Date Published: February 8, 2017
Publisher: Public Library of Science
Author(s): Jens Hellinger, Peter Jägers, Marcel Donner, Franziska Sutt, Melanie D. Mark, Budiono Senen, Ralph Tollrian, Stefan Herlitze, Abhijit De.
Bioluminescence is a fascinating phenomenon occurring in numerous animal taxa in the ocean. The reef dwelling splitfin flashlight fish (Anomalops katoptron) can be found in large schools during moonless nights in the shallow water of coral reefs and in the open surrounding water. Anomalops katoptron produce striking blink patterns with symbiotic bacteria in their sub-ocular light organs. We examined the blink frequency in A. katoptron under various laboratory conditions. During the night A. katoptron swims in schools roughly parallel to their conspecifics and display high blink frequencies of approximately 90 blinks/minute with equal on and off times. However, when planktonic prey was detected in the experimental tank, the open time increased compared to open times in the absence of prey and the frequency decreased to 20% compared to blink frequency at night in the absence of planktonic prey. During the day when the school is in a cave in the reef tank the blink frequency decreases to approximately 9 blinks/minute with increasing off-times of the light organ. Surprisingly the non-luminescent A. katoptron with non-functional light organs displayed the same blink frequencies and light organ open/closed times during the night and day as their luminescent conspecifics. In the presence of plankton non-luminescent specimens showed no change in the blink frequency and open/closed times compared to luminescent A. katoptron. Our experiments performed in a coral reef tank show that A. katoptron use bioluminescent illumination to detect planktonic prey and that the blink frequency of A. katoptron light organs follow an exogenous control by the ambient light.
Bioluminescence is a widespread phenomenon in nature and especially common in the oceanic environment [1–2]. Bioluminescence in the ocean exists in a wide range of genera and is most commonly found in invertebrate species. In contrast to invertebrate, vertebrates lack light emitting structures, with the exception of an abundant range of fish species that either have their own intrinsic photophore system, like hatchetfishes, dragonfishes (Stomiiformes), lanternfishes (Myctophiformes) and sharks [1–6], or host bioluminescent symbiotic bacteria in specialized light organs. Specialized light organs can be found in different fish groups like deep-sea anglerfishes [7–10], ponyfishes (Leiognathidae), e.g. Photoplagios , cardinalfishes (Apogonidae), e.g. Siphamia tubifer  and flashlight fishes (Anomalopidae), e.g. Photoblepharon palpebratum and Anomalops katoptron [13–18]. A recent study reported 27 independent evolutionary events of bioluminescence in marine ray-finned fish .
Very little information is available on the role of the light organ of Anomalops katoptrons for the behavior. In 1975 Morin suggested several functions of light organs in Photoblepharon steinitzii for example in assisting predation, avoiding predation and intraspecific communication based on observations in the field and laboratory . The splitfin flashlight fish A. katoptron live together with P. palpebratus in Indonesia e.g. the Banda Islands [14, 26]. Fast blinking activity in A. katoptron was reported by different authors primarily based on observations in the field [14, 26] and from specimen individually housed in glass jars  or from unpublished data by Morin which described high blink frequencies up to 116 blinks/min . In this article we present the first quantitative study on the light organ blinking activity in the splitfin flashlight fish A. katoptron during the day/night cycle and during hunting under controlled laboratory conditions. The light organs in A. katoptron emit blue/cyan light with a wavelength of approximately 500 nm. Light emission in the blue range around 470 nm is common among many luminescent organisms . Furthermore, we demonstrate that a loss of luminescence in Anomalops is followed by a macroscopic change in light organ anatomy and a reduction of the microscopic regular tubular structure described by Steche and Bassot [14, 28]. Interestingly, the anatomical change had no impact on the complex light organ rotation mechanism in A. katoptron . The loss of luminescence combined with an anatomical change underlines the mutualistic relationship between the vertebrate host A. katoptron and the bacterial symbiont ‘Candidatus Photodesmus katoptron’ .