Long-Wavelength Reflecting Filters Found in the Larval Retinas of One Mantis Shrimp Family (Nannosquillidae).

Abstract

Both vertebrates and invertebrates commonly exploit photonic structures adjacent to their photoreceptors for visual benefits. For example, use of a reflecting structure (tapetum) behind the retina increases photon capture, enhancing vision in dim light [1-5]. Colored filters positioned lateral or distal to a photoreceptive unit may also be used to tune spectral sensitivity by selective transmission of wavelengths not absorbed or scattered bythe filters [6-8]. Here we describe a new category of biological optical filter that acts simultaneously as both a transmissive spectral filter and narrowband reflector. Discovered in the larval eyes of only one family of mantis shrimp (stomatopod) crustaceans (Nannosquillidae), each crystalline structure bisects the photoreceptive rhabdom into two tiers and contains an ordered array of membrane-bound vesicles with sub-wavelength diameters of 153± 5nm. Axial illumination of the intrarhabdomal structural reflector (ISR) invivo produces a narrow band of yellow reflectance (mean peak reflectivity, 572± 18nm). The ISR is similar to several synthetic devices, such as bandgap filters, laser mirrors, and (in particular) fiber Bragg gratings used in optical sensors for awide range of industries. To our knowledge, thestomatopod larval ISR is the first example of a naturally occurring analog to these human-made devices. Considering what is known about these animals' visual ecology, we propose that these reflecting filters may help improve the detection of pelagic bioluminescence in shallow water at night. VIDEO ABSTRACT.