Spatial orientation of rainbow trout to plane-polarized light: The ontogeny of E-vector discrimination and spectral sensitivity characteristics

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1 J Comp Physiol A (1990) 166: dlecmr~l of Seory, e~lnnpac'jbili~.,.,,,,, 9 Springer-Verlag 1990 Spatial orientation of rainbow trout to plane-polarized light: The ontogeny of E-vector discrimination and spectral seitivity characteristics Craig W. Hawryshyn*, Margaret G. Arnold, Elizabeth owering, and Rochelle L. Cole Departments of Psychology and iology, McMaster University, Hamilton, Ontario, L8S 4K1, Canada Accepted September 25, 1989 Summary. The results of this study demotrate that trout (Salmo gairdneri) are capable of orienting to polarized light fields. The spectral composition of the polarized light fields can significantly influence the orientation of trout. Rainbow trout exhibit ontogenetic losses in orientation to polarized light fields which appears coincident with the ontogenetic loss of the UV-seitive cones. Trout were trained to swim to a refuge located at one end of the training tank under a polarized light field. The E-vector of the polarized light field was oriented parallel or perpendicular to the long axis of the training tank. Trained fish were released in a circular test tank and their angular respoe scored. Under a white plus ultraviolet polarized light field, trout oriented in the trained E-vector orientation. For itance, fish trained under a parallel E-vector orientation exhibited angular respoes close to parallel in the test tank. However, when the spectral composition of the polarized light field was manipulated, the accuracy of spatial orientation of the trout varied. Trout weighing about 30 g exhibited accurate orientation to the white plus UV polarized light field. The trout were incapable of orientation at a body weight of 50 to 60 g. Key words: Rainbow trout - Polarization seitivity - Lone photoreceptors - Spatial orientation - Ontogeny Introduction Polarization vision resembles color vision since it depends on the possession of at least two differentially seitive receptor mechanisms which differ in their preference of E-vector. Demotration of polarization sei- * Present address: Department of iology, University of Victoria, P.O. ox 1700, Victoria, ritish Columbia, V8W 2Y2, Canada * To whom offprint requests should be sent tivity or polarotaxis does not necessarily indicate that an animal perceives polarized light as a unique visual quality. Polarization vision requires that an animal discriminates between two plane-polarized stimuli which are matched for brightness. ernard and Wehner (1977) have argued that polarization vision may employ a strategy for information processing similar to color vision. A recent study by Hawryshyn and McFarland (1987) has shown that several of the cone mechanisms participate in polarization indicating that the same neural network mediates both visual processes. The UV-seitive cone mechanism was shown to prefer the vertical E-vector orientation while the green- and red-seitive cone mechanisms preferred the horizontal. Furthermore, the blue-seitive cone mechanism was ieitive to the plane of polarization. Marc and Sperling (1976) have suggested that the blue mechanism plays a role in monitoring veiling radiance. The fact that different cone mechanisms have orthogonal polarization seitivities and overlapping cone absorption spectra suggests that teleosts have the potential to perform discriminatio on the basis of E-vector. The ability of teleosts to respond behaviorally to plane-polarized light has been examined with various innate respoes to plane-polarized stimuli. Kawamura et al. (1981), for itance, found that several species of fish including rainbow trout exhibited bradycardia in respoe to changes in the plane of polarization. Waterman and associates have examined spontaneous orientation of fish to an imposed E-vector field (Waterman and Forward 1972; Forward and Waterman 1973; Forward et al. 1972). Experiments by Forward and Waterman (1973) suggest that the fish were exhibiting polarotaxis but two factors indicate that the problem needs further examination: (i) large within-individual variability in angular respoe. (ii) individuals show respoes parallel with the E-vector on one test and perpendicular on another test. Operant conditioning has been used by Dill (1971) and Davitz and McKaye (1978) to show E-vector discrimination in fish but these studies lack the

2 566 C.W. Hawryshyn et al. : Orientation of trout to polarized light adequate controls to eliminate brightness as a possible confounding cue. ~ PR In the present study, we used a technique developed by Taylor and Adler (1973) which coists of training animals to move the length of the training tank under plane-polarized light. Trained animals are then released in a circular test tank under plane-polarized light and the angular orientation respoe scored. The specific issues we examined were as follows: (1) discriminatio on the basis of E-vector, (2) origin of polarization seitivity, (3) the influence of spectral composition of the plane-polarized light on orientation, and (4) ontogenetic 90cm 9 P changes in orientation. Materials and methods Animals. Rainbow trout (Salmo gairdneri), 15 to 20 g body