Geometric morphometric analysis of head shape variation in four species of hammerhead sharks (Carcharhiniformes: Sphyrnidae)

Abstract

Patterns of head shape variation in four species of hammerhead sharks of the genus Sphyrna (S. lewini, S. tiburo, S. tudes, and S. zygaena) were analyzed using geometric morphometrics methods. The analysis was performed on the coordinates of 8 anatomical landmarks defined on the basis of external morphology and homologous among the species, reconstructed from distance measurements among the landmarks, using a truss scheme and a modified multidimensional scaling algorithm. The landmark coordinates for each specimen were aligned by generalized Procrustes analysis and entered into a thin-plate splines relative warp analysis. Shape differences among the species were investigated using both the uniform and non-uniform shape components. Canonical variates analysis of the partial warp scores were used to compare patterns of interspecific variation and multiple regression analysis of the shape components on centroid size was used to examine for intraspecific variation.

The uniform components accounted for most of the statistically significant shape differences among the species. There were also differences among species with respect to the non-uniform shape components. The first relative warp depicted a lateral expansion and an anterior compression of the head, whereas the second relative warp displayed a lateral compression and posterior expansion of it. Sphyrna. tiburo and S. tudes were separated from S. lewini and S. zygaena along the first relative warp. The major non-affine shape differences (local deformations) were associated to a relatively larger snout and smaller head in S. tiburo and S. tudes. Significant intraspecific shape changes with increasing size were found for two species, S. lewini and S. zygaena. The largest variation observed with size increase was associated to the uniform compression of the head on its anteroposterior axis and a localized lateral compression of the snout.

No clear evolutionary trend could be unequivocally depicted by the interspecific shape differences, though the results of the geometric morphometric analysis may support the existence of multiple head designs in hammerhead sharks.