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Polarized light as an alternative to colour in animal vision

Periodic Reporting for period 1 - PLACAV (Polarized light as an alternative to colour in animal vision)

Reporting period: 2015-03-01 to 2017-02-28

Most animals see the world very differently from us. For example, some are sensitive to colours beyond our own visible spectrum (e.g. bees use ultraviolet colours when viewing flowers). Other animals use a different property of light altogether, its polarization. The polarization of light refers to the angle of the electromagnetic waves of light as they travel through space. By aligning photopigments in the eye, animals such as crabs and some insects can discriminate polarization, and use it for a variety of tasks. Until now it has been unclear how and why some species see in polarization across their whole visual field. This project aimed to address this, focussing on the fiddler crab as a model organism. These animals live in tropical mudflats across the world, and use their compound eyes to detect conspecifics and threats. Understanding how and why these animals see in polarization can inspire and inform future directions in biomimetic technology, such as the development of polarization-sensitive camera chips and the use of polarization-based algorithms to enhance contrast in digital images.
The project had two main objectives. First, to determine how polarization information is incorporated into the contrast vision system of fiddler crabs; and second, to investigate the use of polarization cues to fiddler crabs in their natural environment.
The project was originally designed to run over a 24 month period. However, due to being awarded a prestigious Royal Society University Research Fellowship, I was forced to terminate the project early. As such the research was conducted over a 10 month period from 1st March to 31st Dec 2015. It is important to note that the decision of the Royal Society to award me this fellowship was almost certainly influenced in a positive way by my having already received the Marie Curie fellowship.
During the 10 month period funded by the Marie Curie fellowship, the second objective of the research was achieved as follows: Objective 2; to elucidate the function of the polarization vision of the fiddler crab Uca stenodactulus.
To determine whether fiddler crabs use their polarization vision to enhance the detection of neighbours in their mudflat habitat, I developed a novel system for manipulating their visual environment in situ. Work was conducted during a 3 month fieldtrip to the Republic of Panama, where I collaborated with staff scientist Dr John Christy at the Smithsonian Tropical Research Institute. Here I identified a suitable colony of fiddler crabs, located on the shore of the Pacific entrance to the Panama canal. Resident male fiddler crabs that were defending a home territory around a burrow were approached with a polarized dummy crab that was dragged along the mudflat surface via a pulley system. The response of the male to the approaching dummy was filmed using an overhead video camera. The distance of the dummy from the male crab was recorded to monitor various behavioural responses. This would allow me to identify whether the polarization characteristics of the dummy influenced the ability of the male to detect it. Two iterations of the experiment were conducted. First, polaroid filter was used to construct the stimulus. This proved unsuccessful due to the polarization being masked by silhouetting from the sun. The experiment was then repeated using transparent retarder filter, which alters the polarization but not the brightness of transmitted light. This experiment was successful, and I discovered that an increase in the polarization contrast of the dummy resulted its detection by resident males over a greater distance. The study concluded that this species of fiddler crab does indeed use polarization vision to detect neighbours on the mudflat surface.
These finding were found to be of sufficient quality and broad relevance to merit publication in a high-impact journal. I successfully published the study in the Journal of Current Biology:
How, Christy, Temple, Hemmi, Marshall and Roberts, 2015, Target detection is enhanced by polarization vision in a fiddler crab. Current Biology 25: 3069-3073. doi: 10.1016/j.cub.2015.09.073.
Additional data was collected during this fieldtrip, which has contributed to ongoing research into the polarization ecology of crustaceans. In particular, I started to gather photographic polarization images that quantify the distribution of polarized light in mudflat habitats. These, along with more recent examples, will be collated into a new study to describe the polarization patterns reflected from damp flat surfaces.
Attached to this form are two images: 1 - the fiddler crab Uca stenodactylus; 2 - the polarization pattern reflected from a mudflat panorama.
The findings of this study have opened up a wide range of research questions. In particular, I’ve been able to co-supervise a PhD student, Sam Smithers, on this topic. Over the last few years I have worked with him to gather a wealth of data around objective 1 of my project. Using modified LCD screens we have been able to present fiddler crabs with visual stimuli varying independently in brightness and polarization, allowing us to investigate how these two modalities are combined in the contrast vision system. Last year, Sam modified my field experimental apparatus to investigate the night-time polarization vision of ghost crabs in Hawaii. His data are currently being prepared for publication.
Findings have also been used to develop public engagement tools. In March 2017 I hosted a stand at the Science Museum Lates night, in which we presented a range of polarization-based interactive activities for the public to see.
I have also been successful in obtaining further funding to continue this work, including a Royal Society PhD studentship. I am also currently preparing a BBSRC New Investigator award on the topic.
My main aim for the future is to engage with industry partners, such as the BBC Natural History Unit, Roke Manor Research, and the Air Force Research Lab, to develop polarization imaging devices inspired by the visual system of fiddler crabs.
The fiddler crab Uca stenodactylus
A polarization panorama of the mudflats