Polarization signals in mantis shrimps Thomas W. Cronin *a , Tsyr-Huei Chiou a,b , Roy L. Caldwell c , Nicholas Roberts d , Justin Marshall b a Dept. of Biological Sciences, UMBC, Baltimore, MD, USA 21250; b QBI, Univ. of Queensland, Brisbane, Australia 4072; c Dept. of Integrative Biology, Univ. of California, Berkeley, CA 94720; d The Photon Science Institute, Univ. of Manchester, Manchester, UK M13 9PL ABSTRACT While color signals are well known as a form of animal communication, a number of animals communicate using signals based on patterns of polarized light reflected from specialized body parts or structures. Mantis shrimps, a group of marine crustaceans, have evolved a great diversity of such signals, several of which are based on photonic structures. These include resonant scattering devices, structures based on layered dichroic molecules, and structures that use birefringent layers to produce circular polarization. Such biological polarizers operate in different spectral regions ranging from the near-UV to medium wavelengths of visible light. In addition to the structures that are specialized for signal production, the eyes of many species of mantis shrimp are adapted to detect linearly polarized light in the ultraviolet and in the green, using specialized sets of photoreceptors with oriented, dichroic visual pigments. Finally, a few mantis shrimp species produce biophotonic retarders within their photoreceptors that permit the detection of circularly polarized light and are thus the only animals known to sense this form of polarization. Mantis shrimps use polarized light in species-specific signals related to mating and territorial defense, and their means of manipulating light’s polarization can inspire designs for artificial polarizers and achromatic retarders. Keywords: biophotonic, biomimetic, polarizer, retarder, signal, mantis shrimp, stomatopod, dichroic, birefringent 1. INTRODUCTION Almost all animals with well developed visual systems recognize and use visually-based signals in their communication systems. Such signals incorporate prominent pattern elements, most often based on intensity or color contrast. However, a few species of animals produce signals based on patterns of polarized light 1,2 . Such signals may be preferred in specific lighting or viewing conditions, when color-based signals would be unreliable, and are used for species identification and for aggressive or sexual communication. The circumstances that favor the use of polarization signals, the visual designs that are necessary to interpret them, and the very unusual biological structures that actually produce the polarized-light patterns are inherently interesting, but these aspects of polarization signaling are also of special interest to optical engineers because they suggest approaches to the use of polarization in communication and particularly because some of the biological designs are quite unlike those used in industry to produce and control light’s polarization. In this paper, we will briefly review the contexts within which animals have evolved polarization-based visual signals and touch on the visual detectors they use. We will then discuss in detail a few of the biological polarizers and optical devices used in the production and control of polarized light. 1.1 Photic environments that favor the use of visual signals based on polarized light Even though there are no natural light sources that produce significant amounts of polarized light visible at the earth’s surface, linearly polarized light is abundant in natural scenes 3 . In terrestrial environments, the frequently complex appearance of polarization due to atmospheric scattering and the reflection from shiny surfaces of leaves and water limits the utility of polarization signaling. To date, the only terrestrial animals known to use polarization patterns for communication are a group of tropical butterflies 4 . These butterflies produce polarization reflections using oriented structures in their wing scales, and closely related species either possess or lack such reflections in correlation with their habitats. Butterflies that use polarization signals typically fly under the forest canopy, where polarization noise from local scattering or reflection is nearly absent, so the polarization signal is unique and prominent. Polarization Science and Remote Sensing IV, edited by Joseph A. Shaw, J. Scott Tyo, Proc. of SPIE Vol. 7461, 74610C · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.828492 Proc. of SPIE Vol. 7461 74610C-1