Evidence forE-vector and light intensity pattern discrimination by the teleostDermogenys

Summary

• 1. Azimuth orientation in the halfbeak fishDermogenys was studied in the laboratory to find out whether its spontaneous heading directions in a vertical beam of linearly polarized light involve perception of thee-vectorper se or merely of concomitant light intensity patterns. Responses were tested with polarized and unpolarized light as well as with either a uniform white screen horizontally surrounding the experimental vessel or with one divided into black and white alternating quadrants.

• 2. Measured as counts within 10 °, 45 ° or 90 ° sectors through 180 ° the fish's azimuth orientation was random with unpolarized light and the white surround (Fig. 2).

• 3. In contrast significant preferential orientation was shown in the presence of linearly polarized light and the white surround (Fig. 3). The 10 ° sector centered on the plane of vibration had the most counts (Fig. 3A). Combining the data into four sectors each 45 ° in extent makes clear a significant predominance of orientation parallel to thee-vector (Fig. 3B) as do the total counts for parallel and perpendicular quadrants (Fig. 3D).

• 4. With the black and white quadrants combined with unpolarized light preferential orientation was clearly shown toward the light sectors (Fig. 4A, B). Since maximum differential scattering from a linearly polarized light beam is perpendicular to the plane of vibration the positive sign of this phototactic response ofDermogenys is evidence that the observed orientation parallel to thee-vector cannot also be a similar response to intensity pattern. Hence the plane of vibration must be perceived through a distinct information channel and polarotaxis is different from phototaxis.

• 5. Tests with linear polarized light combined with the black and white surround proved that phototaxis predominated over polarotaxis under our experimental conditions and that the interaction between the two types of behavior in this case was not a simple additive one (Fig. 4C-F).