Toluca silverside Chirostoma riojai is a fish with high cultural and ictiological values. Studies on this species have focused on the type of diet or as part of taxonomic reviews, while few reports are known on the morphological changes during its ontogeny. In this study the shape of C. riojai was evaluated by means of geometric morphometrics from hatchings up to 90 days-old individuals. Specimens of C. riojai were cultivated in controlled conditions, and 171 organisms there fixed, belonging to 11 age intervals included within five phases: free embryo, apterolarva, pterolarva, juvenile, and adult. There were used six landmarks in an initial configuration, and in a second configuration there were outlined two contours (dorsal and ventral) on the anterior region. A principal component analysis showed a strong differentiation in the shape during the transition larva-juvenile, for both an analysis of canonical variables found significant differences (ps<0.001) between the 11 age groups. The linear regression between the logarithm of the size centroid and procustes distances, showed that the individuals maintain a constant change in shape up to the end of the larval phase, at which time the youth stage begins and the changes in its shape decreases. The description of the early development of C. riojai without considering the development of the fins, shows that feature with greatest change is the movement of the anal pore, followed by the lengthening of the cephalic region. The size and shape of C. riojai during its early life could serve to differentiate developmental phases and could be used with other sister species to assess whether their shape is specific in each phase. The morphological changes of C. riojai during his early life could be described in a gradual form up to the the larval period, while saltatory in the change to juvenile.
KEY WORDS: Geometric morphometrics, shape, early ontogy, Chirostoma.