Social signaling via bioluminescent blinks determines nearest neighbor distance in schools of flashlight fish Anomalops katoptron.

The schooling flashlight fish Anomalops katoptron can be found at dark nights at the water surface in the Indo-Pacific. Schools are characterized by bioluminescent blink patterns of sub-ocular light organs densely-packed with bioluminescent, symbiotic bacteria. Here we analyzed how blink patterns of A. katoptron are used in social interactions. We demonstrate that isolated specimen of A. katoptron showed a high motivation to align with fixed or moving artificial light organs in an experimental tank. This intraspecific recognition of A. katoptron is mediated by blinking light and not the body shape. In addition, A. katoptron adjusts its blinking frequencies according to the light intensities. LED pulse frequencies determine the swimming speed and the blink frequency response of A. katoptron, which is modified by light organ occlusion and not exposure. In the natural environment A. katoptron is changing its blink frequencies and nearest neighbor distance in a context specific manner. Blink frequencies are also modified by changes in the occlusion time and are increased from day to night and during avoidance behavior, while group cohesion is higher with increasing blink frequencies. Our results suggest that specific blink patterns in schooling flashlight fish A. katoptron define nearest neighbor distance and determine intraspecific communication.

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.

Magnetic field perception in the rainbow trout Oncorynchus mykiss: magnetite mediated, light dependent or both?

In the present study, we demonstrate the role of the trigeminal system in the perception process of different magnetic field parameters by heartbeat conditioning, i.e. a significantly longer interval between two consecutive heartbeats after magnetic stimulus onset in the salmonid fish Oncorhynchus mykiss. The electrocardiogram was recorded with subcutaneous silver wire electrodes in freely swimming fish. Inactivation of the ophthalmic branch of the trigeminal nerve by local anaesthesia revealed its role in the perception of intensity/inclination of the magnetic field by abolishing the conditioned response (CR). In contrast, experiments with 90° direction shifts clearly showed the normal conditioning effect during trigeminal inactivation. In experiments under red light and in darkness, CR occurred in case of both the intensity/inclination stimulation and 90° direction shifts, respectively. With regard to the data obtained, we propose the trigeminal system to perceive the intensity/inclination of the magnetic field in rainbow trouts and suggest the existence of another light-independent sensory structure that enables fish to detect the magnetic field direction.

Magnetic field perception in the rainbow trout, Oncorhynchus mykiss.

In this study, we present evidence for the perception of different magnetic field parameters in a facultative anadromous fish species of the family Salmonidae. Magnetic field perception of the rainbow trout, Oncorhynchus mykiss, was demonstrated with a heartbeat conditioning test. The electrocardiogram was measured with subcutaneously inserted silver wire electrodes in freely swimming fish. We demonstrate a conditioned response (i.e. a significant longer interval between two heartbeats) to an intensity/inclination shift for three adult and two juvenile rainbow trouts. Moreover, a conditioned response to a 90 degrees direction shift was demonstrated for three adult and two juvenile trouts. These findings support the hypothesis that the rainbow trout is able to perceive different magnetic field parameters. Furthermore, the study demonstrates magnetosensation in different developmental stages in the rainbow trout, i.e. juvenile and adult fish.