Abstracts

Fishes inhabiting the mesopelagic zone (200 - 1000 m depth) often bioluminesce for camouflage purposes - counterillumination. While migrating to the surface at night to feed on zooplankton, ventral light emissions from bioluminescent organs - photophores - match the narrow blue-green light penetrate the ocean waters downward. Luminescence of lanternfishes (Myctophidae) is produced by an oxidative reaction of coelenterazine in bioluminescent cells - photocytes. While other fishes tune counterillumination light to the ambient by pigmented filters, the photophores of lanternfishes are structurally unique, holding a blue-green inner reflector.Collection of myctophid Diaphus watasei took place in fishing ports of Kochi and Mie Prefectures, Japan. Reflection spectra of fresh photophores was obtained by spectrometry. Further analyses included light microscopy, X-ray diffraction and Fourier-transform infrared spectroscopy, to study the photophore histological structure, luciferase purification, and iridophore's chemical nature and morphology.Inner reflector is composed by a monolayer of packed hexagon-shaped iridophores. This unique hexagonal interlock project all the light on a precise direction while achieving low energetic costs from photon losses by minimizing cell gaps. Shaped as a parabolic mirror with photocytes at its focus, this tissue reflects all the light at an accurate vertical angle. By achieving structural colour, the reflector also modulates the spectra of light produced by the photocytes to longer wavelengths. Comparing to photophores of other deep-sea fishes, the lanternfish reflector provides a significant ecological advantage, ensuring not only a precise angle match to the deep-sea light but also adapting the camouflage spectra during vertical migrations.