Sharks acquired bioluminescence in a unique event during the Jurassic Period, after colonising the deep sea. This is shown by a study from the University of Valencia on bioluminescence in sharks, recently published in the journal Palaeontology. The work, which provides the first comprehensive reconstruction of the habitats and lifestyles throughout the evolution of these groups, sheds light on how and why the ability to emit light in the dark appeared in sharks.
Bioluminescence is a phenomenon that occurs in different marine species and an important component in the dynamics of deep-sea ecosystems. As a consequence, there is extensive scientific literature on the causes and the scenario of its origin, but there is no consensus among specialists. The article, which has just appeared in the journal Palaeontology, provides new perspectives on the scenario in which bioluminescence evolved, as well as on the mechanisms of deep-sea colonisation by sharks and vertebrates in general.
According to the study, most of the squaliforms - the order to which bioluminescent sharks belong - originated in neritic environments, that is, in well-lit areas close to the coast. From there they colonised the deep waters at various times during the Jurassic and Cretaceous Periods. And contrary to what was thought until now, this process took place through the benthic zone - that is, on the marine substrate, from coastal areas to great depths devoid of light.
"The evolution of bioluminescence occurred only once in the evolutionary history of sharks, and it did so after they colonised the great deep sea. Therefore, the appearance of bioluminescence, contrary to the scientific consensus that existed to date, most likely played an irrelevant role in the conquest of deep waters", explains Humberto Ferrón, Ramón y Cajal researcher at the Cavanilles Institute of Biodiversity and Evolutionary Biology (ICBiBE) of the University of Valencia, and responsible for the project.
Defensive strategy and signaling for your peers
Bioluminescence is a fundamental component in the dynamics of deep-sea ecosystems. In sharks, its presence is shown in representatives of three different families of the order of the squaliforms -Etmopteridae, Dalatiidae and Somniosidae-, which live both in the body of water -without contact with the bottomand on the substrate, from the 200 to 4,000 meters deep.
According to the work now published, bioluminescence fulfils several key functions in sharks today, from recognising individuals of the same species in the dark to illuminating large spines on their fins as a warning signal for potential predators.
"The pre-established idea was that the first bioluminescent sharks would live in illuminated areas and present bioluminescence in their bellies, making use of this feature to camouflage themselves and go unnoticed by predators lurking below their level", comments H. Ferrón. "The new work calls into question this idea of camouflage by counterillumination, since bioluminescence would be originating in environments of total darkness and in sharks that would live on the same substrate, with no possibility of threats from below", concludes the scientist.
Humberto Ferrón leads the MacroFun Research Group -Macroevolution and Functional morphology research group , as part of the Cavanilles Institute of Biodiversity and Evolutionary Biology. The team focuses on elucidating the main evolutionary transitions of animals, their causes, and consequences, integrating evidence from living organisms and the fossil record through the joint application of a variety of cutting-edge techniques in paleobiology, ecology, evolution, and biomechanics.
Illuminating the evolution of bioluminescence in sharks . Humberto G. Ferrón. Palaeontology . Volume 66, Issue 1 e12641