Hulsey, C.D.*; Wainwright, P.C.; Grubich, J.R.: Theoretical and Empirical Diversity of Anterior Jaw Functional Feeding Morphology

Using both computer simulations and morphometrics of 81 species of labrid fishes, we explored morphospace occupation and functional consequences of design in the anterior jaws four bar linkage. This linkage model abstracts the mechanical properties of the feeding apparatus and is composed of four physical links: the lower jaw, the maxilla, the nasal, and the suspensorium. If one angle between any two of the links and a change in that angle are designated, an index of how motion is transmitted through the linkage, KT, can be calculated. Higher KTs represent increased angular rotation of the maxilla per unit angular change of the lower jaw. By changing single links in a hypothetical morphospace, we found increases in the relative length of the lower jaw or nasal increased KT. Alternatively, increases in the maxilla or suspensorium generally decreased KT. We also modified starting angle and input angle to determine how the configuration of linkages influenced KT. Differences in starting angle, the initial angle between the suspensorium and lower jaw, generally had a large effect on KT. However, changing input angle, the angular rotation of the lower jaw, had little effect on KT. The empirically measured linkages suggest labrid anterior jaws have diversified, with a few notable exceptions, into all mechanically feasible morphologies with KTs between 0.3 to 1.2. We also analyzed the correlation between KT and the principal components (PCA) of all the labrid linkages. We found KT was correlated with PC2 and PC3 but uncorrelated with PC1, interpreted as size. These correlations suggest that the skeletal structure in labrids that transfers motion from the lower jaw to the maxilla has diversified along at least two independent shape axes. We discuss the relevance of this to the diversity of species and feeding modes within labrids.