weight were obtained from Spring Valley Trout Farm, New Dundee, Ontario. were fed standard hatchery trout chow (Martin's Feedmill, Elmira, Ontario) 5 days per week, except during training and testing when food was withheld for 3 days prior to a session. were held in individual 20 1 continuous flow-through tanks, with the system temperature maintained at ~ Apparatus. All experiments were conducted in a room that was painted flat black to minimize spurious brightness cues. All observatio were made from behind a black curtain. Figure 1 shows the training and testing tanks used in these experiments. The Plexiglas training tank coisted of two chambers separated by a motor driven gate: the respoe chamber, which had a refuge placed at the end; and the neutral area, in which the test fish was restricted prior to the initiation of a trial. The training tank was fed continuously with water maintained at 15 ~ During training, the training tank was placed on two boards situated on top of the test tank. Above the position of the training and test tanks was an overhead light source. The test tank was a circular plastic tank painted with flat grey marine paint with an elevated floor. A pulley operated release box was situated in the center of the test tank. The top surface of the release box was covered with white Plexiglas and the sides with black Plexiglas. The light source was a 250 W tungsten halogen Prado projector fitted with: (i) a filter tray for neutral deity and cutoff filters (Corion) and (ii) a UV-grade linear polarizer (Polaroid HNP' series) that could be adjusted through 360 ~ A summary of the light measurements made with a Photodyne radiometer are shown in Fig. 2. Percent polarization of the polarized light field was measured and is described in the results. Training. The photic conditio were set by placing a 1.5 neutral deity filter in the stimulus beam. This produced a spectrally broad-band plane-polarized field with the E-vector orientation parallel or perpendicular to the path of the fish. The training sessio were spaced approximately 1 to 2 days apart. Session # 1: fish was placed in the training tank with the gate open and with one piece of food (beef heart) in the refuge. The fish was allowed to swim freely for a period of 30 min. If the fish successfully found the food, another piece was provided. The main purpose of this session was for the fish to become accustomed to the tank. were only netted in the neutral area. Session # 2: fish was placed in the training tank with the gate open and allowed to swim freely. When the fish approached the refuge it was positively reinforced with food. Training took place for 1 h, using about 15 pieces of food. Session 41: 3: fish was placed in the neutral area with the gate closed. After 5 min, the gate was opened and left open, the fish was gently guided with a rod towards the refuge and presented \ P"l_ ~ P C IR / / 120cm Fig. 1 A,. Experimental apparatus. (A) Side view of the training tank: F refuge; G motorized gate; NC neutral chamber; P linear polarizer; PR projector, RC respoe chamber. () Side view of the circular test tank: PC pulley operated system for release box; R release box; TT circular test tank with a food reward upon approaching the target. Then the fish was guided back to the neutral area and encouraged to start over again for a total training time of 1 h. Session 4~ 4 and following training sessio: fish was placed in the neutral area with the gate closed for a period of 10 to 15 min. The gate was opened and the fish was required to move on its own to the refuge, receive a food reward, and then return to the neutral area and the gate closed. If the fish did not move easily on its own, which was often the case, it was encouraged to do so by gently guiding it with a rod. Each session used 60 trials with a partial reinforcement schedule of food on every 5th trial. Once the fish was moving easily on its own the session length was reduced to 15 trials and the animal was reinforced on every trial. Once the fish performed at a level of 10 out of 15 trials a partial reinforcement schedule of every 3 trials was used. A fish was coidered trained when it could correctly respond on 9 out of 10 trials. The average number of sessio needed to train a fish was 8.0. We attempted to train 25 trout, of these 17 trained successfully. Testing. The lighting conditio were adjusted for the particular experiment. For all tests, the behavior of the test fish was observed from behind a black curtain positioned about 2 feet from the test tank. The fish was placed under the release box for 20 min prior to the experiment. A trial was initiated by lifting the release box; if a fish did not respond within 5 min the release box was pulled from the test tank completely. This almost always resulted in a respoe and was probably due to the fish being situated in the shadow created by the box. The fish was allowed 1 to 3 min in the release box between trials. A respoe was scored when one half of a body length of the test fish crossed a line 5 cm from the wall of the tank. The path of the fish on route to the scoring line was recorded. No food reward was given during testing and therefore the number of trials per session was usually less than 20. Test fish were retrained every 1 to 2 weeks.

