Nervous control of photophores in luminescent fishes

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.

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

Marine organisms are able to produce light using either their own luminous system, called intrinsic bioluminescence, or symbiotic luminous bacteria, called extrinsic bioluminescence. Among bioluminescent vertebrates, Osteichthyes are known to harbor both types of bioluminescence, while no study has so far addressed the potential use of intrinsic/extrinsic luminescence in elasmobranchs. In sharks, two families are known to emit light: Etmopteridae and Dalatiidae. The deep-sea bioluminescent Etmopteridae, Etmopterus spinax, has received a particular interest over the past fifteen years and its bioluminescence control was investigated in depth. However, the nature of the shark luminous system still remains enigmatic. The present work was undertaken to assess whether the light of this shark species originates from a bioluminescent bacterial symbiosis. Using fluorescent in situ hybridization (FISH) and transmission electron microscopy (TEM) image analyses, this study supports the conclusion that the bioluminescence in the deep-sea lanternshark, Etmopterus spinax, is not of bacterial origin.

Luminescence control of Stomiidae photophores

Nervous control of light emission from deep-sea mesopelagic fishes has been documented for several species. Studies on the nervous control of photophores from deep-sea luminescent fish, are mainly restricted to a pharmacological approach. For example, the light organs, called photophores, isolated from Argyropelecus hemygimnus and Maurolicus muelleri show a much higher sensitivity to adrenaline than to noradrenaline. According to these results and other information in different species, catecholamines are considered as main neurotransmitters triggering bioluminescence in deep-sea fishes. The present work is a study of the nervous control of the isolated photophores from two Stomiid fishes, Chauliodus sloani (the viperfish) and Stomias boa (the dragonfish) with the aim to determine the nature of the nervous control by pharmacological, biochemical and morphological approaches. Results show that, although the photophores of both species are sensitive to catecholamines, adrenaline is present in larger amount than noradrenaline in the light organs of C. sloani. Both catecholamines have different immunoreactive (IR) sites, noradrenaline showing a very diffuse localization as compared to adrenaline in C. sloani. On the contrary, only adrenaline is detected in the photocytes chamber and nerves innervating the photophore in S. boa. Knowing that the majority of dragonfishes exhibit a luminescent chin barbel, we also investigated the presence of catecholamines in this specific tissue in S. boa. Immunohistology reveals the presence of adrenaline within the tissue forming the chin barbel; adrenaline-IR is found in the connective tissue surroundings two group of muscle fibers and blood vessels in the stem but also around the multiple blood vessels located within the barbel bulb. Our results strongly support the adrenergic control of light emission in bioluminescent stomiid fishes.

Catecholaminergic System of Invertebrates: Comparative and Evolutionary Aspects in Comparison With the Octopaminergic System

In this review we examined the catecholaminergic system of invertebrates, starting from protists and getting to chordates. Different techniques used by numerous researchers revealed, in most examined phyla, the presence of catecholamines dopamine, noradrenaline, and adrenaline or of the enzymes involved in their synthesis. The catecholamines are generally linked to the nervous system and they can act as neurotransmitters, neuromodulators, and hormones; moreover they play a very important role as regards the response to a large number of stress situations. Nevertheless, in some invertebrate phyla belonging to Protostoma, the monoamine octopamine is the main biogenic amine. The presence of catecholamines in some protists suggests a role as intracellular or interorganismal signaling molecules and an ancient origin of their synthetic pathways. The catecholamines appear also involved in the regulation of bioluminescence and in the control of larval development and metamorphosis in some marine invertebrate phyla.