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Iso-luminance counterillumination drove bioluminescent shark radiation
Claes, J.M.; Nilsson, D.E.; Straube, N.; Collin, S.P.; Mallefet, J. (2014). Iso-luminance counterillumination drove bioluminescent shark radiation. NPG Scientific Reports 4(4328): 7 pp. dx.doi.org/10.1038/srep04328
In: Scientific Reports (Nature Publishing Group). Nature Publishing Group: London. ISSN 2045-2322, more
Peer reviewed article  

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  • Claes, J.M., more
  • Nilsson, D.E.
  • Straube, N.
  • Collin, S.P.
  • Mallefet, J., more

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.

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