African Cichlid Study Finds Evolution Doesn’t Always Mean More Diversification

By on December 1, 2015
Harpagochromis, "two stripe white lip," (top) is an undescribed species of Victorian predator cichlid now extinct in the wild. Harpagochromis, "orange rock hunter," (bottom) is a beautiful predatory cichlid now found in low numbers in Lake Victoria. (Credit: Matthew Mcgee / University of California, Davis)

Though evolutionary changes are largely cast as a good thing, the parts they play can be misleading. As was the case with some African cichlid fish, evolving to have an advantage in consumption — pharyngeal jaws in their throats — ultimately helped rival fish to outcompete them.

The newly evolved traits didn’t immediately yield negative effects for the cichlids. In fact, the strong, chewing jaws they had in their throats let them chow down without discrimination on all sorts of hard-shelled lifeforms in Lake Victoria and Malawi.

This advantage helped them to thrive for decades in the lakes and diversify into different strains of colorful fish, the descendants of which live in many aquariums today.

Scientists with the University of California, Davis, and other schools made the find through a large-scale analysis of changes to the fish’s diets across several generations. This covered the period during which Nile perch were introduced en masse by fishermen around Lake Victoria.

The introduction of Nile perch spelled trouble for a lot of fish and other life forms living in the lake. And so, for some time, it had been thought that the Nile perch must’ve eaten up the cichlids in Lake Victoria like they had everything else.

But thanks to the diet analysis, researchers at UC Davis and elsewhere were able to pinpoint exactly what the perch and cichlids had been chowing down on. And regardless of the fact that perch were eating cichlids, the biggest determining factor of extinction was cichlid type: those that also ate fish.

A young Nile perch. (Credit: John Uhrig)

A young Nile perch. (Credit: John Uhrig)

The scientists found that the fish-eating cichlids had mouths of similar size to the Nile perch but, because of their fused pharyngeal jaws, they could only open theirs half as wide. As a result, the cichlids would have needed many hours to deal with prey that the Nile perch could swallow in just a few minutes. The cichlids were simply outcompeted.

Their evolved throat jaws, which had helped them in the pre-perch era, turned cichlids into inferior competitors. So the specialization that came about due to an evolutionary adaption meant to allow them to eat a broader range of fish actually led to far less diversification in the end.

Researchers say that cichlids aren’t alone in having the evolved pharyngeal jaws. Other groups of fish, including wrasses, damselfish, and flying fish, independently hit on the same innovation.

Scientists have also studied the diets of those fish families to see what kinds of food they specialised in throughout their evolutionary history. The fish were found to be unusually quick to diversify into species that tackle hard-to-eat foods, like tough plants or crunchy animals. By contrast, they were unusually slow to produce species that tackle hard-to-catch foods like other fish. So these other groups were subject to the same trade-off — chewing pharyngeal throat jaws for tough foods or wide mouths for swallowing other fish.

In the past, researchers had seen the pharyngeal jaws of cichlids as a classic example of evolutionary innovation. That’s still true, but the study also reveals that the evolution wasn’t wholly beneficial.

Top image: Harpagochromis, “two stripe white lip,” (top) is an undescribed species of Victorian predator cichlid now extinct in the wild. Harpagochromis, “orange rock hunter,” (bottom) is a beautiful predatory cichlid now found in low numbers in Lake Victoria. (Credit: Matthew Mcgee / University of California, Davis)

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