Evolutionary innovations help animals cross functional barriers and exploit new niches that were inaccessible to their ancestors. These might even promote rapid diversification or reduce extinction rates in lineages that evolve them. But for some fish populations in eastern Africa’s Lake Victoria, their adaptive traits weren’t all that beneficial in the end. Researchers studying the extinction of cichlid species with innovative jaws found that increasingly specialized traits made them less flexible to changes in their surroundings. The findings are published in Science this week.
Several lineages of spiny-finned fishes have independently evolved what’s called pharyngognathy – jaw modifications to help generate high bite force. It’s quite useful for consuming hard-shelled prey items. For decades, the jaws of pharyngognathous fishes have been thought of as a classic example of innovations opening up new niches. And while that’s likely the case, there was also a major tradeoff: These modifications to the back of the throat reduced their pharyngeal gape, limiting the size of prey that can be swallowed easily.
A team led by Matthew McGee from the University of California, Davis, looked at the success of species that have evolved pharyngognathy. Wrasses, surfperches, damselfish, marine halfbeaks, and flyingfishes have always lived in shallow marine habitats alongside normal, non-pharyngognathous fish; cichlids, meanwhile, often occur in tropical freshwaters. In lakes Victoria and Malawi, cichlids with pharyngognathous jaws diversified in the complete absence of any non-pharyngognathous competitors. Some even evolved into predatory, fish-eating niches.
The researchers found that pharyngognathous fishes evolved into niches favoring hard-shelled prey at a much higher rate than other spiny-finned fishes, but they evolved into fish-eating niches more slowly. They all became less adapted to a more general diet, and the limited gape of predatory fish-eating cichlids turned out to be disastrous when the non-pharyngognathous Nile perch (Lates niloticus, pictured above right) was introduced to Lake Victoria in the 1950s. After the voracious invaders showed up, the prevalence of predatory cichlids declined. Competition for prey played a huge role in the mass extinction of many Lake Victoria cichlid species.
When the team compared feeding performance and functional morphology of Nile perch with cichlids, they found that the gape of cichlids was half that of the Nile perch. Cichlids also took many hours to process a prey item that a Nile perch could swallow in a few minutes. The predatory cichlids were easily outcompeted by invasive Nile perch. The evolutionary innovation of pharyngognathy isn't a uniformly beneficial trait, the team writes, but a specialization that can promote competitive inferiority and extinction depending on the ecological context.
This "two stripe white lip" from the genus Harpagochromis is an undescribed species of Victorian predator cichlid that’s now extinct in the wild. Matthew McGee
Image in the text courtesy of John Uhrig