Discrimination of larval Morone geometric shape differences with landmark-based morphometrics

We investigated the use of geometric morphological shape differences to dif ferentiate laboratory-reared larval (4-22 days posthatch, < 10 mm SL) Moron e chrysops, Morone saxatilis, and Morone chrysops <female> x M. saxatilis m ale hybrids. We also examined impacts of allometry on descriptions of indiv idual shape. For validation of a shape-based taxonomic discrimination model , we used cellulose acetate electrophoresis to establish a "known species" test group of field-collected larvae based on banding patterns for the enzy me system esterase. Geometric shape was described with Cartesian coordinate s of 16 anatomical landmarks located along the midsaggital outline of labor atory-reared (n = 373) and field-collected (n = 29) larvae. Coordinate data were reoriented and rescaled to uniform centroid size and analyzed as land mark displacements from a reference form. Discriminant function analysis re sulted in 100% separation of M. chrysqps and M. saxatilis larvae based on g eometric shape. Discrimination success dropped to 87% when more variable da ta from hybrids were included in the discriminant function. Seventy percent and 87%, respectively, of field collected larvae (n = 29) and a randomly s elected laboratory subgroup (n = 40) were correctly classified to taxon. Re sults suggest that taxonomic separation of early-stage larvae based on shap e data is not affected by allometry but is sensitive to larval nutritional condition and handling. Landmark-based morphometrics may provide an improve d, simple method of discrimination among species in sympatric populations o f closely related larval fishes.