Development of the caudal fin skeleton of capelin, Mallotus villosus, is described based on observations of cleared-and-stained specimens from a wild caught growth series ranging in size from 4.2 mm notochord length (NL) to 144 mm standard length (SL). The sequence of chondrification and ossification for each element is described and compared with that of related salmonoid and esocoid fishes. Caudal fin elements are first chondrified in specimens that are 8.9 mm NL and all elements are ossified by 52 mm SL. In general, hypaxial elements (e.g., parhypural, hypurals, hemal arches and spines of preural vertebrae) develop before epaxial elements (e.g., epurals, uroneurals, neural arches and spines of preural vertebrae). Preural centra are among the last elements to form in the caudal skeleton of M. villosus. Interestingly, preural centrum 1 does not form. The caudal fin skeleton morphology of adult M. villosus is nearly identical to that of other osmerid species, and the developmental sequence of the caudal skeleton of M. villosus more closely resembles species of the Salmonoidei than Esocoidei. A ventral-to-dorsal developmental gradient in the caudal fin elements and anterior-posterior gradient in development of vertebral centra are discussed.
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