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Jens Hellinger, Peter Jägers, Katharina Spoida, Linda C. Weiss, Melanie Danelle Mark, Stefan Herlitze

• The flashlight fish \(\textit {Photoblepharon steinitzi}\) inhabit coral reef caves in the Red Sea. During the night they dwell alone or in pairs near their cave entrance, characteristic of territorial like behavior. A special feature of the flashlight fish is a bioluminescent organ located under their eyes, which emits blue green light. This bioluminescence may have various functions including intraspecific communication. To gain a better understanding of how these bioluminescent signals are used during territorial aggression, we investigated the territorial behavior of \(\textit {P. steinitzi}\) in its native environment. Using infrared video recordings at night, we found that \(\textit {P. steinitzi}\) increases its blinking frequency, while attacking intraspecific intruders, different artificial light organs or a fish dummy simulating an intraspecific intruder. All three stimuli presented to \(\textit {P. steinitzi}\) elicited four different types of attack modes (i.e., darting, border crossings, repetitive swimming toward stimuli and aggressive contact with stimuli such as ramming and bites) to varying degrees coupled with high blinking frequencies. These attacks occurred near the entrance of the cave where \(\textit {P. steinitzi}\) mainly resides during the night, suggesting a territorial behavior. Collectively our data show that the intensity of displayed aggression potential in \(\textit {P. steinitzi}\) depends on the signal properties of the intraspecific intruder. A constant glowing light organ dummy increase the aggression level in \(\textit {P. steinitzi}\) whereas a blinking light organ dummy that simulate an intruder and a constant glowing dummy that display the fish shape decrease the aggression level in \(\textit {P. steinitzi}\).