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Congo River fish evo­lu­tion shaped by intense rapids

pub­lished 19 Feb­ru­ary 2017 | mod­i­fied 19 Feb­ru­ary 2017

Cichlids Telegramma brichardiNew DNA-​based research pro­vides com­pelling evi­dence that a group of strange-​looking fish liv­ing near the mouth of the Congo River are evolv­ing due to the intense hydraulics of the river’s rapids and deep canyons. The study, led by sci­en­tists at the Amer­i­can Museum of Nat­ural His­tory, the City Uni­ver­sity of New York, and Ford­ham Uni­ver­sity, reveals that fishes in this part of the river live in ‘neigh­bour­hoods’ that are sep­a­rated from one another by the waters’ tur­bu­lent flow. In some cases, the researchers found that fishes liv­ing less than a mile away from their rel­a­tives are actu­ally exchang­ing very few genes. Many rep­re­sent dis­tinct species, accord­ing to the new study that is pub­lished on 6 Feb­ru­ary in the jour­nal Mol­e­c­u­lar Ecol­ogy.

In this very short sec­tion of the Congo, we find a tremen­dous diver­sity of fishes,” said Melanie Sti­assny, Axel­rod Research Cura­tor in the Museum’s Depart­ment of Ichthy­ol­ogy and an author on the study. “We also know that this part of the river is rel­a­tively young, orig­i­nat­ing only about 3 to 5 mil­lion years ago. So what is it about this sys­tem that makes it such a pump for species?”

Congo River basinCourse and Water­shed of the Congo and Lual­aba River with topog­ra­phy shad­ing.
Image credit: Imagico. Cre­ative Com­mons license (CC BY-​SA 2.5)

For the last 10 years, Sti­assny and her col­leagues, includ­ing hydrol­o­gists and geol­o­gists, have stud­ied the lower Congo River – the final 200-​mile stretch of the fresh­wa­ter river before it emp­ties into the Atlantic Ocean. Most of the world’s great rivers end in a maze-​like delta of trib­u­taries, but not the Congo. Its water hits the Atlantic in a sin­gle nar­row chan­nel, which sci­en­tists believe is more than 750 feet deep at some points. Excep­tional in depth, speed, and tur­bu­lence, the lower Congo is home to the world’s most extreme rapids.

Congo River Rapids

The region is also remark­able for its bio­di­ver­sity; sci­en­tists have iden­ti­fied more than 300 species of fish liv­ing there.

The genetic sep­a­ra­tion between these fishes show that the rapids are work­ing as strong bar­ri­ers, keep­ing them apart
Eliz­a­beth Alter, lead author, The City Uni­ver­sity of New York’s Grad­u­ate Cen­ter and York College »

That diver­sity has long seemed puz­zling to sci­en­tists because the lower Congo appeared to lack phys­i­cal bar­ri­ers which, if dif­fi­cult to tra­verse, are under­stood to drive spe­ci­a­tion by pre­vent­ing ani­mals from either side from breed­ing. Over time, this causes each group to develop separately.

The new study, which focuses on a group of fresh­wa­ter, rock-​dwelling cich­lid fishes of the genus Teleogramma, adds weight to a the­ory long pro­posed by Sti­assny and other experts: that the dynamic forces of the river itself are act­ing like bar­ri­ers, gen­er­at­ing diver­sity by iso­lat­ing cer­tain fishes from oth­ers for so long that their pop­u­la­tions travel down dif­fer­ent evo­lu­tion­ary paths.

The genetic sep­a­ra­tion between these fishes show that the rapids are work­ing as strong bar­ri­ers, keep­ing them apart,” said lead author Eliz­a­beth Alter. “What’s par­tic­u­larly unique about the lower Congo is that this diver­si­fi­ca­tion is hap­pen­ing over extremely small spa­tial scales, over dis­tances as small as 1.5 kilo­me­tres. There is no other river like it.”

The researchers ana­lyzed the genomes of more than 50 indi­vid­ual fishes rep­re­sent­ing each of the dif­fer­ent Teleogramma pop­u­la­tions found in the lower Congo. They found that their species ranges cor­re­spond to geo­graphic regions broadly sep­a­rated by major hydro­log­i­cal and topo­graphic bar­ri­ers, indi­cat­ing that these fea­tures are likely impor­tant dri­vers of diversification.

The authors also note that there are impor­tant con­ser­va­tion impli­ca­tions to this work: about 25 per­cent of the fish in the lower Congo are endemic, or only found in this par­tic­u­lar loca­tion. But the area is cur­rently being pro­posed as a site for major dam development.

Activ­ity like that would majorly inter­rupt the evo­lu­tion­ary poten­tial of this sys­tem,” Sti­assny said.

(Source: Amer­i­can Museum of Nat­ural His­tory press release, 16.02.2017; Condé Nast Trav­eler, The Odd Thing About the World’s Deep­est River by Ken Jen­nings, 28.09.2015)

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