<i>Premnas biaculeatus</i> follow the rotating filter in the full polarization treatment.

<p>Larval position, measured at 2(θ; Y-axis); as the axis changes its direction relative to the sun (α; X-axis). Different colors represent different larvae, under partial and full polarization (panels A & B). Points along the equality line (dashed line) depict larvae swimming towards the sun. Horizontal sequences of points (marked by red arrows) depict larvae that track the polarization axis, as it rotates along with the DISC.</p

Polarized Light Sensitivity and Orientation in Coral Reef Fish Post-Larvae

<div><p>Recent studies of the larvae of coral-reef fishes reveal that these tiny vertebrates possess remarkable swimming capabilities, as well as the ability to orient to olfactory, auditory, and visual cues. While navigation according to reef-generated chemicals and sounds can significantly affect dispersal, the effect is limited to the vicinity of the reef. Effective long-distance navigation requires at least one other capacity–the ability to maintain a bearing using, for example, a sun compass. Directional information in the sun’s position can take the form of polarized-light related cues (i.e., e-vector orientation and percent polarization) and/or non-polarized-light related cues (i.e., the direct image of the sun, and the brightness and spectral gradients). We examined the response to both types of cues using commercially-reared post-larvae of the spine-cheeked anemonefish <i>Premnas biaculeatus</i>. Initial optomotor trials indicated that the post-larval stages are sensitive to linearly polarized light. Swimming directionality was then tested using a Drifting In-Situ Chamber (DISC), which allowed us to examine the response of the post-larvae to natural variation in light conditions and to manipulated levels of light polarization. Under natural light conditions, 28 of 29 post-larvae showed significant directional swimming (Rayleigh’s test p<0.05, R = 0.74±0.23), but to no particular direction. Swimming directionality was positively affected by sky clarity (absence of clouds and haze), which explained 38% of the observed variation. Moreover, post-larvae swimming under fully polarized light exhibited a distinct behavior of tracking the polarization axis, as it rotated along with the DISC. This behavior was not observed under partially-polarized illumination. We view these findings as an indication for the use of sun-related cues, and polarized light signal in specific, by orienting coral-reef fish larvae.</p></div

Environmental factors explaining the variation in the directionality (R_c) of <i>Premnas biaculeatus</i>.

<p>Regression tree of swimming directionality of 28 <i>P. biaculeatus</i> post-larvae, with box plots showing the distribution of R_c in each terminal node.</p

The DISC: Drifting In-Situ Chamber.

<p>The DISC is a cylindrical transparent and symmetrical behavioral chamber, which is set adrift and records the behavior of individual larvae (<a href="https://www.rsmas.miami.edu/users/cparis/instruments.html" target="_blank">https://www.rsmas.miami.edu/users/cparis/instruments.html</a>). (A) The classic DISC setup used in the natural conditions (NC) experiment consisting of a large behavioral chamber. (B) The setup used for the polarization manipulation experiment consisting of: the three layered filter (F), the mesh cover (MC), the behavioral chamber (BC), the acrylic ring (R), and the camera (CAM).</p

View from the behavioral chamber looking upwards, under full and partial polarization (FP and PP, respectively).

<p>The DISC was manually rotated 360° counterclockwise, at increments of ∼45° (A–H), while scuba-diving at a depth of 9 m. Red arrow indicate the filter’s e-vector orientation. Photos were taken on the 18^th of November 2013 at 14∶35–14∶50. Solar elevation at the time was approximately 21° from the horizon, and the sky was clear.</p

Optomotor trials examining sensitivity of <i>Premnas biaculeatus</i> to linearly polarized light.

<p>The proportion of positive optomotor responses (N = 59) to the control, polarized stripes and black-and-white stripes patterns (W, POL and BW; respectively), with different options of considering borderline cases (in which it was hard to decide whether the response was positive or negative). Ratios of fish responding with actual numbers are given in parenthesis. Chi-squared and p-values pertain to a test for the equality of proportions (proportions test); testing the null hypothesis that the probability of responding positively to POL is equal to the expected by chance (W).</p

Mean bearings and directionality of <i>Premnas biaculeatus</i>.

<p>Mean bearings and directionality (R_c) of 28 <i>P. biaculeatus</i> post-larvae, depicted by the direction and the length of the blue lines respectively. Red polygons represent nearby coral-reef patches.</p

The optomotor apparatus.

<p>(A) The optomotor consisting of: a water container (WC), a stationary arena (SA), a rotating platform (RP), an electric motor (M), an experimental pattern (P), light emitting diodes- LEDs (LD). (B) The polarized experimental pattern photographed from the stationary arena looking upwards, (C) same image as B, but viewed through a polarizing filter.</p