A new study has found that river pollution is causing two closely related swordtail fish species in Mexico to interbreed, creating hybrid populations and raising concerns about the loss of biodiversity.
Researchers from Stanford University studied four rivers in Mexico Huasteca region and found that pollution downstream from a small town had disrupted the ability of two native fish species the highland swordtail (Xiphophorus malinche) and the sheepshead swordtail (Xiphophorus birchmanni) to distinguish potential mates.
The research showed that polluted waters contained higher levels of nitrates, phosphates and heavy metals, including copper, lead and cadmium, along with increased turbidity. These environmental changes were linked to physical damage in female fish, particularly to the tiny sensory hairs inside their noses that help detect chemical signals used for mate recognition.
Scientists found that female fish in polluted areas had lost many of these olfactory cilia and developed more mucus producing cells, reducing their ability to identify members of their own species.
Genetic analysis of embryos collected downstream revealed widespread hybridisation, with many offspring carrying genes from both parent species. In contrast populations in cleaner rivers largely maintained distinct species boundaries.
“This research highlights that very recent changes in the environment appear to be causing really dramatic genetic and evolutionary changes in these groups of fish,” said Molly Schumer, associate professor of biology at Stanford University and senior author of the study.
Lead author Ben Moran said while the researchers had no single definitive cause, the evidence strongly suggested pollution was driving increased hybridisation by interfering with the fishes’ sense of smell.
Previous studies have shown that hybrids between the two species can suffer health problems, including melanoma and fatal metabolic disorders.
Researchers warned that continued hybridisation could eventually lead to “species collapse,” where two distinct species gradually merge into a single hybrid population, reducing biodiversity without either species going extinct.
The findings published in the journal Current Biology, suggest that pollution may be reshaping species boundaries in freshwater ecosystems and could have similar effects on other wildlife worldwide.