title: Iso-luminance counterillumination drove bioluminescent shark radiation.
abstract: Counterilluminating animals use ventral photogenic organs (photophores) to mimic the residual downwelling light and cloak their silhouette from upward-looking predators. To cope with variable conditions of pelagic light environments they typically adjust their luminescence intensity. Here, we found evidence that bioluminescent sharks instead emit a constant light output and move up and down in the water column to remain cryptic at iso-luminance depth. We observed, across 21 globally distributed shark species, a correlation between capture depth and the proportion of a ventral area occupied by photophores. This information further allowed us, using visual modelling, to provide an adaptive explanation for shark photophore pattern diversity: in species facing moderate predation risk from below, counterilluminating photophores were partially co-opted for bioluminescent signalling, leading to complex patterns. In addition to increase our understanding of pelagic ecosystems our study emphasizes the importance of bioluminescence as a speciation driver.
authors.affiliation: Laboratoire de Biologie Marine, Earth and Life Institute, Université catholique de Louvain, 1348 Louvain-la-Neuve, Belgium.
authors.forename: Julien M
authors.initials: JM
authors.lastname: Claes
authors.affiliation: Lund Vision Group, Lund University, 22362 Lund, Sweden.
authors.forename: Dan-Eric
authors.initials: DE
authors.lastname: Nilsson
authors.affiliation: Department of Biology, College of Charleston, Charleston, SC 29412, USA.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Straube
authors.affiliation: The School of Animal Biology and The Oceans Institute, The University of Western Australia, Crawley, WA 6009, Australia.
authors.forename: Shaun P
authors.initials: SP
authors.lastname: Collin
authors.affiliation: Laboratoire de Biologie Marine, Earth and Life Institute, Université catholique de Louvain, 1348 Louvain-la-Neuve, Belgium.
authors.forename: Jérôme
authors.initials: J
authors.lastname: Mallefet
mesh_terms: D000818:Animals; D049449:Luminescence; D008954:Models, Biological; D012754:Sharks / Q000502:physiology*