The development of muscle fiber type identity in zebrafish cranial muscles.

Cranial skeletal muscles underlie breathing, eating, and eye movements. In most animals, at least two types of muscle fibers underlie these critical functions: fast and slow muscle fibers. We describe here the anatomical distribution of slow and fast twitch muscle in the zebrafish (Danio rerio) head in the adult and at an early larval stage just after feeding has commenced. We found that all but one of the cranial muscles examined contain both slow and fast muscle fibers, but the relative proportion of slow muscle in each varies considerably. As in the trunk, slow muscle fibers are found only in an anatomically restricted zone of each muscle, usually on the periphery. The relative proportion of slow and fast muscle in each cranial muscle changes markedly with development, with a pronounced decrease in the proportion of slow muscle with ontogeny. We discuss our results in relation to the functional roles of each muscle in larval and adult life and compare findings among a variety of vertebrates.