Mantis shrimps see circularly polarized light
Polarized vision is common in the animal kingdom; however, most organisms that are sensitive to polarized light detect linear polarization. Now a group of researchers has shown that a sea creature called a mantis shrimp can see and make use of circularly polarized light.
The mantis shrimp is not really a shrimp. It is a stomatopod that got its name from the fact that it resembles a combination of a praying mantis and shrimp. For scientists, the stomatopod’s eyes are particularly interesting because of their complexity. For example, they provide tunable, eight-channel color vision. Each eye is mounted on a stalk and moves independently, and evidence exists that the animal can process images from each eye separately.
Stomatopods have very advanced vision — possessing tunable eight-channel color vision, stereo vision in each eye and the ability to see linear polarization. Researchers have found that the creatures also possess the ability to see circularly polarized light. Courtesy of Justin Marshall.
The eyes consist of two main hemispheres separated by the midband, which contains six rows of visual receptor cells. The first four rows of the midband have pigments designed to respond to color vision. Rows five and six, however, have two pigments that capture UV and green light. According to Justin Marshall of the University of Queensland in Australia, 20 years ago Horace B. Barlow of the University of Cambridge in the UK hinted that the receptor cells in these rows had structures that looked as if they might be involved in converting circularly polarized light into the linear form. Marshall and his colleagues from the University of Maryland, Baltimore County, and from the University of California, Berkeley, recently published research on the stomatopods’ vision.
Key to animals’ eyes is a structure called the rhabdom, which is found within the midband rows. Each rhabdom has eight cells (R1 to R8) that contain thousands of microvilli housing light-sensitive rhodopsin molecules. In rows five and six of the midband, these eight cells have a unique arrangement compared with other similar groups of cells elsewhere in a stomatopod’s eyes. The microvilli in R1 through R7 are precisely aligned to create linearly polarized light detectors. R8 lies on top of cells one through seven and acts as a λ/4 plate arranged at 45° to the fast axis of the remaining seven. This orientation converts any incoming circularly polarized light into linearly polarized light. The cells in row six are rotated 90° with respect to row five. This means that the same group of cells in rows five and six of the midband are sensitive to the same left- or right-handed polarity.
The ability of stomatopods to see and respond to both left- and right-handed circularly polarized light might be useful in mating. When viewed through a left-hand circular polarizing filter, certain body parts, such as the telson keel — part of the tail seen here in the lower right — have notable contrast. Photograph by Chrissy Huffard.
According to the researchers, cells R1, R4 and R5 in both rows are sensitive to left-handed circular polarity but not to right-handed. Likewise, cells R2, R3, R6 and R7 are sensitive to right-handed but not to left-handed. As a result, the animal can see both left- and right-handed circular polarization.
”The rotation ensures that the same handedness of circularly polarized light gets handled by the same channel of interneurons to the brain in both rows,” Marshall said. “Ultimately, this is due to the angle of the λ/4 retarder to the underlying detectors and again shows the extraordinary lengths that this eye has gone to see different forms of light.”
The midband (MB) region of the stomatopod eye contains six rows of photoreceptors (above). Rows five and six are responsible for detecting and responding to circularly polarized light (right). The R8 cells in these photoreceptors act as λ/4 plates, converting circularly polarized light into linearly polarized light that is detected by cells R1 through R7. Reprinted with permission of Current Biology.
In addition to studying sectioned stomatopod eyes with a polarization microscope, the group also trained stomatopods to associate left- or right-handed circular polarized light with food. Then they presented the animals with two feeding tubes — one reflecting left-handed polarized light and the other, right-handed polarized light. At rates well above chance, the stomatopods chose the tubes reflecting light matching the polarization they had been trained to respond to, demonstrating that not only can they see circular polarity, but they also can make use of that visual information.
At present, Marshall and his colleagues are not sure what role the circularly polarized vision plays in stomatopods. Certain parts of the organism’s body reflect circularly polarized light, so it may be used for communicating or for mating. It also may be useful for navigation or for helping the animals see through cloudy water.
Besides examining circularly polarized vision in other animals, Marshall said that the researchers may consider the implications of the phenomenon outside of biology. “We hope it will open up a new field of vision research. It will also have relevance to machine vision and biomimetic research.”
Current Biology, March 25, 2008, pp. 429-434.
- linear polarization
- See plane-polarized light; polarization.
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