Parallel evolution of regressive and constructive craniofacial traits across distinct populations of Astyanax mexicanus cavefish

Life in complete darkness has driven the evolution of a suite of troglobitic features in the blind Mexican cavefish Astyanax mexicanus, such as eye and pigmentation loss. While regressive evolution is a hallmark of obligate cave‐dwelling organisms, constructive (or augmented) traits commonly arise as well. The cavefish cranium has undergone extensive changes compared with closely‐related surface fish. These alterations are rooted in both cranial bones and surrounding sensory tissues such as enhancements in the gustatory and lateral line systems. Cavefish also harbor numerous cranial bone asymmetries: fluctuating asymmetry of individual bones and directional asymmetry in a dorsal bend of the skull. This asymmetry is mirrored by the asymmetrical patterning of mechanosensory neuromasts. We explored the relationship between facial bones and neuromasts using in vivo fluorescent colabeling and microcomputed tomography. We found an increase in neuromast density within dermal bone boundaries across three distinct populations of cavefish compared to surface‐dwelling fish. We also show that eye loss disrupts early neuromast patterning, which in turn impacts the development of dermal bones. While cavefish exhibit alterations in cranial bone and neuromast patterning, each population varied in the severity. This variation may reflect observed differences in behavior across populations. For instance, a bend in the dorsal region of the skull may expose neuromasts to water flow on the opposite side of the face, enhancing sensory input and spatial mapping in the dark. Three geographically‐distinct populations of cavefish exhibit bone fragmentation and an increase in the number of SO3 superficial neuromasts. Astyanax mexicanus exist as surface‐dwelling morphs inhabiting river systems throughout Mexico and cave‐dwelling morphs found in caves throughout the El Abra mountain range (A). Fragmentation, as defined by the number of separate suborbital #3 (SO3) bony elements, was assessed for surface fish and three populations of cavefish (B; Chica, Tinaja, and Pachón). The number of superficial neuromasts within the SO3 bony region were scored for each population (C). Pairwise t tests were performed and populations were grouped by statistical significance (a–d). White scale bar set to 3 mm. Map adapted from Mitchell et al. (1977). Research Highlights We characterized facial traits that evolved across multiple populations of cavefish. Changes to the lateral line mirror alterations