Functional morphology and developmental biology of zebrafish: reciprocal illumination from an unlikely couple

Functional morphology has benefited greatly from the input of techniques and thinking from other disciplines. This has been especially productive in situations where each discipline has made significant contributions to a particular research topic. A combination of methodologies from functional morphology and developmental biology has allowed us to characterize feeding mechanics of first-feeding larval zebrafish (Danio rerio). Contrary to kinematic patterns commonly seen in adult teleosts, larval zebrafish showed no lateral abduction during the expansive phase of a suction-feeding event. Instead, dorsoventral expansion of the buccal chamber, more typical of patterns seen in primitive fishes, characterized the expansive phase. Moreover, a pronounced preparatory phase during which the buccal chamber is constricted by the protractor hyoideus was consistently seen in first-feeding larval kinematics. Key kinematic variables associated with first feeding correlated significantly with the hydrodynamic regime as measured by the Reynolds number. Using the tools of both functional morphology and developmental biology we have not only determined which cranial muscles are important for successful feeding but also uncovered important physiological differences in muscle structure. Muscles necessary for the rapid dorsoventral expansion of the head are composed primarily of fast-twitch fibers while those involved in more tonic contractions such as hyoid protraction have more slow-twitch muscle fibers. While most evolutionary developmental studies have examined mechanisms responsible for large evolutionary changes in morphology, we propose that the type of data uncovered in functional studies can lead to the generation of hypotheses concerning the developmental mechanisms responsible for smaller intra- and/or interspecific changes.