts do not seem to go together, yet one Australian freshwater fish has built its home in the country’s arid and semi-arid regions. A investigation of how it achieves this discovered desert rainbowfish (Melanotaenia splendida tatei) populations living in environments where evolutionary theory would say they shouldn’t. The findings not only bode well for this subspecies’ chances of living in a hotter environment, but they may also alter our perceptions of others who have persevered in the face of adversity, Years without rain in Central Australia might followed by massive floods. Trucks are taking a 3,000-kilometer (1,860-mile) detour to locate a dry route to transport food to distant settlements since portions of the Australian heartland are so wet right now.
The survival of rainbowfish in remote waterholes piqued the interest of Flinders University’s Professor Luciano Beheregaray and his team. Rainbowfish genotypes compared at 18 places across Australia’s heartland. Beheregaray and co-authors disclose something surprising in their study “Fish out of Water” in the journal Evolution. Beheregaray told IFLScience the genetic evidence suggests, “There are not 18 populations, but eight”. That is, there is no crossing of the boundaries between the eight unique catchment areas where the fish lives for thousands of years. Fish have recently interbred between waterholes separated by hundreds of kilometers of dry terrain inside these catchments.
The rainbowfish lives in somewhat permanent rivers in the eastern half of its range; large rainfall events provide additional opportunities, but even dry years provide adequate habitat to support huge numbers. Only the deepest waterholes survive the drought further west, inhabited by populations so small that they predicted to be strongly inbred. Genetic diversity is generally lacking in small, isolated groups. Living beings, according to evolutionary theory and most examples, require stable settings in such situations since they have almost completely lost their ability to adapt to change.
The rainbowfish, on the other hand, is the polar opposite. Even small populations have genetic diversity and can adapt to geographical change. “This flips the common notion that tiny populations are evolutionary dead ends on its head. Even in the most harsh and unexpected environments on Earth, life finds a way,” Beheregaray said in a statement.
On the rare occasions when floodwaters cover wide portions of the flat ground, the fish’s secret is to travel between waterholes. They are more likely to breed at these times, according to the authors. As a result, while a rainbowfish may only have siblings and cousins to mate with for a few years, they disperse and pair with far more distant members of their species when the opportunity arises, guaranteeing genetic diversity for future generations. Rainbowfish in western catchment areas appear to have acquired characteristics that make them particularly well suited to boom-bust cycles. They have a variant of a gene that governs taste, smell, and light sensitivity, as well as the ability to discern salinity.
The western gene variety, according to Beheregaray and co-authors, may cause the fish to move more when conditions are favorable, sustaining population links. If given to eastern populations who are increasingly suffering from a drying environment, this could be a lifesaver.
Some marine birds and southern Australia’s pygmy perch have shown clues of such small-population resilience, according to Beheragaray, but never with the clarity exhibited here. The discovery could indicate that other small populations are healthier than they appear and that we should not dismiss uncommon species as unsalvageable, as some have proposed. “We should not interpret this incorrectly,” Beheregaray cautioned IFLScience. The majority of species require big populations.”
