American Society of Naturalists

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“Head shape modulates diversification of a classic cichlid pharyngeal jaw innovation”

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Edward D. Burress, Milton Tan, and Peter C. Wainwright (Nov 2019)

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Shape disparity and rate of evolution of a major cichlid pharyngeal jaw innovation is dependent upon head shape

<i>Gymnogeophagus terrapurpura</i> from a stream in southern Uruguay. This species is an example of a head shape that is associated with high pharyngeal jaw shape disparity in Neotropical cichlid fishes.<br/>(Credit: Edward Burress)
Gymnogeophagus terrapurpura from a stream in southern Uruguay. This species is an example of a head shape that is associated with high pharyngeal jaw shape disparity in Neotropical cichlid fishes.
(Credit: Edward Burress)

Key innovations are morphological, physiological, or behavioral traits that facilitate the exploitation of new resources and are often proposed to explain why some groups of organisms are diverse while others are not. However, groups that share such an innovation may also exhibit unevenly distributed ecological diversity, raising the possibility that the utility of innovations may be dependent upon other factors. In this study, Edward Burress, Milton Tan, and Peter Wainwright at the University of California-Davis test whether head shape influences diversification of cichlid pharyngeal jaws, which provide an enhanced ability to crush, chew, and grasp prey. The authors show that wide heads evolve in association with feeding upon items that require extensive processing by the pharyngeal jaws such as snails, likely to accommodate large pharyngeal jaw bones and associated musculature. However, head width is negatively correlated with pharyngeal jaw shape diversity and rates of shape evolution. Species with wide heads exhibit only a fraction of observed pharyngeal jaw shapes, whereas species with narrow heads exhibit more pharyngeal jaw shapes and exploit correspondingly more types of prey. The authors determine that head shape modulates diversification of pharyngeal jaws and subsequently the ecological diversity of cichlids. This study provides an example of how a simple aspect of morphology can have broad evolutionary and ecological implications by interacting with a major innovation.


Functional innovations are often invoked to explain the uneven distribution of ecological diversity. Innovations may provide access to new adaptive zones by expanding available ecological opportunities and may serve as catalysts of adaptive radiation. However, diversity is often unevenly distributed within clades that share a key innovation, highlighting the possibility that the impact of the innovation is mediated by other traits. Pharyngognathy is a widely recognized innovation of the pharyngeal jaws that enhances the ability to process hard and tough prey in several major radiations of fishes, including marine wrasses and freshwater cichlids. We explored diversification of lower pharyngeal jaw shape, a key feature of pharyngognathy, and the extent to which it is influenced by head shape in Neotropical cichlids. While pharyngeal jaw shape was unaffected by either head length or head depth, its disparity declined dramatically with increasing head width. Head width also predicted the rate of pharyngeal jaw evolution such that higher rates were associated with narrow heads. Wide heads are associated with exploiting prey that require intense processing by pharyngeal jaws that have expanded surfaces for the attachment of enlarged muscles. However, we show that a wide head constrains access to adaptive peaks associated with several trophic roles. A constraint on the independent evolution of pharyngeal jaw and head shape may explain the uneven distribution of ecological diversity within a clade that shares a major functional innovation.