Etmopteridae bioluminescence: dorsal pattern specificity and aposematic use

Area of Science:

• Marine Biology
• Deep-sea Ecology
• Bioluminescence Research

Background:

• Many deep-sea organisms, including sharks, use bioluminescence for counterillumination camouflage.
• Dorsal luminescence patterns are rare in deep-sea species, with lateral marks in Etmopteridae sharks suggesting recognition functions.
• The function of dorsal bioluminescence in sharks remains largely unexplored.

Purpose of the Study:

• To investigate the presence and function of dorsal luminescence patterns in three species of lanternsharks: *Etmopterus spinax*, *Etmopterus molleri*, and *Etmopterus splendidus*.
• To determine if dorsal bioluminescence serves a defensive purpose, potentially related to their dorsal spines.

Main Methods:

• In situ imaging and video analysis to observe and record dorsal light emission patterns.
• Morphological and histological analyses of photophores associated with dorsal spines.
• MRI and CT scans to investigate the presence of venomous glands in spine-associated structures.

Main Results:

• Confirmed species-specific dorsal luminescence patterns in *Etmopterus spinax*, *Etmopterus molleri*, and *Etmopterus splendidus*, characterized by lines of photophores on the rostrum, dorsal area, and spine periphery.
• Demonstrated that dorsal photophore patterns contrast with ventral counterillumination, suggesting a different function.
• Validated the defensive function of dorsal spines using in situ video footage, with photophores potentially signaling their presence.

Conclusions:

• The study reveals a novel aposematic (warning) function for dorsal bioluminescence in lanternsharks, highlighting the presence of defensive dorsal spines.
• The defensive use of dorsal spines in these species has been documented in situ for the first time.
• Absence of venomous glands in Etmopteridae suggests that the defensive strategy relies on the visual warning provided by bioluminescence and the physical spines themselves.