Explosive evolution in tropical lakes

Chris Martin, a fourth-year graduate student at UC Davis, waded into a lake in 100-degree weather. Though the water was only two feet deep, he had to wear a full-body wet suit to avoid the pain of stinging algae.

Chris Martin, a fourth-year graduate student at UC Davis, waded into a lake in 100-degree weather. Though the water was only two feet deep, he had to wear a full-body wet suit to avoid the pain of stinging algae.

All of this trouble in order to collect tiny pupfish.

Two groups of small fish have been found to exhibit the fastest rates of evolution known in any organism, according to a new UC Davis study.

One group of the rapidly-evolving pupfish is from a Caribbean island and the other group is from the Yucatan peninsula of Mexico. There are about 50 species of pupfish in the world and they are found in many varying environments. Most of these groups of pupfish are fairly similar in appearance and behavior.

However, in Lake Chichancanab in Mexico and the inland salt lakes of San Salvador in the Caribbean, pupfish have undergone extraordinary evolutionary change.

Martin studied adaptive radiation of pupfish for his dissertation research. Adaptive radiation occurs when a species undergoes rapid evolution, resulting in great diversity within the species. If divergence continues it could result in a splitting of the lineage to make different species. Charles Darwin studied adaptive radiation in his famous study of the diversity of the finches on Galapagos Island.

“When I learned there were two places showing adaptive radiation [in pupfish], I wanted to study how it happened,” Martin said.

Most pupfish, no matter where they live, are ecological generalists: they eat algae, decaying vegetation and, occasionally, insects. Pupfish species can be distinguished by subtle differences in coloration or fin shape, but their anatomy is very similar, especially in jaw shape.

Since having so many pupfish species occupying the same niche would overwhelm a habitat, each habitat is only home to a single pupfish species.

This is not the case in Lake Chichancanab or the lakes of the island of San Salvador.

“You have a situation where in a single lake, a single species that got into the lake rapidly evolved to be morphologically different,” said Peter Wainwright, a professor in the department of evolution and ecology and head of the lab in which Martin did his research.

The island of San Salvador in the Bahamas is only 11 miles long, but the pupfish have evolved two distinct groups that each have unusual features; one feeds entirely upon the scales of other pupfish, while the other has a thick jaw for eating snails. The second group has also evolved a nose, an unusual feature for a fish.

Scale-eating has evolved independently in many different kinds of fish, but this particular species in San Salvador is the only pupfish known to eat scales. Martin described the scale-eater as a “pup-tiger” in how it stalks its prey before striking.

The other pupfish in San Salvador have a protrusion above their shortened jaw that looks like an upper lip, but is in fact a fish version of a nose. At this point, Martin is uncertain why this particular group has nasal tissue, but he is currently exploring hypotheses such as that the nose aids the fish in identifying females.

In Lake Chichancanab, pupfish took a different evolutionary pathway. The species of pupfish that occupy this lake are very different in size and diet. One species is the largest known pupfish species and is the only pupfish to eat other fish, while another species is the second smallest pupfish and feeds on plankton.

The fact that these fish evolved different specialties wasn’t as extraordinary to the researchers as the speed of the change.

“Something in the lake is causing very rapid divergence much more quickly than we would have expected,” Wainwright said.

According to their research, it wasn’t just lake conditions that caused the rapid diversification of the pupfish.

The lakes in San Salvador and Mexico are also home to mosquito fish. Martin also studied these fish and how they were hardly evolving at all.

“They [mosquito fish] don’t do anything special, even though they got there at about the same time,” said Martin. “The easy explanation is that the pupfish got there a bit first.”

Martin wonders if there’s something different about pupfish that causes them to become specialists.

The reasons for the rapid evolution of pupfish is still an outstanding question; Wainwright and Martin still want to find out what conditions led to such quick divergence and whether it was a single original species that got into the lake as opposed to multiple invasions by different species.

AMY STEWART can be reached at science@theaggie.org.