FESSLER, J.L.*; WESTNEAT, M.W.; CHERNOFF, B.; Univ. of Illinois, Chicago; Field Museum; Field Museum: Morphometrics and Biomechanics: Functional Implications of Shape Change in the Labridae

The Labridae is a large family of marine fishes (>600 sp.) that are diverse in trophic morphology, ecology, and prey capture biomechanics. Although the functional morphology of feeding in adult fishes has been extensively studied, few have investigated the biomechanical implications of morphological changes that occur during development. Morphometric data on cranial skeletal and muscular structure were analyzed in a diversity of labrid fishes and used to construct a morphospace through which we tracked the ontogenetic trajectories of two species. We examined size series of two species of wrasse thought to be sister-species. Gomphosus varius undergoes great elongation of the snout as it matures, whereas Thalassoma lunare retains a relatively constant head shape. We tested two hypotheses. First, we predicted that lever and linkage design would be more variable in G. varius than in T. lunare. Results show that this was true for levers, but false for linkages. Lever mechanical advantage decreased from 0.35 to about 0.2 in G. varius but was about 0.4 across ontogeny in T. lunare. We also expected that functionally-relevant landmarks would vary the least through development. We found that the anterior jaw four-bar linkages remained constant (maxillary KT= 1.0) and the linkage systems for these closely-related species covaried similarly, despite feeding and modeled force differences. Finally, the youngest G. varius and T. lunare were in close proximity in shape space but then diverged through ontogeny. The G. varius trajectory spanned nearly the entire labrid morphospace. We conclude that combining biomechanical models and morphometrics offers a more complete picture of developmental and evolutionary changes in fish skulls.