3 C.W. Hawryshyn et al. : Orientation of trout to polarized light 567 Results WHITE /- ~.~ ~.~_.~..~ Orientation under a white polarized light field >- U,I Z i. Z d IZ o I LOG UNIT y ~5oLP / i i i I L i i t I i L I i l i I I i I I I WAVELENGTH (nm) II '6~~11 \ / X_ II Fig. 2A,. Characterization of the light field. (A) Spectral energy distribution of the polarized light fields used in these experiments: Top curve: white (no filters); second curve : 450 nm long pass cutoff filter; third curve: 450 nm short pass cutoff filter; fourth curve: 500 nm short pass cutoff filter. () Irradiance of the polarized light field. Irradiance was measured along 8 arms in the test tank at the water surface. Arrows indicate the orientation of the E-vector Analysis. To calculate a mean axis of orientation for the bimodal data, vector analysis was performed subsequent to using the doubling angles procedure outlined in atschelet (1981). Note that the data for Fig. 8 is unimodal and thus these angles were not doubled. The V-test was used to test whether the observed angles have a tendency to cluster around a predicted angle (atschelet 1981). Figure 3 illustrates the angular respoes of trout trained in polarized light fields with the E-vector orientation parallel and perpendicular to the axis of the training tank. The results shown in Fig. 3 were based on blind experiments in which the experimenter did not know the training history of the trout. The fish, 7 in each condition, were tested under a white polarized light field with the E-vector set in one orientation. Figure 3A shows the results of the fish trained under the parallel polarized light field. The E-vector is represented by the solid arrowheads outside of the circle in all figures (and all subsequent figures). Note that the mean angular respoe (solid line double-headed arrow shown in figures with statistically significant distributio) was significantly oriented in the predicted direction for 5 of the 7 trout (see Table 1). Figure 3 shows the results of the fish trained under the perpendicular polarized light field. Note that the predicted direction was orthogonal to the E-vector orientation. The mean angular respoe was significantly oriented in the predicted direction for 7 of the 7 trout (see Table 1). oth Fig. 3A and indicate that trout are capable of distinguishing one E-vector orientation from another and that the orientation of the fish was likely based on the polarized light field. This suggests that the trout were not using brightness cues for this task. In fact Jander and Waterman (1960) have been able to demotrate through the use of different photic conditio that the orientation respoes of fish depend on the plane of polarization and must be mediated by a mechanism other than brightness detector (see Jander and Waterman 1960 for more exteive discussion). Figure3C shows the distributio which were determined with a depolarized light field. A diffusing screen was placed after the polarizer itead of in front of the polarizer. There was no change in inteity, only in the degree of polarization (polarized with diffuser in front of polarizer: 360 nm=89, 460 nm=87, 540 nm=85, 620 nm= 88% polarized at the water's surface versus depolarized with diffuser after polarizer: 360, 460, 540, 620 nm all < 1% polarized at the water's surface). In the depolarized condition none of the 5 trout had a mean angular respoe which was significantly oriented in the predicted direction. Therefore, the presence of a plane-polarized light field seems to be a necessary condition for accurate orientation in these experiments. Figure 4 shows the results of 3 trout trained to a parallel plane-polarized light field that were tested at a given E-vector orientation and then retested with the E-vector rotated by 90 ~ All fish in both Fig. 4A and were significantly oriented to the predicted direction. This suggests that trout were capable of discriminating E-vector. Note that the only difference between the two sessio was a change in E-vector orientation, not in inteity or in any other photic parameter. If the test trout were performing orientation tasks using brightness

