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, 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.