Variation in lanternfish (Myctophidae) photophore structure: A comprehensive comparative analysis.

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The deep-sea open ocean habitat (below 200 m depth) is comprised of little-to-no light, near freezing temperatures, and vastly connected stratified waters. Bioluminescence is often linked to the success and diversification of fishes in these dark deep-sea habitats, which are host to many species-rich and morphologically diverse clades. Fish bioluminescence takes many forms and is used in a variety of behaviors including counterillumination, prey detection and luring, communication, and predator avoidance. This study focuses on lanternfishes (Myctophidae), a diverse group (252 spp. in 34 genera) of deep-sea fishes in which bioluminescence has played a critical role in their diversification. Using histological techniques, we provide new morphological analyses of the complex structure of the primary photophores of representative species from 17 genera in which photophore morphology has not previously been described. We combine this information with data from prior studies to compare primary photophore characteristics for species representing all 34 lanternfish genera. Although we find that lanternfish primary photophores are similar in many of their structural components, including the possession of a modified scale cup, photocytes, pigment, and reflector layers, we observe significant variation among species in other aspects of photophore morphology. Observed morphological differences include variation in pigmentation and in the calcification and thickness of the modified scale cup. We also find reflectors that are very thin or absent in gymnoscopeline and lampanyctine species, relative to the robust reflectors present in myctophine species. We find evidence of secondary reflectors and secondary pigment layers in six lanternfish species and observe major differences in scale-lens thickness and mineralization across the assemblage. Lastly, Scopelopsis multipunctatus is the only species analyzed lacking a photophore cup. Obtaining finer detail of light organ morphology across this species-rich lineage provides much-needed insight into the factors that have contributed to the remarkable diversity of lanternfishes in the deep open ocean.