4 568 C.W. Hawryshyn et al. : Orientation of trout to polarized light A PARALLEL PERPENDICULAR C DEPOLARIZED /X 10 II.L C / C x7 II s / s n b '7 II b 12 / II s ~7 9 II /X 8 J..L. d Z~ V 3 II 7 / J- a 2 II 6.L 1 II 5.L V Fig. 3A-C. Orientation respoes of rainbow trout to a white polarized light field. In this figure as well as all others, the solid line with double-headed arrows represents the mean angular respoe (MAR). In all figures, the E-vector orientation is noted by the solid arrow-heads outside of the circle. (A) Angular respoe of trout trained in a polarized light field with the E-vector oriented parallel to the long axis of the tank. In this test situation, the predicted direction was parallel with the E-vector orientation. () Angular respoes of trout trained in a polarized field with the E-vector oriented perpendicular to the long axis of the training tank. In this test situation, the predicted direction was perpendicular to E-vector orientation. (C)Angular respoes of trout in a depolarized field. A diffuser sheet was placed over the polarizer

5 Table 1. Statistical analysis of orientation data # Trials N Mean angular respoe Mean vector length Mean angular deviation E-Vector setting V-Test P Proportion oriented Figure 3 Fig 3A Parallel Fig. 3 Perpendicular Fig. 3 C Depolarized Figure 4 Orientation i Fig. 4A Orientation 2 Fig. 4 Figure 5 Pineal Patch Figure 6 Fig. 6A 450 nm LP Fig nm SP Fig. 6C 500 nm SP Figure 7 Fig. 7A Immature Fig. 7 Mature Figure 8 Light bulb Cue 10// //a //b // // // // & //b //a // // // // // // s // // // // / A_ //a // // // // // _1_ s // // // // // & // // // // // = =0.01 =0.025 =0.04 = =0.05 as = = = = = as as = =0.025 = =0.025 =0.03 =0.01 = =0.005 =0.02 =0.01 = as as = = "12/14 0/5 *3/3 * 3/3 * 3/3 3/11 *4/6 * 8/9 * 3/3 0/3 * 2/2

6 570 C.W. Hawryshyn et al. : Orientation of trout to polarized light A ORIENTATION 1 ORIENTATION 2 Z~ 12.L PINEAL PATCH.IX D ~7 1 II D ~x 14 II 4 / ~ m ~ 14 II < L:) Fig. 4A,. Comparison of the mean angular respoes of trout tested at one E-vector orientation (A) with those of the same trout at an E-vector orientation rotated 90 ~ () cues one would expect fish to orient in the same direction independent of E-vector orientation. This was clearly not the case. However, this does not fully rule out the possibility that polarization-induced brightness patter are being utilized (this will be more fully addressed in the Discussion. The data from Fig. 3A and, using different fish in the two groups, and Fig. 4A and, using the same fish for the two sessio, support the contention that trout are using plane-polarized light cues rather than simple brightness patter, to guide orientation. Origin of polarization seitivity To test the hypothesis that the orientation behavior of trout was mediated by an ocular and not a pineal receptor system we placed a patch of diffusing screen on the skin over a large area including and surrounding the pineal. Three out of 3 trout in Fig. 5 were significantly oriented to the predicted direction (see Table 1). If the orientation of trout to plane-polarized light was principally mediated by a pineal mechanism, then the presence of the patch should have diminished orientation accuracy. The fact that trout were oriented in the predicted direction indicates that the orientation behavior of trout is mediated by ocular photoreceptors. This result is coistent with Kawamura et al. (1981) who showed that 10 II Fig. 5. Origin of the respoe to polarized light. Mean angular respoes of the trout with a patch over the pineal heart-rate respoes of rainbow trout to plane-polarized light were not diminished when the pineal was covered. Spectral characteristics of the polarized light field Having demotrated that trout were capable of orienting to a white polarized light field, we were interested in determining which spectral region was most critical for orientation. In Fig. 6A, a 450 nm long pass filter was used to block the tramission of UV light and test the hypothesis that UV plane-polarized light was an essential cue for orientation. Only 3 out of 11 trout showed a significant angular respoe under a white minus UV polarized light field (see Table 1). This result indicates that UV polarized light is critical for orientation. In the next experiment, a 450 nm short pass filter was used to evaluate whether UV polarized light alone was a sufficient cue for orientation. Figure 6 shows that 4 out of 6 trout were oriented to the predicted direction (see Table 1) indicating that UV polarized light was not completely sufficient for orientation. Figure 6C shows the orientation of trout using a 500 nm short pass filter to produce UV plus blue polarized light field. The UV plus blue field resulted in 8 out of 9 trout showing significant oientation to the predicted direction (see Table 1). Thus it appears necessary to stimulate the blueseitive cone mechanism in addition to the UV-seitive cone mechanism of trout to observe proper orientation of trout to the polarized light field. Ontogenetic changes All the results described so far have involved experiments which used relatively small and immature trout

