Meeting Abstract
Under the directional light field of the mesopelagic and the bioluminescent searchlights of potential predators, transparent animals may be revealed by reflections from their body surface. However, thin films of intermediate refractive index may reduce such reflections. We investigated the cuticle surfaces of seven species of hyperiids (Crustacea; Amphipoda) using scanning electron microscopy and found that their cuticles were covered with a monolayer of putative spherical bacteria (diameters ranging from 52 ± 7 nm S.D. on Cystisoma spp. to 320 ± 15 nm S.D. on Phronima spp.). Optical modeling demonstrated that these layers can reduce reflectance by over 100-fold, depending on wavelength, angle of the incident light, and thickness of the gradient layer. Additionally, we found that the refractive index of bacteria (1.44) is close to optimal to serve as a single coating anti-reflection scheme for a crustacean’s chitinous cuticle in seawater. Although we only consider surface reflectance and not internal light scattering, the calculated Weber contrasts of a hyperiid cuticle with and without the spheres show that they decrease the reflectance of downwelling light enough to reduce the visibility of the cuticle. Preliminary sequencing identifies these spheres as Prochlorococcus marinus and a nanobacteria in the genus Mycoplasma. Bacteria and zooplankton in the open ocean have often been viewed as belonging to different, indirectly-linked food webs, but our study shows a direct association between nanobacteria and the cuticles of transparent planktonic crustaceans that may function to enhance crustacean camouflage.
