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.