Abstract
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 Premnas biaculeatus. 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.
Highlights
Animals are often required to navigate a straight course, either towards or away from a particular location [1,2]
The proportion of positive responses elicited by the polarized cue (POL) was somewhat lower than that elicited by the BW cue, but significantly higher than the proportion expected ‘by chance’ (W)
Drifting In-Situ Chamber (DISC) – Natural Light Conditions (NC) Rc was larger than Ro for 67% of the post-larvae (29 of 43 postlarvae)
Summary
Animals are often required to navigate a straight course, either towards or away from a particular location [1,2]. A guiding compass would benefit most actively mobile organisms; including the pelagic larvae of coral reef fishes For these dispersive elements, reaching an unobservable reef on which to settle is a matter of life or death; with far-reaching demographic and ecological consequences [4]. Despite their minute size, coral-reef fish larvae are known to be effective swimmers [5]. Coral-reef fish larvae are known to be effective swimmers [5] They have been shown to possess sensory capabilities that could locate distant targets for settlement The effective range of the sensory capabilities is largely unknown; as is the capacity for compass-orientation, which would render larvae effective navigators
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