Nanostructures and Monolayers of Spheres Reduce Surface Reflections in Hyperiid Amphipods

Transparent zooplankton and nekton are often nearly invisible when viewed under ambient light in the pelagic zone [1–3]. However, in this environment, where the light field is directional (and thus likely to cause reflections), and under the bioluminescent searchlights of potential predators, animals may be revealed by reflections from their body surface [4–7]. We investigated the cuticle surfaces of seven species of hyperiids (Crustacea; Amphipoda) using scanning electron microscopy and found two undocumented features that may reduce reflectance. We found that the legs of Cystisoma spp. (n = 5) are covered with an ordered array of nanoprotuberances 200 ± 20 nm SD in height that function optically as a gradient refractive index material [6, 8, 9]. Additionally, we observed that Cystisoma and six other species of hyperiids are covered with a monolayer of homogenous spheres (diameters ranging from 52 ± 7 nm SD on Cystisoma spp. to 320 ± 15 nm SD on Phronima spp.). Optical modeling using effective medium theory and transfer matrix methods demonstrated that both the nanoprotuberances and the monolayers reduce reflectance by as much as 100-fold, depending on the wavelength and angle of the incident light and the thickness of the gradient layer. Even though we only consider surface reflectance and not internal light scattering, our study demonstrates that these nanoprotuberances and spheres can improve crypsis in a featureless habitat where the smallest reflection can render an animal vulnerable to visual predation. •The appendages of the hyperiid Cystisoma have arrays of cuticular nanoprotuberances•The cuticles of seven hyperiid species have monolayers of subwavelength-sized spheres•Both features reduce surface reflections from these transparent oceanic species Bagge et al. use microscopy and optical modeling to show that hyperiid amphipods (oceanic crustaceans that are mostly transparent) have two previously undocumented features—cuticular nanoprotuberances and monolayers of subwavelength-sized spheres—that reduce their surface reflections, making their cuticle less visible to potential predators.