Functional physiology of lantern shark (<i>Etmopterus spinax</i>) luminescent pattern: differential hormonal regulation of luminous zones

Lantern sharks are small deep-sea sharks that harbour complex species-specific luminescent photophore patterns. The luminescent pattern of one of these sharks, Etmopterus spinax, is made up of nine luminous zones. Previous experiments revealed that in the largest of these zones (ventral zone), photophores are under hormonal control, light being triggered by both melatonin (MT) and prolactin (PRL). In this study, we analysed the luminescent responses to MT and PRL in five other luminous zones from 12 female and eight male E. spinax specimens. The results showed that all luminous zones respond to both hormones, with each zone having its own kinetic parameters (maximum light intensity, Lmax; total light emitted, Ltot; time from stimulation to Lmax, TLmax), which confirms the multifunctional character of this shark's luminescence. Ltot and Lmax were found to be directly dependent on the photophore density (PD) of the luminous zone, while TLmax varied independently from PD. In addition, we demonstrate a sexual dimorphism in the luminescent response to PRL, with male specimens having smaller Ltot and TLmax in the luminous zones from the pelvic region. As this region also harbours the sexual organs of this species, this strongly suggests a role for the luminescence from these zones in reproduction