The lower Congo River. © Melanie Stiassny
A new study shows how climate change affected the diversity of Congo River fish during glacial periods, crucial knowledge for understanding modern threats to fish in this species-rich region.
The lower Congo River, the final 200-mile stretch of the iconic waterway, is the deepest river in the world and exceptional in its speed and turbulence. It’s also known for its great fish diversity, with more than 300 known species.
“It is an excellent natural laboratory for understanding complex speciation and population diversification processes,” said Naoko Kurata, a research associate in the Museum’s Department of Ichthyology and a postdoctoral associate at Cornell University.
The intense hydraulics of the river’s rapids and its deep canyons have been shown to contribute to its rich biodiversity by walling off fish populations into what amount to “islands” in the water. But Kurata and colleagues, including Melanie Stiassny, the Axelrod Research Curator in the Museum’s Department of Ichthyology, and Elizabeth Alter, professor at California State University, Monterey Bay, thought that there might be more to this evolutionary story.
Over the last about 2.6 million years, polar ice caps have continuously expanded and retracted during glacial and interglacial cycles. This is thought to be a significant driver of biodiversity on land, but less is known about its effect on freshwater systems, especially in the Congo River.
Using molecular tools, Kurata and colleagues focused on four species of fish endemic to the lower Congo River, known as lamprologine cichlids, including an unusual blind fish, Lamprologus lethops.
The research team found that these fish emerged during dry periods, when the river discharge level dropped, specifically during the Early-Middle Pleistocene, one of the Earth’s major climate transformation periods.
This speciation likely occurred because, as the river’s water level decreased, populations of fish were isolated into smaller pockets of water, leading to the rise of new species over time. The findings are published in the journal Integrative and Comparative Biology.
“This work shows that the hyper-diverse fish assemblages we see in the lower Congo River today likely reflect the synergistic effects of multiple drivers fueling complex evolutionary processes through time,” Kurata said.
Because many of the fish in the lower Congo are endemic, or only found in this particular location, there are important conservation elements to this work.
“Understanding the historical and contemporary climate impacts on river fish in Central Africa is needed to predict their evolutionary trajectory and prevent the loss of the incredible biodiversity in this region,” Stiassny said.