Reconstruction of Light Organ in Squid With The Histological Method of Electron Transmission Microscope

The light organ is an electronic device that can emit light. However, there are light organs in animals that can produce light naturally. Loligo duvaucelii is a species whose biolumenesence comes from fluorescent bacteria that live in symbiosis in its ink sacs. This study aims to determine in detail the construction of the squid light organ using the transmission electron microscopy (TEM) method. The results showed that this type of squid has a pair of light organs attached to the dorso-lateral ink sac. The light organ is spherical, some are found on the surface and some are embedded on the wall of the ink sac. It consists of a lens that is located on the outer surface of the ink sac, and a sac of light organs (embedded on the wall of the ink sac) with channels connecting the pocket to the mantle cavity. The wall of the sac of the light organ consists of three layers, namely the innermost layer which is multi-fold with microvilli on the cell surface and between the folds of the sac populated with bacteria, the dense layer that acts as a reflector, and the pigment layer. Cilia are observed on the surface of the duct connecting the sac with the mantle cavity. This study concluded that the construction of the squid light organ has a convex-shaped lens structure and is muscular. In the pockets of light organs, a dense population of bacteria is found. The reflector consists of many layers, and the pigment layer contains many granules.

Social signaling via bioluminescent blinks determines nearest neighbor distance in schools of flashlight fish Anomalops katoptron

The schooling flashlight fish Anomalops katoptron can be found at dark nights at the water surface in the Indo-Pacific. Schools are characterized by bioluminescent blink patterns of sub-ocular light organs densely-packed with bioluminescent, symbiotic bacteria. Here we analyzed how blink patterns of A. katoptron are used in social interactions. We demonstrate that isolated specimen of A. katoptron showed a high motivation to align with fixed or moving artificial light organs in an experimental tank. This intraspecific recognition of A. katoptron is mediated by blinking light and not the body shape. In addition, A. katoptron adjusts its blinking frequencies according to the light intensities. LED pulse frequencies determine the swimming speed and the blink frequency response of A. katoptron, which is modified by light organ occlusion and not exposure. In the natural environment A. katoptron is changing its blink frequencies and nearest neighbor distance in a context specific manner. Blink frequencies are also modified by changes in the occlusion time and are increased from day to night and during avoidance behavior, while group cohesion is higher with increasing blink frequencies. Our results suggest that specific blink patterns in schooling flashlight fish A. katoptron define nearest neighbor distance and determine intraspecific communication.

Bioluminescence of the Largest Luminous Vertebrate, the Kitefin Shark, Dalatias licha: First Insights and Comparative Aspects

Bioluminescence has often been seen as a spectacular yet uncommon event at sea but considering the vastness of the deep sea and the occurrence of luminous organisms in this zone, it is now more and more obvious that producing light at depth must play an important role structuring the biggest ecosystem on our planet. Three species of deepwater sharks (Dalatias licha, Etmopterus lucifer, and Etmopterus granulosus) were collected from the Chatham Rise, off New Zealand, and for the first time, we documented their luminescence. Comparison of glowing shark pictures, combined with histological description of light organs and hormonal control analysis, highlight the evolutive conservation of the bioluminescence process within Dalatiidae and Etmopteridae. A special emphasis is placed on the luminescence of D. licha, the largest known luminous vertebrate. This first experimental study of three luminous shark species from New Zealand provides an insight into the diversity of shark bioluminescence and highlights the need for more research to help understand these unusual deep-sea inhabitants: the glowing sharks.

