7 2 Magnetoreception Thermoreception As their name suggests electromagnetic waves such as light consist of an electric field and a magnetic field The electric field is made of charged particles that travel in transverse waves which oscillate with a particular frequency This frequency of oscillation corresponds to a particular color due to the frequency specificity of the opsin proteins These waves can oscillate in many different directions or planes Normal sunlight is called unpolarized light because the waves oscillate in all different planes However when waves are reflected off of objects or refracted by passing from one medium to another many of the reflected or refracted waves can line up and oscillate in the same plane This reflected or refracted light is now polarized because it has a particular direction of oscillation If light is extremely polarized and many of the waves are oscillating in the same direction the animal may see a glare such as light being reflected off of a pond or refracted through a window at a particular angle Polarized sunglasses are filters that block light oscillating horizontally as it does when it is reflected off of objects If you turn polarized sunglasses 90 they are no longer effective because they will selectively filter out light oscillating vertically their tint will still reduce the light s intensity but it will affect all waves equally see animation The degree to which the light will be polarized depends on the angle at which the light hits the surface of the object or medium Thus the amount of polarized light around us is a proxy for the position of the sun If one were to simply follow the sun one would follow a moving target as the sun appears to move from east to west throughout the course of a day By quantifying the amount of light polarization and thus the relative angle of the sun one could possess a time compensated sun compass Both polarized and unpolarized light activate ciliary and rhabdomeric photoreceptors but some animals are able to distinguish between the two allowing them to perceive relative quantities of polarization The photopigments in rod type ciliary photoreceptors are arranged semi randomly in several planes so most vertebrate and some invertebrate eyes cannot distinguish polarized from unpolarized light However recent research suggests that there are several adaptations in cones that could make vertebrates polarization sensitive as several species of fish amphibians and birds are In rhabdomeric photoreceptors the photopigments are neatly arranged in the microvilli at consistent angles to each other creating a system that can analyze different planes of polarized light Instead of being frequency specific these photoreceptors are direction specific different cells respond preferentially to different directions of oscillation Like other photoreceptors the intensity of light is encoded by the graded potential of the photoreceptor Polarization vision is common in arthropods and cephalopods For example many insects have a specialized set of ommatidia for detecting polarized light The schematic below shows a dorsal from above the animal view of a compound eye with the polarization sensitive ommatidia spread throughout the middle The orientation of each line indicates the preferred direction of oscillation that is detected by the opsin in that ommatidia s photoreceptor Some ants are able to integrate this celestial compass with a step counter to determine how far they have traveled from their nest and in what direction to expedite the trip home Animals can use relative amounts of polarization as a time compensated sun compass allowing them to adjust their behavior to the circadian time of day and for navigation and orientation Cephalopods are able to rapidly and dramatically change their color iridescence and patterns allowing them to camouflage and signal to other animals A few examples of squid skin are shown below This is even more remarkable considering that cephalopods have rhabdomeric photoreceptors strongly suggesting that they are color blind Recent research suggests that in addition to color and iridescence some cephalopods can control the amount that their bodies reflect light potentially using polarized light as a means of communication to other animals that have polarization vision A false color image of a cuttlefish demonstrating the relative amounts of polarized light that reflect off of its body is also shown below M thger et al 2009 As the Earth orbits around the sun the polarization pattern of reflected sunlight also changes over the course of a year Thus some animals that exhibit circannual behavior such as migration can also use changes in polarized light Alternatively some migratory animals use the magnetic field around the Earth to orient themselves and navigate their annual routes Several migratory species of bird turtle butterfly and fish are magnetoreceptive able to detect and respond to magnetic fields Magnetic fields like electric fields contain transverse waves that travel in a particular direction and oscillate in a particular plane As you can see in the figure below both of these directions can be used by the animal to orient and navigate Magnetoreception may be the most elusive sense we know of There seem to be several mechanisms by which an animal could detect magnetic fields 1 Induction Moving a coil across a magnetic field induces current flow in the coil This mechanism is limited to fast swimming marine organisms which can exploit the conductivity of water such as sharks and rays 2 Compass needles Chains of magnetite grains oriented head to tail NS NS NS NS NS NS serve as a single sensitive unit Magnetite is a naturally occurring magnetic mineral that responds to magnetic fields Evidence for this mechanism has been found in insects honeybees rainbow trout and many birds located in the upper beak 3 Paramagnetic molecules or crystals Paramagnetic substances have a magnetic moment of their own that aligns itself with the external field They are activated only by an interaction with short wavelength e g UV light Evidence for this mechanism has been found in birds and insects Drosophila butterflies The signal transduction mechanisms linking each of these to changes in neuronal conductivity are largely unknown and remain an active field of research as animals are able to use this information often integrated with visual landmark cues time elapsed changes in polarized light and others to travel
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