Ontogeny and Allometry of Body Shape in the Blacktail Shiner, Cyprinella venusta

Ontogenetic changes in body shape and its associated allometry were studied in the Blacktail Shiner, Cyprinella venusta, using geometric morphometric methods. We used a single, large sample (n = 397; 182 males, 215 females), collected in Catahoula Creek, Jourdan River drainage, Hancock County, Mississippi. Ten body landmarks were digitized from each specimen, which yielded partial warp scores that were used as shape variables to describe body shape change during ontogeny, assess sexual dimorphism, and investigate the relationship between reproductive status and ontogenetic body shape change. We also assessed the effect of sexual dimorphism on size and body shape. The null hypothesis of isometry during ontogeny was strongly rejected by multivariate regression of shape on size for both sexes (males, P < 0.0001, F = 21.970; females, P < 0.0001, F = 16.238). We found large, highly significant sexually dimorphic differences in the body shapes of males and females (MANOVA for overall shape, P < 0.0001, F = 7.535, Wilks' lambda, 0.758), which remained significant using MANCOVA with size as a covariate (log SL, P < 0.0001, F = 34.872, Wilks' lambda, 0.438; log CS, P < 0.0001, F = 34.829, Wilks' lambda, 0.439). Moreover, the ontogeny of body shape differs between males and females. There were highly significant shape differences among reproductive classes within males and females. These findings suggest that change in reproductive status may occur in concert with body shape change.

Fig. 1.

Landmarks used to capture body shape variation in Cyprinella venusta; all landmarks were located on midline of the body in each specimen: (1) origin of dorsal fin, basal junction of first dorsal fin ray; (2) nape of neck, posterior boundary of supraoccipital bone; (3) tip of snout, upper margin of mouth; (4) posterior margin of opercular series; (5) origin of pelvic fin, basal junction of first pelvic fin ray; (6) origin of anal fin, basal junction of first anal fin ray; (7) origin of anal fin, basal junction of the last anal fin ray; (8) origin of caudal fin, basal junction of ventral-most caudal fin ray; (9) origin of caudal fin, basal junction of dorsal-most caudal fin ray; (10) origin of dorsal fin, basal junction of the last dorsal fin ray. A series of 103 specimens were digitized twice (three months between capture sessions) to evaluate measurement error associated with these landmarks. Paired t-tests of centroid size (CS) on these repeated digitizing sessions were nonsignificant (P = 0.954, t = 0.058), root mean square error (RMS) was 0.034 mm and percent incongruence (PI) 0.055% (Hildebolt and Vannier, 1988)

Fig. 2.

Ontogenetic shape changes in males and females decomposed into overall, uniform, and nonuniform components. These visualizations reflect shape change between the consensus of males and consensus of females with all specimens included in the shape space (n = 397 specimens)

Fig. 3.

Ontogenetic shape changes within males and females. Visualizations reflect shape changes within females (with only females included in the shape space, n = 215) along a multivariate regression of small to large specimens, and a similar set of visualizations within males (with only males included in the shape space, n = 182)

Accepted: 21 Jun 1999