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.
The lighting of the underwater environment is constantly changing due to attenuation by water, scattering by
suspended particles, as well as the refraction and reflection caused by the surface waves. These factors pose a great
challenge for marine animals which communicate through visual signals, especially those based on color. To escape this
problem, certain cephalopod mollusks and stomatopod crustaceans utilize the polarization properties of light. While the
mechanisms behind the polarization vision of these two animal groups are similar, several distinctive types of polarizers
(i.e. the structure producing the signal) have been found in these animals. To gain a better knowledge of how these
polarizers function, we studied the relationships between fine structures and optical properties of four types of polarizers
found in cephalopods and stomatopods. Although all the polarizers share a somewhat similar spectral range, around 450-
550 nm, the reflectance properties of the signals and the mechanisms used to produce them have dramatic differences. In
cephalopods, stack-plates polarizers produce the polarization patterns found on the arms and around their eyes. In
stomatopods, we have found one type of beam-splitting polarizer based on photonic structures and two absorptive
polarizer types based on dichroic molecules. These stomatopod polarizers may be found on various appendages, and on
the cuticle covering dorsal or lateral sides of the animal. Since the efficiencies of all these polarizer types are somewhat
sensitive to the change of illumination and viewing angle, how these animals compensate with different behaviors or fine
structural features of the polarizer also varies.
Body parts that can reflect highly polarized light have been found in several species of stomatopod crustaceans (mantis shrimps). These polarized light reflectors can be grossly divided into two major types. The first type, usually red or pink in color to the human visual system, is located within an animal's cuticle. Reflectors of the second type, showing iridescent blue, are located beneath the exoskeleton and thus are unaffected by the molt cycle. We used reflection spectropolarimetry and transmission electron microscopy (TEM) to study the reflective properties and the structures that reflect highly polarized light in stomatopods. For the first type of reflector, the degree of polarization usually changes dramatically, from less than 20% to over 70%, with a change in viewing angle. TEM examination indicates that the polarization reflection is generated by multilayer thin-film interference. The second type of reflector, the blue colored ones, reflects highly polarized light to all viewing angles. However, these reflectors show a slight chromatic change with different viewing angles. TEM sections have revealed that streams of oval-shaped vesicles might be responsible for the production of the polarized light reflection. In all the reflectors we have examined so far, the reflected light is always maximally polarized at around 500 nm, which is close to the wavelength best transmitted by sea water. This suggests that the polarized light reflectors found in stomatopods are well adapted to the underwater environment. We also found that most reflectors produce polarized light with a horizontal e-vector. How these polarized light reflectors are used in stomatopod signaling remains unknown.
Although natural light sources produce depolarized light, patterns of partially linearly polarized light appear in the sky due to scattering from air molecules, dust, and aerosols. Many animals, including bees and ants, orient themselves to patterns of polarization that are present in daytime skies, when the intensity is high and skylight polarization is strong and predictable. The halicitid bee Megalopta genalis inhabits rainforests in Central America. Unlike typical bees, it forages before sunrise and after sunset, when light intensities under the forest canopy are very low, and must find its way to food sources and return to its nest in visually challenging circumstances. An important cue for the orientation could be patterns of polarization in the twilight sky. Therefore, we used a calibrated digital camera to image skylight polarization in an overhead patch of sky, 87.6° across, before dawn on Barro Colorado Island in Panama, where the bees are found. We simultaneously measured the spectral properties of polarized light in a cloudless patch of sky 15° across centered on the zenith. We also performed full-sky imaging of polarization before dawn and after dusk on Lizard Island in Australia, another tropical island. During twilight, celestial polarized light occurs in a wide band stretching perpendicular to the location of the hidden sun and reaching typical degrees of polarization near 80% at wavelengths >600 nm. This pattern appears about 45 minutes before local sunrise or disappears 45 minutes after local sunset (about 20 minutes after the onset of astronomical twilight at dawn, or before its end at dusk) and extends with little change through the entire twilight period. Such a strong and reliable orientation cue could be used for flight orientation by any animal with polarization sensitivity that navigates during twilight.
