Etmopterus spinax, the velvet belly lanternshark, does not use bacterial luminescence

The language of light: a review of bioluminescence in deep-sea decapod shrimps

In the dark, expansive habitat of the deep sea, the production of light through bioluminescence is commonly used among a wide range of taxa. In decapod crustaceans, bioluminescence is only known in shrimps (Dendrobranchiata and Caridea) and may occur in different modes, including luminous secretions that are used to deter predators and/or from specialised light organs called photophores that function by providing camouflage against downwelling light. Photophores exhibit an extensive amount of morphological variation across decapod families: they may be internal (of hepatic origin) or embedded in surface tissues (dermal), and may possess an external lens, suggesting independent origins and multiple functions. Within Dendrobranchiata, we report bioluminescence in Sergestidae, Aristeidae, and Solenoceridae, and speculate that it may also be found in Acetidae, Luciferidae, Sicyonellidae, Benthesicymidae, and Penaeidae. Within Caridea, we report bioluminescence in Acanthephyridae, Oplophoridae, Pandalidae, and new observations for Pasiphaeidae. This comprehensive review includes historic taxonomic literature and recent studies investigating bioluminescence in all midwater and deep benthic shrimp families. Overall, we report known or suspected bioluminescence in 157 species across 12 families of decapod shrimps, increasing previous records of bioluminescent species by 65%. Mounting evidence from personal observations and the literature allow us to speculate the presence of light organs in several families thought to lack bioluminescence, making this phenomenon much more common than previously reported. We provide a detailed discussion of light organ morphology and function within each group and indicate future directions that will contribute to a better understanding of how deep-sea decapods use the language of light.

Successful delivery of viviparous lantern shark from an artificial uterus and the self-production of lantern shark luciferin

Our recent success in the long-term maintenance of lantern shark embryos in artificial uterine systems has provided a novel option for the medical treatment of premature embryos for captive viviparous elasmobranchs. The remaining issue with this system is that the embryos cannot survive the abrupt change in the chemical environment from artificial uterine fluid (AUF) to seawater during delivery. To overcome this issue, the present study developed a new protocol for seawater adaptation, which is characterized by a long-term and stepwise shift from AUF to seawater prior to delivery. This protocol was employed successfully, and the specimen survived for more than seven months after delivery, the longest captive record of the species. During the experiment, we unexpectedly detected bioluminescence of the embryonic lantern shark in the artificial uterus. This observation indicates that lantern sharks can produce luciferin, a substance for bioluminescence. This contradicts the recent hypothesis that lantern sharks lack the ability to produce luciferin and use luciferin obtained from food sources.

Head anatomy of a lantern shark wet-collection specimen (Chondrichthyes: Etmopteridae)

In this study, we apply a two-step (untreated and soft tissue stained) diffusible iodine-based contrast-enhanced micro-computed tomography array to a wet-collection Lantern Shark specimen of Etmopterus lucifer. The focus of our scanning approach is the head anatomy. The unstained CT data allow the imaging of mineralized (skeletal) tissue, while results for soft tissue were achieved after staining for 120 h in a 1% ethanolic iodine solution. Three-dimensional visualization after the segmentation of hard as well as soft tissue reveals new details of tissue organization and allows us to draw conclusions on the significance of organs in their function. Outstanding are the ampullae of Lorenzini for electroreception, which appear as the dominant sense along with the olfactory system. Corresponding brain areas of these sensory organs are significantly enlarged as well and likely reflect adaptations to the lantern sharks' deep-sea habitat. While electroreception supports the capture of living prey, the enlarged olfactory system can guide the scavenging of these opportunistic feeders. Compared to other approaches based on the manual dissection of similar species, CT scanning is superior in some but not all aspects. For example, fenestrae of the cranial nerves within the chondrocranium cannot be identified reflecting the limitations of the method, however, CT scanning is less invasive, and the staining is mostly reversible and can be rinsed out.

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.

Acquisition of bioluminescent trait by non-luminous organisms from luminous organisms through various origins

Bioluminescence is a natural light emitting phenomenon that occurs due to a chemical reaction between luciferin and luciferase. It is primarily an innate and inherited trait in most terrestrial luminous organisms. However, most luminous organisms produce light in the ocean by acquiring luminous symbionts, luciferin (substrate), and/or luciferase (enzyme) through various transmission pathways. For instance, coelenterazine, a well-known luciferin, is obtained by cnidarians, crustaceans, and deep-sea fish through multi-level dietary linkages from coelenterazine producers such as ctenophores, decapods, and copepods. In contrast, some non-luminous Vibrio bacteria became bioluminescent by obtaining lux genes from luminous Vibrio species by horizontal gene transfer. Various examples detailed in this review show how non-luminescent organisms became luminescent by acquiring symbionts, dietary luciferins and luciferases, and genes. This review highlights three modes (symbiosis, ingestion, and horizontal gene transfer) that allow organisms lacking genes for autonomous bioluminescent systems to obtain the ability to produce light. In addition to bioluminescence, this manuscript discusses the acquisition of other traits such as pigments, fluorescence, toxins, and others, to infer the potential processes of acquisition.

