Colour and polarization visual pathways of the northern anchovy: from neuroethology to machine sensor technology

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In addition to wavelength (colour) and intensity (number of photons), many invertebrates can detect the polarization of light and use it for a variety of purposes including orientation, prey detection, predator avoidance, and communication. The polarization of light refers to the predominant plane of electric field oscillations of the photons that comprise it. If all the photons have electric fields that oscillate in the same plane, light is termed 100% linearly polarized in that plane (the E-vector plane). In vertebrates, polarization vision, including a cellular mechanism explaining polarization detection, has only been established for some species of anchovies. In these animals, a set of retinal cone photoreceptors are axially dichroic, i.e., they are sensitive to the polarization (E-vector orientation) of incident light that enters the eye. This is due to the fact that, unlike the cones in other vertebrates examined so far, these axially dichroic cones have lamellae oriented parallel to the length of the cell. Two such cones (short and long) have lamellae that are perpendicular to each other forming the inputs to two retinal channels with orthogonal polarization sensitivity. ***I have recently shown that the northern anchovy, Engraulis mordax, has a ventral retina that is compartmentalized for colour and polarization sensitivity, the first discovery of its kind among vertebrates. The ventro-temporal retina has axially dichroic cones that mediate polarization but not colour sensitivity at the level of the optic nerve, whereas the ventro-nasal retina has triple cones (with lamellae transverse to the photoreceptor's length, as in other vertebrates) and is not polarization-sensitive but colour-sensitive. Such functional segregation of visual modalities is only known in insects where a specialized area of the compound eye processes polarization information whereas the rest of the eye is devoted to colour vision. The properties of cells that make up the colour and polarization visual pathways of anchovies are unknown. Using a combination of molecular (qPCR, in-situ hybridization), electrophysiological (intracellular/patch clamp), and mathematical modeling techniques, this research will attempt to unveil the properties of cells that underlie the colour and polarization visual pathways of the northern anchovy. To understand the animal's use of polarization vision in nature, behavioural studies will be carried out to test the hypothesis that polarization sensitivity enhances target contrast perception of zooplankton by the anchovy, thereby improving its foraging performance. Lastly, this research will optimize novel polarization sensors inspired by optical structures found in the eyes of anchovies. These sensors are far superior to those presently found in the market place and could revolutionize the industry through mass applications in devices like cameras and cell phones.