Inter- and intraspecific morphometric divergences in a Seychelles endemic gecko genus.

Accurate species delimitation is essential for understanding biodiversity patterns and guiding conservation, yet cryptic species with minimal morphological variation often obscure the assessment of true species diversity, making the identification of reliable diagnostic characters critical. Island radiations exemplify this challenge by concentrating endemic diversity in restricted areas, which accentuates both the urgency of conservation and the need for accurate taxonomy, while often presenting species divergences masked by subtle morphological differences. Here, we investigated patterns of morphological differentiation in Ailuronyx geckos, a genus endemic to the Seychelles archipelago, comprising deeply divergent lineages and unresolved, possibly cryptic species boundaries. We applied 3D and 2D morphometric analyses across the range (16 islands) of the three recognised species: the bronze-eyed gecko A. seychellensis, dwarf bronze gecko A. tachyscopaeus, and giant bronze gecko A. trachygaster. 3D geometric morphometric analyses of cranial and mandibular landmark data from 42 geckos revealed strong species-level differences. Ailuronyx trachygaster exhibits extreme skull sculpturing with rugose textures on up to ten cranial and three mandibular bones, while the other two species have smooth skull surfaces. Skull elongation and narrowing increased with decreasing body size from the larger A. trachygaster, via the medium-sized A. seychellensis, to the smaller A. tachyscopaeus. Analysis of body size-corrected 2D measurements of ten external morphological traits from 567 geckos revealed similar differences in head dimensions among the species, but also showed changes in mass and toe length. While we observed substantial morphological variation between islands, we did not find support for the previously identified northern and southern island group divergence within either A. seychellensis or A. tachyscopaeus. Nevertheless, we found strong support for the proposed divergence between northern and southern Mahé populations of A. tachyscopaeus, with variation in mass and head size between the two populations. We provide morphometric parameters to guide species identification within this gecko genus and highlight distinct population units that warrant further genomic investigation and prioritisation for conservation. Our findings illustrate the complexity of morphological diversification in archipelagos, where island-specific processes may outweigh regional biogeographic patterns. This underscores the need for integrative approaches to species delimitation and conservation in insular systems.