7 C.W. Hawryshyn et al. : Orientation of trout to polarized light 571 A 450 rim LP 450 nm $p C rim SP 12.L LX Z~ 9 II z~ 5.L Z~ Z~ V '~7 Z~ Lx... _,,.... o,, 7 / ~ Flsh2#l ~ 9 8.L 533,,5)... v %z... 5) v,~7 A -0 0 Fig. 6 A-C. Mean angular respoes of trout under spectrally variable polarized light fields. (A) A 450 nm long pass filter was used to eliminate UV light from the field. () A 450 nm short pass filter was used to produce a UV polarized light field. (C) A 500 nm short pass filter was used to produce a UV plus blue background with a body weight of 30 to 40 g. These individuals show coistent orientation to a white polarized light field (Fig. 3A, ). Three fish from this group were followed over the duration of study. Figure 7 shows the orientation of 3 of these trout at the beginning and end of the study. At the beginning of the study 3 out of 3 trout oriented to the predicted direction. Note that when the average body weight of the trout was 50 to 60 g, none of the trout were capable of orienting to the polarized light field (see Table 1). Did the loss of orientation observed in Fig. 7 result from the ontogenetic loss of UV-seitive cones, or perhaps was it due to an ontogenetic loss of a more general 'cognitive' ability for spatial orientation? We proceeded to rule out the possibility that the fish simply forgot the task. The fish were retrained under the white polarized light field with the E-vector oriented parallel to the training tank. When retested, none of the 3 trout were significantly oriented to the predicted direction. Secondly, trout not capable of orienting to a polarized light field were trained to approach a discrete light source in the training tank and then released in the test tank with the light source present. If the loss of orientation was due to a general loss in orientation ability one would expect the trout to be incapable of orienting to the light source. Figure 8 shows the results of the light source test. Clearly, these trout were orienting to the

8 572 C.W. Hawryshyn et al. : Orientation of trout to polarized light A IMMATURE F,.h3,, 0 2 II ~ 2 II MATURE Fig. 7A,. Ontogenetic changes in the orientation of trout. (A) Mean angular respoes of trout at a mean body weight of 46.9 g. Note that in all previous curves where orientation was evident mean body weight varied between 30.4 and 43.6 g. ()Angular respoes of trout with a mean body weight of 54.5 g 2 8 / 5) II ~ LIGHT UL CUE Fig. 8. Mean angular respoes to a light bulb cue. were trained to a simple light bulb cue subsequent to showing a developmental loss of polarized light orientation predicted direction (see Table 1), thus indicating that trout of this developmental stage have not lost their general ability to orient. Discussion This study has shown that rainbow trout were capable of orienting to certain polarized light fields. We suggest that rainbow trout can perform this task by discriminating on the basis of E-vector orientation. When UV radia- tion is eliminated from the polarized light field, trout lose their ability to discriminate E-vector and orient. This is probably due to the limiting of stimulation to green- and red-seitive cone mechanisms which have been shown to share a single class of polarization seitivity (Hawryshyn and McFarland 1987). Rainbow trout which exhibit orientation to the polarized light field in the immature stage, lose this ability at a developmental stage which is characterized by the disappearance of UVseitive cones (owmaker and Kunz 1987; Hawryshyn et al. 1989). Moreover, this loss of orientation is not due to a developmental change in general spatial orientation ability. There are 4 lines of evidence which suggest that trout were making E-vector discriminatio in these experiments rather than following brightness patter: (1) Two groups of fish were trained under a