Microscopic and Genetic Characterization of Bacterial Symbionts With Bioluminescent Potential in Pyrosoma atlanticum

The pelagic tunicate pyrosome,Pyrosoma atlanticum, is known for its brilliant bioluminescence, but the mechanism causing this bioluminescence has not been fully characterized. This study identifies the bacterial bioluminescent symbionts ofP. atlanticumcollected in the northern Gulf of Mexico using several methods such as light and electron microscopy, as well as molecular genetics. The bacteria are localized within the pyrosome light organs. Greater than 50% of the bacterial taxa present in the tunicate samples were the bioluminescent symbiotic bacteria Vibrionaceae as determined by utilizing current molecular genetics methodologies. A total of 396K MiSeq16S rRNA reads provided total pyrosome microbiome profiles to determine bacterial symbiont taxonomy. After comparing with the Silva rRNA database, aPhotobacteriumsp. r33-like bacterium (which we refer to as “PhotobacteriumPa-1”) matched at 99% sequence identity as the most abundant bacteria withinPyrosoma atlanticumsamples. Specifically designed 16S rRNA V4 probes for fluorescencein situhybridization (FISH) verified thePhotobacteriumPa-1 location as internally concentrated along the periphery of each dual pyrosome luminous organ. While searching for bacterialluxgenes in two tunicate samples, we also serendipitously generated a draft tunicate mitochondrial genome that can be used forPyrosoma atlanticumidentification. Scanning (SEM) and transmission (TEM) electron microscopy confirmed the presence of intracellular rod-like bacteria in the light organs. This intracellular localization of bacteria may represent bacteriocyte formation reminiscent of other invertebrates.

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.

Identification of a Transcriptomic Network Underlying the Wrinkly and Smooth Phenotypes of Vibrio fischeri

ABSTRACT Vibrio fischeri is a cosmopolitan marine bacterium that oftentimes displays different colony morphologies, switching from a smooth to a wrinkly phenotype in order to adapt to changes in the environment. This wrinkly phenotype has also been associated with increased biofilm formation, an essential characteristic for V. fischeri to adhere to substrates, to suspended debris, and within the light organs of sepiolid squids. Elevated levels of biofilm formation are correlated with increased microbial survival of exposure to environmental stressors and the ability to expand niche breadth. Since V. fischeri has a biphasic life history strategy between its free-living and symbiotic states, we were interested in whether the wrinkly morphotype demonstrated differences in its expression profile in comparison to the naturally occurring and more common smooth variant. We show that genes involved in major biochemical cascades, including those involved in protein sorting, oxidative stress, and membrane transport, play a role in the wrinkly phenotype. Interestingly, only a few unique genes are specifically involved in macromolecule biosynthesis in the wrinkly phenotype, which underlies the importance of other pathways utilized for adaptation under the conditions in which Vibrio bacteria are producing this change in phenotype. These results provide the first comprehensive analysis of the complex form of genetic activation that underlies the diversity in morphologies of V. fischeri when switching between two different colony morphotypes, each representing a unique biofilm ecotype. IMPORTANCE The wrinkly bacterial colony phenotype has been associated with increased squid host colonization in V. fischeri. The significance of our research is in identifying the genetic mechanisms that are responsible for heightened biofilm formation in V. fischeri. This report also advances our understanding of gene regulation in V. fischeri and brings to the forefront a number of previously overlooked genetic networks. Several loci that were identified in this study were not previously known to be associated with biofilm formation in V. fischeri.

Systematic review and phylogeny of the firefly genus Dilychnia (Lampyridae: Lampyrinae), with notes on geographical range

Dilychnia (Lampyridae: Lampyrinae) is a poorly known Neotropical genus of lampyrid firefly. Descriptions of Dilychnia species were traditionally based on differences in colour pattern, which are usually not reliable for species discrimination. Here, we review and redescribe the genus Dilychnia and its species, after examining the type material of all species, and we present the first phylogenetic analysis for the group, including 70 morphological characters and 15 species. After the proposal of new taxonomic combinations, Dilychnia was recovered as monophyletic with high support. We provide a key to species identification, updated geographical ranges and illustrations of diagnostic features for all species. We transfer Lucidota propinqua and Photinus succensus to Dilychnia and describe the new species Dilychnia dumasi sp. nov. We report, for the first time, that these three taxa are nocturnal and flash with an orange-yellow light. Phenotypic diversity within the genus is most pronounced in characters involved in mate tracking (i.e. antennae, eyes and light organs).

