Reflector of the body photophore in lanternfish is mechanistically tuned to project the biochemical emission in photocytes for counterillumination

The filter in photophores of the deep-sea fish Neoscopelus (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.

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

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.

Localization of multiple opsins in ocular and non-ocular tissues of deep-sea shrimps and the first evidence of co-localization in a rhabdomeric R8 cell (Caridea: Oplophoroidea)

Bioluminescence is a prevalent phenomenon throughout the marine realm and is often the dominant source of light in mesophotic and aphotic depth horizons. Shrimp belonging to the superfamily Oplophoroidea are mesopelagic, perform diel vertical migration, and secrete a bright burst of bioluminescent mucous when threatened. Species in the family Oplophoridae also possess cuticular light-emitting photophores presumably for camouflage via counter-illumination. Many species within the superfamily express a single visual pigment in the retina, consistent with most other large-bodied mesopelagic crustaceans studied to date. Photophore-bearing species have an expanded visual opsin repertoire and dual-sensitivity visual systems, as evidenced by transcriptomes and electroretinograms. In this study, we used immunohistochemistry to describe opsin protein localization in the retinas of four species of Oplophoroidea and non-ocular tissues of Janicella spinicauda. Our results show that Acanthephyra purpurea (Acanthephyridae) retinas possess LWS-only photoreceptors, consistent with the singular peak sensitivity previously reported. Oplophoridae retinas contain two opsin clades (LWS and MWS) consistent with dual-sensitivity. Oplophorus gracilirostris and Systellaspis debilis have LWS in the proximal rhabdom (R1-7 cells) and MWS2 localized in the distal rhabdom (R8 cell). Surprisingly, Janicella spinicauda has LWS in the proximal rhabdom (R1-7) and co-localized MWS1 and MWS2 opsin paralogs in the distal rhabdom, providing the first evidence of co-localization of opsins in a crustacean rhabdomeric R8 cell. Furthermore, opsins were found in multiple non-ocular tissues of J. spinicauda, including nerve, tendon, and photophore. These combined data demonstrate evolutionary novelty and opsin duplication within Oplophoridae, with implications for visual ecology, evolution in mesophotic environments, and a mechanistic understanding of adaptive counter-illumination using photophore bioluminescence.

Tracing the evolution of bioluminescent light organs across the deep-sea shrimp family Sergestidae using a genomic skimming and phylogenetic approach

Deep-sea shrimp of the family Sergestidae Dana, 1852 provide a unique system for studying the evolution of bioluminescence. Most species within the family possess autogenic bioluminescent photophores in one of three distinct forms: lensed photophores; non-lensed photophores; or internal organs of Pesta. This morphological diversity across the Sergestidae has resulted in recent major taxonomic revisions, dividing the two major genera (Sergia Stimpson, 1860 and Sergestes Milne Edwards, 1830) into 15. The present study capitalises on molecular data to construct an updated genus-level phylogeny of sergestid shrimp. DNA was successfully extracted from ~87 individuals belonging to 13 of the 15 newly proposed genera. A ‘genome skimming’ approach was implemented, allowing the capture of mitochondrial genomic data across 19 sergestid species. Additional individuals have been incorporated into the phylogeny through Sanger sequencing of both nuclear (H3 and NAK) and mitochondrial (16S and COI) genes. The resulting molecular phylogeny is compared with previous morphological trees with specific attention to genus-level relationships. The -sergestes group was rendered non-monophyletic and the -sergia group was recovered as monophyletic. Ancestral state reconstructions of light organ type indicate that organs of Pesta is the ancestral state for the family. Non-lensed photophores evolved once across the -sergia group, but were later lost in the deepest living genus, Sergia. Lensed photophores also evolved once within the genera Prehensilosergia Vereshchaka, Olesen & Lunina, 2014, Lucensosergia Vereshchaka, Olesen & Lunina, 2014 and Challengerosergia Vereshchaka, Olesen & Lunina, 2014. These findings identify preliminary patterns across light organ type and species’ depth distributions; however, future research that incorporates finer-scale depth data and more species is needed to confirm our findings.

Etmopterus lantern sharks use coelenterazine as the substrate for their luciferin-luciferase bioluminescence system

The lantern shark genus Etmopterus contains approximately 40 species of deep-sea bioluminescent cartilaginous fishes. They emit blue light mainly from the ventral body surface. The biological functions of this bioluminescence have been discussed based on the luminescence patterns, but the bioluminescence mechanism remains uncertain. In this study, we detected both coelenterazine and coelenterazine-dependent luciferase activity in the ventral photophore tissue of Etmopterus molleri. The results suggested that bioluminescence in lantern sharks is produced using coelenterazine as the substrate for the luciferin-luciferase reaction, as some luminous bony fishes.

Multi-level convergence of complex traits and the evolution of bioluminescence

Evolutionary convergence provides natural opportunities to investigate how, when, and why novel traits evolve. Many convergent traits are complex, highlighting the importance of explicitly considering convergence at different levels of biological organization, or 'multi-level convergent evolution'. To investigate multi-level convergent evolution, we propose a holistic and hierarchical framework that emphasizes breaking down traits into several functional modules. We begin by identifying long-standing questions on the origins of complexity and the diverse evolutionary processes underlying phenotypic convergence to discuss how they can be addressed by examining convergent systems. We argue that bioluminescence, a complex trait that evolved dozens of times through either novel mechanisms or conserved toolkits, is particularly well suited for these studies. We present an updated estimate of at least 94 independent origins of bioluminescence across the tree of life, which we calculated by reviewing and summarizing all estimates of independent origins. Then, we use our framework to review the biology, chemistry, and evolution of bioluminescence, and for each biological level identify questions that arise from our systematic review. We focus on luminous organisms that use the shared luciferin substrates coelenterazine or vargulin to produce light because these organisms convergently evolved bioluminescent proteins that use the same luciferins to produce bioluminescence. Evolutionary convergence does not necessarily extend across biological levels, as exemplified by cases of conservation and disparity in biological functions, organs, cells, and molecules associated with bioluminescence systems. Investigating differences across bioluminescent organisms will address fundamental questions on predictability and contingency in convergent evolution. Lastly, we highlight unexplored areas of bioluminescence research and advances in sequencing and chemical techniques useful for developing bioluminescence as a model system for studying multi-level convergent evolution.