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. 2017 Feb 8;12(2):e0170489.
doi: 10.1371/journal.pone.0170489. eCollection 2017.

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

Jens Hellinger  1 , Peter Jägers  1 , Marcel Donner  1 , Franziska Sutt  1 , Melanie D Mark  1 , Budiono Senen  2 , Ralph Tollrian  3 , Stefan Herlitze  1
Affiliations

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

Jens Hellinger et al. PLoS One. .

Abstract

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.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Field recordings on the Banda Islands nights.
(a) Observation sites marked by black burgees. Samples are indicated by A-G. Map adapted from OpenStreetMap-contributor (Open Database Licence (ODbL) 1.0). (b) Number of individuals in schools at different study sites A: Pulau Naira, B: Banda Naira, C: Pulau Banda, D-G: Hatta. Circles indicate individual schools. A total of 31 schools (1 recording per school) were recorded at the study sites. (M) represents the mean value of fish per school ± SEM. (c) Orientation of schooling A. katoptron on the coral reef flat during the night. Pictures were recorded with an internal camera flash. Black bar (1) indicates specimens orientated in one direction (see example in c). Grey bar (2) indicates the opposite direction. White bar (3) indicates specimens in perpendicular orientation. (d) Schooling behavior in A. katoptron on a reef flat during a moonless night. Flash photograph of an A. katoptron school showing the majority of fish moving in the same direction, and a minority positioned in the opposite or perpendicular direction. (e) Bioluminescence of A. katoptron light organs in complete darkness Error bars indicate ± SEM.
Fig 2
Fig 2. Bioluminescence, anatomical position and structure of the light organs in the splitfin flashlight fish (Anomalops katoptron).
(a) Bioluminescence of A. katoptron during the night. Front view of both subocular light organs. The photograph was taken in a reef tank during the night. (b) Approximated bioluminescence wavelength (498 nm) of the emitted light. (c) Habitus, subocular position and structure of the light organ of the splitfin flashlight fish (Anomalops katoptron) shown for one luminescent and one non-luminescent specimen with degenerated light organs. The oval light organ appears as a white patch because of the guanine crystal reflector on the backside of the light organ and photography using a camera flash. The degenerated non-luminescent light organ illustrates a loss of blood vessels on the surface and a change in shape. (d) Number of tubules in luminous (n = 4) and non-luminous (n = 11) specimens of A. katoptron. Error bars indicate ± SEM (e) Sagittal 3D-photomicrographs of one luminescent and one non-luminescent light organ. The images show the tubules (tu) where A. kataptron host the bioluminescent bacterial symbionts, the reflector (ref), and the cartilaginous light organ attachment (ca) located at the frontal apex.
Fig 3
Fig 3. Blinking behaviour in A. katoptron under graded dim light conditions, in darkness and while feeding defrosted zooplankton in darkness.
(a) Blink frequency (blinks/min) in A. katoptron with luminous light organs (A1-A5) and non-luminous light organs (Anl1-Anl3). Luminous and non-luminous fish rotated the light organs with a comparable frequency under 3 decreasing dim light conditions. White bars (1–3) indicate blink frequencies under dim light (i3-i1) conditions. Light grey bars indicate blink frequencies at night. Dark grey bars indicate blink frequencies while feeding frozen zooplankton. (b) Summarized percentiles of blink frequencies in A. katoptron with luminous (solid grey lines: A1-A5, n = 5; blue line: average A1-A5) and non-luminous light organs (dark grey lines: Anl1-Anl3, n = 3; red line: average Anl1-Anl3) under the conditions dim light i1-i3, dark and feeding with frozen zooplankton in the dark. (c) Mean time intervals based on single open & closed times of light organs during dim light (dl i1-dl i3; white squares), darkness (d, light grey squares), and feeding in the dark (f, dark grey squares) conditions. (d) Total relative distribution of open & closed light organs during dim light (dl i1-dl i3), darkness and feeding in the dark experiments. Icons illustrate color coding in c-d. Black indicates closed light organs for both luminous and non-luminous light organs. Blue indicates luminous open light organs. Red indicates non-luminous open light organs. Error bars indicate ± SEM.
Fig 4
Fig 4. Blink frequency and catch efficiency in A. katoptron feeding on currently hatching cleaner shrimp larvea (L. amboiensis).
(a) Blink frequency percentiles for specimens with luminous (blue line A1-A5, n = 5) and non-luminous (red line Anl1-Anl3, n = 3) light organs during 2 subsequent 5 minute recording sessions (minute 1–5 & minute 6–10) to illustrate the changes during decreasing larvae density. (b) Average open & closed time of light organs for individual specimens with luminous light organs (A1-A5, n = 5) and non-luminous light organs (Anl1-Anl3, n = 3). Pooled data from two subsequent observation periods, (min 1–5: directly after hatching; min 6–10: observation period directly after min 1–5) each of five minutes. (c) Percentile distribution of open & closed times of light organs in A. katoptron, summary of the 10 min feeding period. Icons illustrate color coding for bar graphs 4b and 4c. Icons illustrate color coding in c. Black indicates closed light organs for both luminous and non-luminous light organs. Blue indicates luminous open light organs. Red indicates non-luminous open light organs. Error bars indicate ± SEM.
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