Timeline summary of cranial development in <i>Astyanax mexicanus</i> cavefish.

<p>The formation of the chondrocranium is complete by 6–8dpf. Cranial bones begin to develop around 20mm standard length, and the osteocranium is completely ossified by 35mm standard length. Previous studies reported morphological asymmetry [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0177419#pone.0177419.ref020" target="_blank">20</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0177419#pone.0177419.ref022" target="_blank">22</a>] and genetic asymmetry [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0177419#pone.0177419.ref045" target="_blank">45</a>] for individual cranial bones in adult cavefish. Gross <i>et al</i>. (2016) showed that cranial sensory neuromasts located on the suborbital #3 bone were more asymmetric in cavefish than surface fish [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0177419#pone.0177419.ref050" target="_blank">50</a>]. Here, we demonstrate normal chondrocranial development, followed by divergence of cranial shape and fluctuating asymmetry at the level of individual bones, as well as directional asymmetry associated with dorso-cranial bending toward the left.