Etmopterus spinax, the velvet belly lanternshark, does not use bacterial luminescence

Marine organisms are able to produce light using either their own luminous system, called intrinsic bioluminescence, or symbiotic luminous bacteria, called extrinsic bioluminescence. Among bioluminescent vertebrates, Osteichthyes are known to harbor both types of bioluminescence, while no study has so far addressed the potential use of intrinsic/extrinsic luminescence in elasmobranchs. In sharks, two families are known to emit light: Etmopteridae and Dalatiidae. The deep-sea bioluminescent Etmopteridae, Etmopterus spinax, has received a particular interest over the past fifteen years and its bioluminescence control was investigated in depth. However, the nature of the shark luminous system still remains enigmatic. The present work was undertaken to assess whether the light of this shark species originates from a bioluminescent bacterial symbiosis. Using fluorescent in situ hybridization (FISH) and transmission electron microscopy (TEM) image analyses, this study supports the conclusion that the bioluminescence in the deep-sea lanternshark, Etmopterus spinax, is not of bacterial origin.