Subterranean Fishes of the World

3250

2017

AIM 2017

Craniofacial alterations in cavefish were among the first morphological traits to be characterized during their discovery in the 1930’s. Alvarez (1947) described “fragmentations” and “fusions” that occur in the circumorbital bones surrounding the collapsed eye orbit in multiple cavefish populations. These features are not present in extant surface-dwelling forms of the same species, Astyanax mexicanus, providing a comparative paradigm for morphological traits. In addition to bone fragmentation and fusion, other cranial alterations impact the size, shape and positioning of cranial bones, and include bilateral asymmetries. The developmental mechanisms contributing to cranial asymmetries in cavefish remain unknown. We sought to understand when, during ontogeny, cranial asymmetries arise in cavefish. Toward that end, we implemented a geometric morphometric approach to evaluate shape variation of the chondrocranium and osteocranium across life history in surface fish and two distinct cavefish populations. Interestingly, measures of cartilaginous skull in juveniles were bilaterally symmetric and shape was conserved across all three populations. In contrast, osteocranial shape segregated into significantly distinct groups in adults. Cavefish demonstrated significant asymmetry for bones surrounding the collapsed eye orbit, and the opercle bone posterior to the orbit. Interestingly, we discovered that cavefish exhibit directional asymmetry in the form of a dorsal-cranial bend, which was biased to the left. This observation may relate to observed asymmetries with the lateral line sensory system and putative ‘handedness’ associated with swimming behavior. Further experimentation will reveal the extent to which sensory system expansion correlates to cranial alterations in cavefish. In sum, this work reveals dynamic changes to the craniofacial complex over the course of cavefish life history, which has evolved in response to extreme environmental pressures of the cave.

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