Geometric morphometrics for taxonomy: an examination of opercular plate shapes of twelve species of Amphibalanus Pitombo, 2004 (Cirripedia: Balanidae)

Abstract

The taxonomy of the barnacle genus Amphibalanus has long been obscured by morphological plasticity and overlapping diagnostic traits, particularly within the amphitrite complex. Using landmark-based geometric morphometrics of the opercular plates (tergum and scutum), we examined 12 species to assess their taxonomic distinctiveness. Principal component analyses highlighted consistent shape variation in the articular and adductor ridge regions, and canonical variate analyses showed that the tergum provides reliable separation when sample sizes are adequate. In particular, A. amphitrite, A. eburneus, A. improvisus and A. reticulatus (the four species with the largest sample sizes) formed distinct morphometric clusters with high cross-validated accuracy (>80%). The main shape differences among these species occurred on the tergum, especially in the spur and basal margin. In comparison, species with low replication showed greater overlap due to limited statistical power; therefore, their results were considered exploratory. Our findings indicate that geometric morphometrics strengthens the diagnostic value of opercular plates in Amphibalanus, offering a complementary tool for species delimitation.