The filter in photophores of the deep‐sea fish <i>Neoscopelus</i> (Neoscopelidae: Myctophiformes) and its role in counterillumination spectra

Neoscopelus is a genus of deep-sea fishes with ventral-lateral photophores, likely used for counterillumination. In this study, we report a novel functional structure of spectral filter in the bioluminescence mechanism of Neoscopelus microchir. Photocytes are innervated and located inside a new type of photophore filter composed of web-like branched chambers filled with red-pigmented cells. The branches extend beyond the photophore along the epidermis. The blue light produced in the photocytes is red-shifted to blue-green by the filter effect, matching the light emitted by the photophore to the deep-sea downwelling light spectrum. The variation in pigmentation density, ranging from dark red to pale yellow, is a result of the filter thickness and the number of pigment layers within it. These factors influence light transmittance across the photophores, probably for adaptation to various light environments during vertical migration. We also compared the photophore structures of two Neoscopelus species. N. microchir exhibits a greater area occupied by photocytes with a thinner layer of pigment filter in comparison to Neoscopelus porosus. These structural distinctions may elucidate the species-specific adaptations of counterillumination to the differing light conditions at the depths where each Neoscopelus species resides.