A Stanford study reveals how pollution alters the sense of smell of swordtail fish, pushing them to hybridize and threatening biodiversity.

The river pollution It not only alters ecosystems, but can even go as far as modifying mating behaviors and species genetics Animals. We saw this happen from the 1980s onwards with the cichlids of Lake Victoria, whose increasingly polluted waters—combined with the commercial introduction of Nile perch—led to the loss of between 200 and 300 fish species within a decade. The increasingly murky water had deprived the females of their visual pathways—the ones that allowed them to identify the males of their species.

According to a new study led by Stanford researchers, something very similar appears to be happening to other small aquarium fish, the sword holderThis time, however, the hybridization seems to be caused by the loss of the sense of smell.

A pollution capable of mixing genes

Known in their aquarium version as xiphoid or sword holder, for their long sword-shaped caudal fin, the small fish belonging to the genus Xiphophorus They are native to the freshwater basins of Central America. The ornamental trade has made some species very common, such as the Xiphophorus hellerii and X. maculatus, and various hybridizations of them. genetic mixing Between fish of different species, however, it also seems to occur in nature. This is due to pollution. This is what emerges from a new study conducted by Stanford researchers, just published in Current Biology.

The study examined two species: Xiphophorus malinche, or highland swordtail, which retains the characteristic tip on the caudal fin, and Xiphophorus birchmanni, which is clearly differentiated from the other by its sail-shaped dorsal fin. The researchers took water and fish samples from at least ten points along four rivers of the Huasteca region, in Mexico. This region, they explain, located north of Mexico City and inland from the Gulf of Mexico, features diverse landscapes, including pristine wilderness, cattle ranches, and farms.

One of the rivers analyzed, in particular, crosses a town of about 3.000 inhabitantsAnd it is near this urbanized area that the researchers discovered the most interesting things. Meanwhile, downstream from the city the water was found to be (predictably) polluted: the researchers point out that the samples taken in this area appeared more cloudy and showed higher levels of chemicals such as nitrates and phosphates, as well as heavy metals such as copper, lead and cadmium, compared to those taken upstream. What's worse, however, is that this type of pressure is already showing its effects on small fish of the genus Xiphophorus.

Damaged noses and broken species barriers

Analysis of swordtail specimens living downstream from the city revealed that the females lacked olfactory cilia, the hair-like sensory structures in their noses that allow them to perceive smells. They also had a greater number of mucus-producing cells, which means, in a sense, that they were congested, just like humans who get colds. Genetic analysis of fish embryos, the researchers explain, revealed that many of the born specimens had characteristics intermediate between the two fish species. Ahybridization.

"The research highlights how recent environmental changes appear to be causing truly drastic genetic and evolutionary changes in these groups of fish. It's likely that this phenomenon is occurring on a large scale across many different species and ecosystems."

explains Molly Schumer, senior author of the study and associate professor of biology at Stanford's School of Humanities and Social Sciences.

The results of the new study thus add to the growing evidence that human impact on the environment is threatening biodiversity not only through extinction, but also through hybridization. As the research states, this trend could be caused precisely by the damage that pollution causes to the noses of Xiphophorus:

“We hypothesize that downstream hybrid swarming is driven by a chemical alteration of olfaction that influences mating preferences.”

It almost seems like we're seeing the Lake Victoria cichlid drama, in East Africa, whose females, no longer able to distinguish the bright colors of the males in the polluted waters that had become turbid, began to mate with males of all species leading to the formation of various hybrid populations.

Hybridization and health: a threat to biodiversity

How do you explain Ben Moran, first author of the study and former doctoral student in Schumer's laboratory, the main factor disturbing the reproductive isolation of the two species would be pollution:

"We don't have irrefutable proof, but we do have very good circumstantial evidence that hybridization is causing a loss of biodiversity in these fish. And it's likely linked to how humans are impacting the environment."

has explained.

Previous research from Schumer's lab had shown that hybridization between these two species can be harmful, causing melanoma in some fish and a lethal metabolic disorder in others. The new findings, therefore, could point to a potential long-term threat for these little fish.

"One concern is the reduction in population health. But from a conservation perspective, hybridization could lead to a sort of species collapse, where first there were two species, then eventually only one remains and biodiversity is lost."

concludes Professor Schumer.

Here are three insights that might interest you:

Biodiversity: The future of freshwater begins in cities
Is there really a sea walnut invasion in the Baltic?
Fish biodiversity in Switzerland, a unique heritage in the world

Researchers compared the genetic makeup of swordfish embryos upstream and downstream from the city to gain more insight into their origins (Photo: Ben Moran)