Anatomy and evolution of bioluminescent organs in the slimeheads (Teleostei: Trachichthyidae)

Bacterial bioluminescent organs in fishes have a diverse range of tissues of origin, patterns of compartmentalization, and associated light-conducting structures. The morphology of the perianal, bacterial bioluminescent organ of Aulotrachichthys prosthemius was described previously, but the light organ in other species of slimeheads, family Trachichthyidae, is poorly known. Here, we describe the anatomy of the bioluminescent organs in trachichthyids and places the evolution of this light-producing system in the context of a new phylogeny of the Trachichthyoidei to test the hypothesis that bioluminescence evolved twice in the suborder and that the light-producing component derives from the perianal ectoderm. We use gross and histological examination to provide the first description of the bioluminescent organ of Paratrachichthys and four additional species of Aulotrachichthys. Observations also strongly suggest the presence of a perianal bioluminescent organ in Sorosichthys ananasa. The updated phylogeny of the Trachichthyoidei is the first to combine morphological and DNA-sequence (11-gene fragments) evidence, and supports a monophyletic Trachichthyidae with component subfamilies Hoplostethinae and Trachichthyinae, supporting continued recognition of the family Anoplogastridae. All bioluminescent trachichthyoids share a similar bioluminescent-organ structure with elongate chambers filled with bacteria and connected to collecting ducts that, in turn, connect to superficial ducts that lead to and have lining epithelia continuous with the epidermis. In the context of the phylogeny, the bioluminescent organ of trachichthyids is inferred to have evolved as an elaboration of the proctodeum in the ancestor of Aulotrachichthys, Paratrachichthys, and Sorosichthys independently from the structurally similar cephalic bioluminescent organs in Anomalopidae and Monocentridae.

A Third Luminous Shark Family: Confirmation of Luminescence Ability for Zameus squamulosus (Squaliformes; Somniosidae)

Since recently, shark's bioluminescence has been recorded from two Squaliformes families, the Etmopteridae and Dalatiidae. Pictures of luminescence, light organ morphologies and physiology of the luminous control have been described for species of the Etmopteridae and Dalatiidae families. In 2015, a third luminous family, Somniosidae, was assumed to present a bioluminescent species, Zameus squamulosus. Up to now, confirmation of the luminous abilities of Z. squamulosus is lacking. Here, the luminescence of Z. squamulosus was in vivo recorded for the first time confirming the bioluminescence status of the third luminescent shark family. Additionally, photophore histology revealed the conservation of the light organ morphology across the luminous Squaliformes. Light transmittance analysis through the placoid scale added information on the luminescence efficiency and highlighted a new type of bioluminescent-like squamation. All these data reinforced the likelihood that the common ancestor of Dalatiidae, Etmopteridae and Somniosidae may already have been luminescent for counterillumination purpose.

The megamouth shark, Megachasma pelagios, is not a luminous species

Despite its five meters length, the megamouth shark (Megachasma pelagios Taylor, Compagno & Struhsaker, 1983) is one of the rarest big sharks known in the world (117 specimens observed and documented so far). This filter-feeding shark has been assumed to be a luminous species, using its species-specific white band to produce bioluminescence as a lure trap. Another hypothesis was the use of the white band reflectivity to attract prey or for social recognition purposes. However, no histological study has ever been performed to confirm these assumptions so far. Two hypotheses about the megamouth shark's luminescence arose: firstly, the light emission may be intrinsically or extrinsically produced by specific light organs (photophores) located either on the upper jaw white band or inside the mouth; secondly, the luminous appearance might be a consequence of the reflection of prey luminescence on the white band during feeding events. Aims of the study were to test these hypotheses by highlighting the potential presence of specific photophores responsible for bioluminescence and to reveal and analyze the presence of specialized light-reflective structures in and around the mouth of the shark. By using different histological approaches (histological sections, fluorescent in situ hybridization, scanning electron microscopy) and spectrophotometry, this study allows to unravel these hypotheses and strongly supports that the megamouth shark does not emit bioluminescence, but might rather reflect the light produced by bioluminescent planktonic preys, thanks to the denticles of the white band.