polarized light field with the E-vector parallel and perpendicular to the training tank. During testing, both groups showed orientation coistent with the trained direction. If the trout were using brightness cues related to the polarized light field, then one would expect the two groups to orient independently of the test tank E-vector. Furthermore, one would expect the orientation of the two groups of trout to be coistent with one another. This was not the case in this study since trout in the two groups oriented in opposite directio. (2) Trout which were tested at one E-vector orientation performed equally well at an E-vector orientation shifted by 90 ~. This merely confirms point 1. (3) Trout which were tested under a polarized light field lacking UV radiation (450 nm long pass filter) did not orient to the predicted direction. Indeed, if the trout were orienting to brightness patter produced by the polarized light field they should have oriented under these conditio. This observation has additional importance since it indicates that in the event the UV-seitive cones are not stimulated, and the blue-, green- and red-seitive cones are stimulated, only a single class of polarization seitivity is operational (one seitive to the horizontal E-vector). Hence, under these conditio, the fish is incapable of E-vector discrimination. If the observed orientation was based on brightness patter, then trout should have oriented to the polarized light field. (4) Trout at the developmental stage when UV-seitive cones disappear were not capable of orienting to the polarized light field. This reinforces the previous point that without the UV-seitive cones being stimulated or present, polarization seitivity of trout is mediated by detectors seitive to only one plane of polarization. Hence they are incapable of performing discriminatio on the basis of E-vector. Therefore, mature rainbow trout do not orient to the polarized light field. This is the critical evidence agait the brightness hypothesis since the mature trout are incapable of orienting to the polarized light field even though the light field contai UV plus white polarized light. Collectively, these 4 observatio indicate that trout use E-vector discrimination to orient and that the UV-seitive cones play a vital role in this process. What is the potential mechanism of E-vector discrim-

9 C.W. Hawryshyn et al. : Orientation of trout to polarized light 573 ination? We know that orthogonal polarization seitivity (at least 2 differentially seitive receptors) is a necessary requirement for the discrimination of different E- vector orientatio. Hawryshyn and McFarland (1987) have shown that cyprinids, possessing UV photoreception, exhibit orthogonal polarization seitivity. Further experiments have demotrated that the beta-band of the red-seitive cone mechanism shows a preference for the horizontal E-vector of UV plane-polarized stimuli (Hawryshyn, unpublished). This is coistent with the finding that alpha band stimulation of the red-seitive mechanism results in horizontal E-vector preference. The absorption spectra of cone photopigrnents overlap in the UV part of the spectrum, thus a UV stimulus under certain photic conditio may stimulate coincidentally a number of cone mechanisms. If the cone mechanisms stimulated have orthogonal polarization seitivity, such as the UV- and red-seitive (or green-seitive) cone mechanisms, one would expect E-vector discrimination for UV plane-polarized stimuli. Clearly, evidence in Figs. 3 and 4 indicate that discrimination was taking place. However, Fig. 6 illustrates what happe when the spectral quality of the polarized light field was limited. The use of a 450 nm long pass filter restricted illumination to a single class of polarization detectors, thereby precluding the possible interaction of cone mechanisms with orthogonal polarization seitivity. When the polarized light field was limited to the shortwave (500 nm short pass filter) spectrum, a region of cone mechanism overlap, stimulation of orthogonally seitive cone mechanisms permitted the discrimination of E-vector and hence correct orientation to the polarized light field. Furthermore, when trout ontogenetically lose UV photoseitivity (owmaker and Kunz 1987; Hawryshyn et al. 1989), the ability to orient to the polarized light field also disappears. This indicates even more convincingly