Discrimination of Larval Morone Geometric Shape Differences with Landmark-Based Morphometrics

We investigated the use of geometric morphological shape differences to differentiate laboratory-reared larval (4–22 days posthatch, < 10 mm SL) Morone chrysops, Morone saxatilis, and Morone chrysops ♀ × M. saxatilis ♂ hybrids. We also examined impacts of allometry on descriptions of individual 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 enzyme system esterase. Geometric shape was described with Cartesian coordinates of 16 anatomical landmarks located along the midsaggital outline of laboratory-reared (n = 373) and field-collected (n = 29) larvae. Coordinate data were reoriented and rescaled to uniform centroid size and analyzed as landmark displacements from a reference form. Discriminant function analysis resulted in 100% separation of M. chrysops and M. saxatilis larvae based on geometric shape. Discrimination success dropped to 87% when more variable data from hybrids were included in the discriminant function. Seventy percent and 87%, respectively, of field collected larvae (n = 29) and a randomly selected laboratory subgroup (n = 40) were correctly classified to taxon. Results suggest that taxonomic separation of early-stage larvae based on shape data is not affected by allometry but is sensitive to larval nutritional condition and handling. Landmark-based morphometrics may provide an improved, simple method of discrimination among species in sympatric populations of closely related larval fishes.

Fig. 1.

Landmarks used for description of shape change in larval Morone: (1) medial tip of snout, (2) intersection of the opercular membrane with the ventral outline, (3) inflection point marking dorsal-anterior terminus of the head, (4–9) dorsal and ventral insertion of first (4,5) sixth (6,7), and 10th (8,9) preanal myoseptum, (10–15) dorsal and ventral insertion of the first (10,11), sixth (12,13), and last (14,15) postanal myoseptum, and (16) intersection of the last postanal myoseptum with the notochord. Standard length was recorded as the distance between landmark 1 and landmark 16

Fig. 2.

Comparison of posterior density (PD) distributions for Morone chrysops, Morone saxatilis, and M. chrysops × M. saxatilis hybrids generated by discriminant function analysis. Taxonomic classification for individual points correspond with the axis closest to each point. Circles indicate data clusters in high discrimination regions (1,0,0), (0,1,0), and (0,0,1). Circle size indicates cluster size but does not correspond to any metric of data variance

Fig. 3.

Difference in shape deformation among (A) Morone saxatilis, (B) Morone chrysops × M. saxatilis, (C) M. chrysops larvae relative to the reference form. Deformation increased 4× for clarity

Fig. 4.

Shape deformation of Morone saxatilis larvae (2–18 dph) with increasing centroid size (CS). (A) CS = 22.1, (B) CS = 55.8, (C) CS = 86.9. Deformation increased 4× for clarity

Accepted: 09 May 2000