Illuminating the evolution of bioluminescence in sharks

The evolutionary context in which shark bioluminescence originated is poorly understood, despite it being critical to uncovering influential factors in the evolutionary history and diversity of living chondrichthyans as well as the mechanisms of deep-water colonization by vertebrates. This study provides the first joint reconstruction of the habitats, lifestyles, and occurrence of bioluminescence in the evolution of squalomorph sharks using ancestral state estimation analysis to resolve the timing of deep-sea colonization, the evolutionary origin of bioluminescence and the ancestral ecologies of this group. The results suggest that most squalomorphs originated in neritic environments from where they colonized deep waters on several independent occasions during the Late Jurassic and Early Cretaceous, predating most of the previous estimates of the timing of this event. The colonization of the deep sea took place via the benthic zone, in contrast to the view that an intermediate mesopelagic stage occurred during this ecological transition. Finally, the analyses accounting for uncertainty of the presence of bioluminescence strongly support that this trait evolved only once among sharks in a bathydemersal ancestor. This study reveals that shark bioluminescence evolved in a complex scenario that combines elements of several previous proposals, and enriches our perspective on the sequence of events that characterized the vertebrate conquest of the deep sea.