that with the loss of one class of polarization receptor, orthogonal polarization seitivity is lost and hence E-vector discrimination is not possible. The results obtained in Fig. 6 using the 450 nm short pass filter are at first glance somewhat puzzling, since under such a polarized light field one would expect stimulation of overlapping orthogonal mechanisms. However, under these conditio 4 out of 6 trout were oriented. It was only when the field included a substantial amount of blue light that the fish exhibited a clearer orientation respoe to the polarized light field. The 500 nm short pass polarized light field was 54% more effective than the 450 nm short pass polarized light field in stimulating the blue-seitive mechanism. Why is it necessary to stimulate the blue-seitive mechanism to enable orientation to a polarized light field? Hawryshyn and McFarland (1987) demotrated that the blue-seitive mechanism is ieitive to polarized light in cyprinids. We also know from Ivanoff and Waterman (1958) that the minimum degree of polarization in the marine ecosystem is at 450 nm close to the absorption maximum of the blue-seitive mechanism. In clear freshwater lakes, the underwater light field (downwelling) is dominated by blue light (progressively so with depth; Tyler and Smith 1970). Furthermore, Marc and Sperling (1976) have suggested that the blue-seitive cones possibly play a role in monitoring veiling radiance or ambient illumination. With regard to Fig. 6, it is possible that the blue-seitive mechanism was iufficiently stimulated by the 450 nm short pass polarized light field. Thus under these conditio the fish could be operating as if the photic conditio were comparable to those during nocturnal periods. Recent experiments have shown that under dark-adapted conditio cyprinids were not seitive to the plane of polarization (Hawryshyn, unpublished). We would argue, therefore, that under a polarized light field limited to the UV spectrum the blueseitive mechanism was iufficiently stimulated. As a result, fish tested under these conditio were somewhat limited in their ability to orient to plane-polarized light. It is a well established fact that both Pacific and Atlantic salmonids exhibit migratio during their life history. Anadromous salmonids hatch and forage in the freshwater environment until they smoltify, and then they go out to sea. These fish then spend a period of time in the open ocean feeding and growing, and finally return (time of return is species specific) to their natal streams. The distance traveled varies from species to species and stock to stock but it should be emphasized that the distances are vast, possibly thousands of miles. There have been a variety of theories postulated to explain homing in fish, including sun-compass orientation (Schwassmann and Hasler 1964), olfactory imprinting (Hasler et al. 1978), and celestial and magnetic cues (Quinn and rannon 1982). However, it remai to be demotrated how the actual seory systems of a fish receive, encode and process the relevant cues for spatial guidance. We believe our study conclusively links, for the first time, movements relative to a polarized light field to the ultraviolet-polarized light seitivity of the rainbow trout. Furthermore, when the UV-seitive cones disappear trout lose their orientation ability. We are uncertain whether trout exhibit this cone loss permanently or whether UV-seitive cones are reincorporated into the photoreceptor mosaic at a future point in their life history. The orientation of trout to a polarized light field could be important evidence supporting the hypothesis of that fish utilize a sun compass navigational system. This does not imply that the sun compass system is the only navigational system used by fish. Rather, fish are likely to use a multimodal navigational system for migration. Acknowledgements. We thank Ms. Anne olger and Ms. Aileen runett for technical assistance. Thanks to Dr. Jerry Waldvogel for critically reading the manuscript and Dr. John Phillips for helpful discussion. This research was supported by a Natural Sciences and Engineering Research Council of Canada Operating Grant (# U0535) and University Research Fellowship to C.W.H. References atschelet E (1981) Circular statistics in biology. Academic Press, London New York Toronto

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