Today our paper came out in Science investigating the actual causes of the Lake Victoria cichlid extinction event. Lake Victoria was home to around several hundred endemic cichlid species. However, introductions of the Nile perch (Lates niloticus) into Lake Victoria in the 1950’s caused a mass extinction of roughly 200 species of cichlids. As the Nile perch is a voracious predator that grows to six feet in length, it was long thought high predation on the Lake Victorian cichlids (the largest of which only grow slightly over 12 inches in length) by Nile perch best explained the extinction of so many species.
Our research group found that predation alone doesn’t explain the Lake Victorian cichlid extinction as we found that there was a non-random pattern of which cichlid species went extinct. Specifically, we found that the cichlids that were most impacted by the Nile perch were piscivorous (fish eating) species. The reason for this is that cichlids have highly modified pharyngeal jaws in their throat that function as a second set of jaws. These pharyngeal jaws are essentially a fused gill arch supported by muscles. Cichlids and other fish with this trait are known as “pharyngognathous fishes”. It has long been thought that pharyngognathy is evolutionary innovation as it allows the oral jaws to specialize in prey capture and the pharyngeal jaws in prey processing. When measuring the diameter of the oral jaws and the pharyngeal jaws we saw that the pharyngeal jaws were much smaller and imposed a limit on the prey size that could be swallowed by cichlids due to their bulkiness. Nile perch, which lack these specialized pharyngeal jaws had much larger gapes, meaning that they could swallow much larger prey.
Additionally, when we performed prey processing trials to see how long it took Lake Victorian cichlid piscivores and Nile perch to swallow fish prey, it took Victorian cichlids hours to process and swallow a fish that of comparable size would take Nile perch only several minutes! This indicates that when competing for the same prey resources, Nile perch are far superior to the native cichlids.
So what then explains the Lake Victorian extinction? While predation by Nile perch certainly played a role, our study highlights that competition for prey between Lake Victorian cichlid piscivores and Nile perch also played a critical role. So why then were so many cichlids in Lake Victoria piscivores if pharyngeal jaws limit them? There is pretty good evidence that the ancestor to all Lake Victorian cichlids was either a generalist or insectivore. Piscivory likely evolved in the lake because it was an open niche that had an abundance of prey resources. As the lake is dominated by cichlids, the main competitor to other cichlid piscivores before the Nile perch invasion was other piscivorous cichlids, so it was a leveled playing field in relation to prey processing. We show support for this when looking at marine reef systems and evolutionary transitions to piscivory verses processing intensive prey (algae, mollusks, echinoderms) between other pharyngognathous fishes (wrasses, parrotfish, surf perches, damselfishes) and non-pharyngognathous fishes. We find that non-pharyngognathous fish have far more transitions to piscivory than pharyngognathous fishes, indicating that pharyngognathy does restrict piscivory in marine systems as well. However, pharyngognathous fishes more commonly transition to processing intensive prey, highlighting that the pharyngeal jaws are extremely useful for processing these hard and tough to process items.
Paper Citation & Link : McGee MD, Borstein SR, Neches RY, Buescher HH, Seehausen O, and Wainwright PC. 2015. A pharyngeal jaw evolutionary innovation facilitated extinction in Lake Victoria cichlids. Science 350:1077-1079. http://science.sciencemag.org/content/350/6264/1077