Using landmark-based geometric morphometrics for holotype selection in cryptic species: a case study of Western Australian Halicyclops (Copepoda, Cyclopoida)

Abstract

Abstract Holotype designation is mandatory for animal species descriptions, yet we are lacking studies on its selection. Landmark-based geometric morphometrics (LBGM) can be used to detect subtle phenotypic variations and to calculate mean shapes of groups for certain characters, which should provide reliable tools for selecting holotypes that are most representative even of cryptic species. Western Australian copepods from the genus Halicyclops Norman, 1903 (Halicyclopidae) were studied here in a subterranean archipelago of calcretes by a combination of molecular phylogenetics (using mitochondrial COI and 12S markers) and LBGM, on the same specimens. Bayesian analyses of molecular markers resulted in five deeply divergent and congruent clades, supported with maximum posterior probabilities. ANOVA analysis of LBGM data showed that the effect of these clades was a much larger contributor to shape variation than the effects of locality or individual. Therefore, five new endemic species were described, employing a condensed format appropriate for cryptic species. All of them are destined for extinction due to approved uranium mining in the region. Four morphological structures used for LBGM analyses provided different degrees of phenotypic species separation in both sexes, and representative holotypes and allotypes for one structure were sometimes close to species’ extremes of variability for another structure.