Visual tuning in the flashlight fish Anomalops katoptron to detect blue, bioluminescent light.

Bioluminescence is a fascinating phenomenon and can be found in many different organisms including fish. It has been suggested that bioluminescence is used for example for defense, prey attraction, and for intraspecific communication to attract for example sexual partners. The flashlight fish, Anomalops katoptron (A. katoptron), is a nocturnal fish that produces bioluminescence and lives in shallow waters, which makes it ideal for laboratory studies. In order to understand A. katoptron's ability to detect bioluminescent light (480 to 490 nm) at night, we characterized the visual system adaptation of A. katoptron using phylogenetic, electrophysiological and behavioral studies. We found that the retinae of A. katoptron contain rods and sparse cones. A. katoptron retinae express two main visual pigments, rhodopsin (RH1), and to a lesser extent, rhodopsin-like opsin (RH2). Interestingly, recombinant RH1 and RH2 are maximally sensitive to a wavelength of approximately 490 nm light (λmax), which correspond to the spectral peak of in vivo electroretinogram (ERG) measurements. In addition, behavioral assays revealed that A. katoptron is attracted by low intensity blue but not red light. Collectively, our results suggest that the A. katoptron visual system is optimized to detect blue light in the frequency range of its own bioluminescence and residual starlight.

The Flashlight Fish Anomalops katoptron Uses Bioluminescent Light to Detect Prey in the Dark.

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.