Morphology and control of photogenic structures in a rare dwarf pelagic lantern shark (Etmopterus splendidus)

The shark genus Etmopterus encompasses numerous deep-sea species that are widely distributed throughout the world's oceans and share the capability to emit light thanks to numerous tiny epidermal photogenic organs called photophores. Despite the potential wide ecological interest of this light emission, it is still a poorly studied aspect of shark biology, mostly due to the challenges inherent to the study of uncommon deep-sea animals. During a collection trip in waters around Okinawa Island, we had the opportunity to collect, maintain and study specimens of Etmopterus splendidus, a small pelagic lantern shark that was not previously known from this area. Analyses show that (i) the photophore density of this species varies according to the different parts of the body, which led to a heterogeneous photogenic pattern; (ii) photophore harbour the classical structure found in other etmopterid sharks, i.e. a cluster of photocytes enclosed in a pigmented sheath and surmounted by pigmented and lens cells; (iii) the physiological control of these photophores appears similar to what was found in the distantly related Etmopterus spinax, i.e. including hormonal and neural inputs as well as the action of pigmented cells overlying the photocytes. These results indicate that E. splendidus luminescence is probably used for more than one purpose, and support the idea that the physiological control of lantern shark photophores was selected early in the evolution of these sharks.

► We investigated living specimens of a rare dwarf pelagic luminous shark.
► We described the organization of its luminescent structures.
► We investigated the physiological control of its luminescence.
► Its luminescence is probably used in camouflage and intraspecific behaviours.
► An early origin of hormonally controlled photogenic organs in sharks is suggested.