Early stages of cranial skeletogenesis in the Mexican tetra Astyanax mexicanus (Teleostei: Characiformes)

Astyanax mexicanus is a small freshwater fish from Mexico. It is of particular interest because in addition to the ancestral surface morph, A. mexicanus has several independently evolved cave populations. These cave morphs have a strikingly different cranial morphology, most notably in the regression of the eyes. Detailed information is available on the development, ossification and morphology of the jaws and on the differences in the sensory systems that underlie the patterns of fusion and fragmentation of the infraorbital series in the skulls of cave (troglomorphic) populations of the species. However, there is limited information on the skeletogenesis of the remaining skull components in the species. The present work aims to fill this gap by providing a detailed comparative description of cranial skeletogenesis in surface and Pachón cave populations of Astyanax mexicanus. We find that, anatomically, cavefish larvae are significantly larger than surface larvae at early stages of development, developing overall larger head structures than surface individuals. Pachón cavefish also follow a heterochronic development with respect to surface populations with slower growth and a delayed ossification process compared to surface morphs.