Reviews and syntheses: Bacterial bioluminescence – ecology and impact in the biological carbon pump

Around 30 species of marine bacteria can emit light, a critical characteristic in the oceanic environment is mostly deprived of sunlight. In this article, we first review current knowledge on bioluminescent bacteria symbiosis in light organs. Then, focusing on gut-associated bacteria, we highlight that recent works, based on omics methods, confirm previous claims about the prominence of bioluminescent bacterial species in fish guts. Such host–symbiont relationships are relatively well-established and represent important knowledge in the bioluminescence field. However, the consequences of bioluminescent bacteria continuously released from light organs and through the digestive tracts to the seawater have been barely taken into account at the ecological and biogeochemical level. For too long neglected, we propose considering the role of bioluminescent bacteria and reconsidering the biological carbon pump, taking into account the bioluminescence effect (“bioluminescence shunt hypothesis”). Indeed, it has been shown that marine snow and fecal pellets are often luminous due to microbial colonization, which makes them a visual target. These luminous particles seem preferentially consumed by organisms of higher trophic levels in comparison to nonluminous ones. As a consequence, the sinking rate of consumed particles could be either increased (due to repackaging) or reduced (due to sloppy feeding or coprophagy/coprorhexy), which can imply a major impact on global biological carbon fluxes. Finally, we propose a strategy, at a worldwide scale, relying on recently developed instrumentation and methodological tools to quantify the impact of bioluminescent bacteria in the biological carbon pump.

Carbon dioxide-induced bioluminescence increase in Arachnocampa larvae

ABSTRACTArachnocampa larvae utilise bioluminescence to lure small arthropod prey into their web-like silk snares. The luciferin–luciferase light-producing reaction occurs in a specialised light organ composed of Malpighian tubule cells in association with a tracheal mass. The accepted model for bioluminescence regulation is that light is actively repressed during the non-glowing period and released when glowing through the night. The model is based upon foregoing observations that carbon dioxide (CO2) – a commonly used insect anaesthetic – produces elevated light output in whole, live larvae as well as isolated light organs. Alternative anaesthetics were reported to have a similar light-releasing effect. We set out to test this model in Arachnocampa flava larvae by exposing them to a range of anaesthetics and gas mixtures. The anaesthetics isoflurane, ethyl acetate and diethyl ether did not produce high bioluminescence responses in the same way as CO2. Ligation and dissection experiments localised the CO2 response to the light organ rather than it being a response to general anaesthesia. Exposure to hypoxia through the introduction of nitrogen gas combined with CO2 exposures highlighted that continuity between the longitudinal tracheal trunks and the light organ tracheal mass is necessary for recovery of the CO2-induced light response. The physiological basis of the CO2-induced bioluminescence increase remains unresolved, but is most likely related to access of oxygen to the photocytes. The results suggest that the repression model for bioluminescence control can be rejected. An alternative is proposed based on neural upregulation modulating bioluminescence intensity.

Social signaling via bioluminescent blinks drives schooling behavior in the flashlight fish Anomalops katoptron

The bioluminescent flashlight fish Anomalops katoptron live in schools of several hundred specimens. To understand how flashlight fish, integrate bioluminescent signaling into their schooling behavior, we analyzed movement profiles and blink frequencies. Isolated specimen of A. katoptron show a high motivation to align with fixed or moving artificial light organs. Depending on presented frequencies A. katoptron responds with a reduction in swimming speed and its own blink frequency. Higher presented blink frequencies reduce the nearest neighbor distance. In the natural environment A. katoptron is changing its blink frequencies and nearest neighbor distance in a context specific manner. Blink frequencies are increased from day to night and during avoidance behavior, while nearest neighbor distance is decreased with increasing blink frequencies. A. katoptron changes its blink frequencies by modifying light organ occlusion. Our results suggest that visually transmitted information via specific blink patterns determine intraspecific communication and group cohesion in schooling A. katoptron.