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Foraging and prey-search behaviour of small juvenile rainbow trout (Oncorhynchus mykiss) under polarized light

Iñigo Novales Flamarique^1,2,3,* and Howard I. Browman^1,3

¹ Institut Maurice Lamontagne, Fisheries and Oceans Canada, CP 1000, 850 route de la Mer, Mont-Joli, Quebec, Canada, G5H 3Z4,
² Simon Fraser University, Department of Biological Sciences, 8888 University Drive, Burnaby, British Columbia, Canada V5A 1S6 and
³ Institute of Marine Research, Austevoll Aquaculture Research Station, N-5392 Storebø, Norway

View larger version (71K): [in a new window]   Fig. 1. Images of copepods taken under different spectral and polarized light illuminations. All images were taken using an Ektachrome-type colour slide film with nearly uniform sensitivity in the range 320–700nm (Kodak Color Films and Paper Manual, 1986). (A–D) The copepod Metridia pacifica (length=2.6mm) was photographed with a Zeiss Universal microscope equipped with Balzer filters with the following peak transmissions: (A) ultraviolet (max=368nm), (B) short-wavelength (max=432nm), (C) middle-wavelength (max=544nm) and (D) long-wavelength (max=646nm) light. The light source was a halogen lamp. All photographs share the same exposure (70s); this exposure was selected because it gave the highest contrasts under middle- and long-wavelength illumination (with the progressive disappearance of the copepod at lower or higher exposures). The copepod is most conspicuous under ultraviolet- and short-wavelength illumination; under these conditions, the animal remained highly visible from 1–90s of exposure. Scale bar, 0.5mm. (E–H) The copepods Calanus pacificus (larger animal, length=2.6mm) and Aetidius divergens (smaller animal, length=1.6mm) were photographed using the same microscope but equipped with (E) a polarizer but no analyzer, (F) a polarizer oriented perpendicular to the analyzer, (G) a polarizer oriented perpendicular to the analyzer but with a quarter-wave plate (Zeiss Quarz 1, total retardation 140nm) oriented at 45° to the polarizer and prior to the specimen, and (H) a polarizer oriented parallel to the analyzer with the quarter-wave plate in the same configuration. The light source here was a short-arc mercury vapor lamp (Osram). The animals are more conspicuous when an analyzer of polarized light is present in the light path. In this fashion, prey may become more conspicuous to a polarization-sensitive predator by diffusing (depolarizing) the incident polarization and, for animals that possess pre-retinal analyzers (e.g. the iridescent corneas of some fishes, perhaps) (see Lythgoe, 1975), by the changes in intensity and colour patterns that may arise from transduction of polarization in the animal’s visual system. Scale bar, 1mm.

View larger version (24K): [in a new window]   Fig. 2. Frequency distributions of (A) prey location distance, (B) the vertical component of prey location angle and (C) the lateral component of prey location angle for rainbow trout foraging under 100% linearly polarized or unpolarized (diffuse, 0%) light. Individual frequencies are percentage totals from three trials per light condition.

View larger version (33K): [in a new window]   Fig. 3. Frequency distributions of (A) prey location distance, (B) the vertical component of prey location and (C) the lateral component of prey location angle for rainbow trout foraging under short-wavelength light of varying percent polarization (97%, 85%, 71% or 55%). Individual frequencies are percentage totals from three trials per light condition.