Proc. SPIE. 5329, Genetically Engineered and Optical Probes for Biomedical Applications II
KEYWORDS: Proteins, Biomedical optics, Green fluorescent protein, Tissues, Luminescence, Organisms, Ocean optics, In vivo imaging, Fluorescence correlation spectroscopy, Fluorescence resonance energy transfer
In recent years, a variety of Green Fluorescent Protein (GFP)-like pigments have been discovered from corals and other marine organisms. They are widely used to expand the range of available GFP-type proteins in imaging applications, such as in vivo markers for gene expression and protein localization studies, FRET-based (Förster resonance energy transfer) multicolor imaging and biosensors. They have known diverse optical and biochemical properties but their in vivo spectral properties and biological function in marine organisms is only beginning to be understood. We have investigated their spectral diversity, optical properties and cellular microstructure in corals of the Great Barrier Reef with the aim of elucidating their photo-biological function/s as well as to identify novel proteins suitable for GFP-based technologies. We found numerous spectral variants, with emissions covering almost the full range of the visible spectrum. Many of these GFP-like proteins, especially in corals from the more extreme habitats, such as sun-exposed shallows or in deep water, showed a range of light-related spectral characteristics: high photostability, spectral tuning for energy transfer and dynamic photo-induced transformation properties. Intra-cellularly they were organized into spectral donor-acceptor pairs or even arrays, tuned for FRET. Coral color proteins thus offer an exciting potential to expand the use of the available GFPs in bio-imaging applications and as a basis for improved protein engineering.
Although natural light sources produce depolarized light, partially linearly polarized light is naturally abundant in the scenes animal view, being produced by scattering air or water or by reflection from shiny surfaces. Many species of animals are sensitive to light's polarization, and use this sensitivity to orient themselves using polarization patterns in the atmosphere or underwater. A few animal species have been shown to take this polarization sensitivity to another level of sophistication, seeing the world as a polarization image, analogous to the color images humans and other animals view. This sensory capacity has been incorporated into biological signals by a smaller assortment of species, who use patterns of polarization on their bodies to communicate with conspecific animals. In other words, they use polarization patterns for tasks similar to those for which other animals use biologically produced color patterns. Polarization signals are particularly useful in marine environments, where the spectrum of incident light is variable and unpredictable. Here, cephalopod mollusks (octopuses, squids, and cuttlefish) and stomatopod crustaceans (mantis shrimps) have developed striking patterns of polarization used in communication.
Many animals, both marine and terrestrial, are sensitive to the orientation of the e-vector of partially linearly polarized light (PLPL). This sensitivity is used for navigation, spatial orientation, and detection of large bodies of water. However, it is not clear what other information animals may receive from polarized light. Natural light fields, both in the sky and underwater, are known to be partially polarized. Additionally, natural objects reflect light that is polarized at specific orientations. Sensors capable of measuring the characteristics of PLPL, namely partial polarization and orientation, throughout an image are not yet available. By placing 2 twisted nematic liquid crystals (TNLCs) and a fixed polarizing filter in series in front of a video camera, and by controlling the angles of rotation of the orientation of polarization produced by the TNLCs, we are able to fully analyze PLPL throughout a full image on a single pixel basis. As a recording device we use a small camcorder. The sensor can be operated autonomously, with the images analyzed at a later stage, or it can be connected (in a future phase) via a frame grabber to a personal computer which analyzes the information online. The analyzed image can be presented as a false color image, where hue represents orientation of polarization and saturation represents partial polarization. Field measurements confirm that PLPL is a characteristic distributed both under water and on land. Marine background light is strongly horizontally polarized. Light reflected from leaves is polarized mainly according to their spatial orientation. Differences between PLPL reflected from objects or animals and their background can be used to enhance contrast and break color camouflage. Our sensor presents a new approach for answering questions related to the ecology of vision and is a new tool for remote sensing.