Terrestrial and marine bioluminescent organisms from the Indian subcontinent: a review

The inception of bioluminescence by Harvey (1952) has led to a Nobel Prize to Osamu Shimomura (Chemistry, 2008) in biological research. Consequently, in recent years, bioluminescence-based assays to monitor toxic pollutants as a real-time marker, to study various diseases and their propagation in plants and animals, are developed in many countries. The emission ability of bioluminescence is improved by gene modification, and also, search for novel bioluminescent systems is underway. Over 100 species of organisms belonging to different taxa are known to be luminous in India. However, the diversity and distribution of luminous organisms and their applications are studied scarcely in the Indian scenario. In this context, the present review provides an overview of the current understanding of various bioluminescent organisms, functions, and applications. A detailed checklist of known bioluminescent organisms from India's marine, terrestrial, and freshwater ecosystems is detailed. This review infers that Indian scientists are needed to extend their research on various aspects of luminescent organisms such as biodiversity, genomics, and chemical mechanisms for conservation, ecological, and biomedical applications.

Bioluminescence in lanternsharks: Insight from hormone receptor localization

As part of the study of their bioluminescence, the deep-sea lanternshark Etmopterus spinax and Etmopterus molleri (Chondrichthyes, Etmopteridae) received growing interest over the past ten years. These mesopelagic sharks produce light thanks to a finely tuned hormonal control involving melatonin, adrenocorticotropic hormone and α-melanocyte-stimulating hormone. Receptors of these hormones, respectively the melatonin receptors and the melanocortin receptors, are all members of the G-protein coupled receptor family i.e. coupled with specific G proteins involved in the preliminary steps of their transduction pathways. The present study highlights the specific localization of the hormonal receptors, as well as of their associated G-proteins within the light organs, the so-called photophores, in E. spinax and E. molleri through immunohistofluorescence technic. Our results allow gaining insight into the molecular actors and mechanisms involved in the control of the light emission in Etmopterid sharks.

From extraocular photoreception to pigment movement regulation: a new control mechanism of the lanternshark luminescence

The velvet belly lanternshark, Etmopterus spinax, uses counterillumination to disappear in the surrounding blue light of its marine environment. This shark displays hormonally controlled bioluminescence in which melatonin (MT) and prolactin (PRL) trigger light emission, while α-melanocyte-stimulating hormone (α-MSH) and adrenocorticotropic hormone (ACTH) play an inhibitory role. The extraocular encephalopsin (Es-Opn3) was also hypothesized to act as a luminescence regulator. The majority of these compounds (MT, α-MSH, ACTH, opsin) are members of the rapid physiological colour change that regulates the pigment motion within chromatophores in metazoans. Interestingly, the lanternshark photophore comprises a specific iris-like structure (ILS), partially composed of melanophore-like cells, serving as a photophore shutter. Here, we investigated the role of (i) Es-Opn3 and (ii) actors involved in both MT and α-MSH/ACTH pathways on the shark bioluminescence and ILS cell pigment motions. Our results reveal the implication of Es-Opn3, MT, inositol triphosphate (IP₃), intracellular calcium, calcium-dependent calmodulin and dynein in the ILS cell pigment aggregation. Conversely, our results highlighted the implication of the α-MSH/ACTH pathway, involving kinesin, in the dispersion of the ILS cell pigment. The lanternshark luminescence then appears to be controlled by the balanced bidirectional motion of ILS cell pigments within the photophore. This suggests a functional link between photoreception and photoemission in the photogenic tissue of lanternsharks and gives precious insights into the bioluminescence control of these organisms.

Adrenocorticotropic Hormone and Cyclic Adenosine Monophosphate are Involved in the Control of Shark Bioluminescence

Among Etmopteridae and Dalatiidae, luminous species use hormonal control to regulate bioluminescence. Melatonin (MT) triggers light emission and, conversely, alpha melanocyte-stimulating hormone (α-MSH) actively reduces ongoing luminescence. Prolactin (PRL) acts differentially, triggering light emission in Etmopteridae and inhibiting it in Dalatiidae. Interestingly, these hormones are also known as regulators of skin pigment movements in vertebrates. One other hormone, the adrenocorticotropic hormone (ACTH), also members of the skin pigmentation regulators, is here pharmacologically tested on the light emission. Results show that ACTH inhibits luminescence in both families. Moreover, as MT and α-MSH/ACTH receptors are members of the G-protein coupled receptor (GPCR) family, we investigated the effect of hormonal treatments on the cAMP level of photophores through specific cAMP assays. Our results highlight the involvement of ACTH and cAMP in the control of light emission in sharks and suggest a functional similarity between skin pigment migration and luminescence control, this latter being mediated by pigment movements in the light organ-associated iris-like structure cells.