Diversity and development of the hemibacula of croaking geckos (Sphaerodactylidae: Aristelliger)

Among squamates, hemipenes are known to evolve rapidly and exhibit diverse shapes, sizes, and ornamentation. Croaking geckos (Aristelliger) are unique among geckos in exhibiting mineralized structures (hemibacula) in their hemipenes. We here describe the gross morphology of the hemibacula of each currently recognized species of Aristelliger, document hemibacular histology, and report on hemibaculum development. We confirm the presence of hemibacula in all currently recognized species and demonstrate that three distinct morphologies correspond to three putative clades in the genus. Histology revealed that hemibacula are superficially similar to chondroid bone and composed of mineralized dense connective tissue covered in a thin layer of epidermis with alcian-positive cells embedded within a mineralized matrix. Additionally, we demonstrate that hemibacula do not develop until past the onset of sexual maturity and that hemibaculum length scales isometrically with body size. We hypothesize that hemibacula of Aristelliger develop via peramorphosis, a phenomenon also expressed in the cranial morphology of this genus. Additionally, we speculate on the functional significance of these enigmatic structures.