The lantern shark's light switch: turning shallow water crypsis into midwater camouflage

Abstract

Bioluminescence is a common feature in the permanent darkness of the deep-sea. In fishes, light is emitted by organs containing either photogenic cells (intrinsic photophores), which are under direct nervous control, or symbiotic luminous bacteria (symbiotic photophores), whose light is controlled by secondary means such as mechanical occlusion or physiological suppression. The intrinsic photophores of the lantern shark Etmopterus spinax were recently shown as an exception to this rule since they appear to be under hormonal control. Here, we show that hormones operate what amounts to a unique light switch, by acting on a chromatophore iris, which regulates light emission by pigment translocation. This result strongly suggests that this shark's luminescence control originates from the mechanism for physiological colour change found in shallow water sharks that also involves hormonally controlled chromatophores: the lantern shark would have turned the initial shallow water crypsis mechanism into a midwater luminous camouflage, more efficient in the deep-sea environment.

Figure 1.

Mechanism for luminescence control in Etmopterus spinax. (a) Photophore luminescence recordings after melatonin (MT) and prolactin (PRL) application (arrows). (b) Evolution of the ‘transparency index’ of photophores (dots, mean data points calculated on six different photophores) after hormonal stimulation. Serial pictures (top) illustrate individual photophore evolution after MT and PRL stimulation. (c) Photophore-containing skin patch before hormonal stimulation (i) and at luminescence peak (ii). The same spiny dermal denticle (white arrow) and group of photophores (white box) are represented in both panels. Scale bar, 1 mm. (d) Ventral luminous pattern of an adult E. spinax specimen. Scale bar, 2 cm.