Controlled iris radiance in a diurnal fish looking at prey

publishedVersio

DAlvise_pathogens_Cq-values

Pathogen detection by quantitative PCR: Cq-values of all samples and assay

Data from: Controlled iris radiance in a diurnal fish looking at prey

Active sensing using light, or active photolocation, is only known from deep sea and nocturnal fish with chemiluminescent "search" lights. Bright irides in diurnal fish species have recently been proposed as a potential analogue. Here, we contribute to this discussion by testing whether iris radiance is actively modulated. The focus is on behaviourally controlled iris reflections, called "ocular sparks". The triplefin Tripterygion delaisi can alternate between red and blue ocular sparks, allowing us to test the prediction that spark frequency and hue depend on background hue and prey presence. In a first experiment, we found that blue ocular sparks were significantly more often "on" against red backgrounds, and red ocular sparks against blue backgrounds, particularly when copepods were present. A second experiment tested whether hungry fish showed more ocular sparks, which was not the case. Again, background hue resulted in differential use of ocular spark types. We conclude that iris radiance through ocular sparks in T. delaisi is not a side effect of eye movement, but adaptively modulated in response to the context under which prey are detected. We discuss the possible alternative functions of ocular sparks, including an as yet speculative role in active photolocation

<i>Tripterygion delaisi</i> showing ocular sparks in the field (video) from Controlled iris radiance in a diurnal fish looking at prey

Free-roaming <i>T. delaisi</i> individuals filmed while foraging on natural substrates in 5 m depth in Stareso, Corsica. Individuals show blue and red ocular sparks frequently and switch between them, or turn them off, by small eye movements. Ocular sparks require the fish to sit in a sun-lit environment and are generated by focusing downwelling light on the iris or skin below the pupil. See also figure 2 in the main text

Comparison between four common reflector types (figure) from Controlled iris radiance in a diurnal fish looking at prey

Comparison between four common reflector types. A. Focusing eyes work as a retroreflector. By focusing light from an object onto the back of the eye structure, the reflected light is sent back to the source. The brightness of the returned light depends on the reflectiveness of the layer behind the lens (e.g. a tapetum). B. Specular mirrors reflect the light at an angle that is identical to the incoming angle. They send light back to the source only when it arrives orthogonal to the mirror's surface. Silvery fish scales have specular properties. C. Diffuse reflectors scatter incoming light in all directions. Matt structures are diffuse reflectors. D. Reflective cups show complex patterns of specular reflection, but have a higher probability to send light back to the source than a flat specular mirror. Although this may explain the strength and directionality of the reflections seen in the eyes of some invertebrates such as copepods, the actual reflective properties of copepod eyes remain to be investigated

Experimental design (2 figures, A and B) from Controlled iris radiance in a diurnal fish looking at prey

<b>Figure S5-A:</b> Frontal view of three of the 21 experimental tanks in the setup, each section with its own blue LED source. A <i>Tripterygion delaisi</i> individual is visible at the front window of the middle tank. The top image shows the section with manual white balance, which approximates how humans perceive the setup once adapted to the blue light. Setting the camera to automatic white balance (below) illustrates how the setup appears to a human immediately upon entering the room from a regular, broad-spectral environment. We do not know how fish perceive colour in a blue environment like this – but a certain degree of colour constancy (neural compensation for a skewed ambient spectrum), as in the top image, is expected (images taken with a Nikon AW130 by Gregor Schulte).<b>Figure S5-B:</b> Side view of the experimental setup (not drawn to scale) including a copepod chamber (front view shown at the top)

Eyeshine in the scorpionfish <i>Scorpaena porcus</i> induced by a diffuse white reflective strip of paper in the field (video) from Controlled iris radiance in a diurnal fish looking at prey

Eyeshine can be induced by moving a strip of white paper in front of a scorpionfish (<i>Scorpaena porcus</i>), illustrating that even weak sources can induce a perceptible effect. An important property of <i>retro</i>-reflective eyeshine is that the effect is only visible when the source (the paper) is in line with the viewing angle of the camera. <b>Methods:</b> The scorpionfish is shaded, only illuminated by natural side-welling light and natural downwelling light diffusely reflected off a narrow strip of underwater paper (white at the side pointing at the scorpionfish, black at the back). The strip measures approx. 1.5 mm across at the position of the scorpionfish's eye and was held about 5 cm away. The effect weakens with distance, but can be seen by a human observer over a distance of up to 40 cm. Recording made at 15 m depth under natural field conditions in Calvi, Corsica using a Nikon AW130 compact camera with manual white balance, no filter or other light source. The sequence is shown forward and reversed, repeated 3 times (video by N.K.M.)

Live copepods (<i>Tigriopus californicus</i>) under diffuse, coaxial illumination (video) from Controlled iris radiance in a diurnal fish looking at prey

The recording demonstrates the reflectiveness of copepod ocelli. As can be seen, the reflected beam is not consistently sent back to the source, but sometimes also in other directions. Although this is not true retroreflection, the directionality and intensity of the reflection confirms a form of specular reflection, as expected for the guanine crystals that cover the inside of <i>T. californicus</i> cup-like ocelli [1] (see also ESM S1) (video by N.K.M.).1. Martin, G. G., Speekmann, C., Beidler, S. 2000 Photobehavior of the harpacticoid copepod Tigriopus californicus and the fine structure of its nauplius eye. Invertebr Biol. 119, 110-124. (10.1111/j.1744-7410.2000.tb00179.x

Species list for figure 1 (text) from Controlled iris radiance in a diurnal fish looking at prey

List of all the species shown in figure 1

Ocular sparks in the laboratory (video) from Controlled iris radiance in a diurnal fish looking at prey

This clip shows a red and blue ocular spark under the laboratory conditions of the experiment. The ambient light consisted of one blue LED source above each tank (one fish per tank). Videos recorded using a Nikon D4 with a 105 mm macro lens through a LEE spring yellow filter by N.K.M