The Effects of Bioluminescence on Anglerfish and their Symbionts
The Effects of Bioluminescence on Anglerfish and their Symbionts
By: Sophia Hardy
Introduction
Anglerfish are known for the glowing apparatus that extends from their heads, which is composed of the illicium and the esca (Pietsch, 2009). The light that is emitted from the escal light organ is a product of bioluminescent bacteria. The bacteria and the anglerfish maintain a symbiotic relationship with each other to survive in the dangerous environment of the deep. This light is a product of bioluminescence, a chemical reaction that is universal among all bioluminescent organisms. These reactions are exergonic, involving molecular oxygen, but vary in their substrates, or luciferins, and enzymes, or luciferases (Wilson and Hastings, 1998). This paper will dive into how bioluminescence is produced and how these organisms use them for survival.
Anatomy
Anglerfish of various species have the illicium and esca prominently residing on top of their heads. The illicium is the spine-like dorsal fin and the esca is the light organ that rests at the end of the illicium (Declan, 2014). This fishing, lure-like apparatus is important for attracting both mates and prey as it is the place where anglerfish emit light. Escal light organs, the primary organs behind the glowing lure of the anglerfish, are only present in females. It is not present at birth but develops as the juveniles migrate into the deep sea. The esca invaginates during this period and populates with the symbiotic luminescent bacteria (Freed et. al., 2019).
There is great variety in the esca's appearance in the anglerfish species, but all esca share similar anatomical structures. The escal organ, from outside in, starts with an outer layer of epithelium, followed by an inner connective tissue layer with nerves (Munk, 1999). Located near the bottom of the esca is the light proof cup that encloses the light gland proper (Munk, 1999) Superior to the light gland proper is a layer of thin branched tubules and above this resides the vestibule, which, in larger anglerfish, contains excretory goblet cells. The light gland proper and vestibule are connected by the structure located on top of the vestibule, the ceratiid (Munk, 1999). Each of these parts of the escal light organ is vital in emitting light produced by the bioluminescent symbionts that reside in this organ.
Symbiotic Relationship
Many theories surround the symbiotic relationship between bacteria and anglerfish. Four genera of bacteria, in particular, are known to be abundant within the esca: Moritella, Pseudoalteromonas, Enterovibrio, and Vibrio (Freed et. al., 2019). Since not much is known about the creatures of the deep sea, there were two hypotheses concerning how anglerfish acquire these symbionts. The first hypothesis is that anglerfish mothers inoculate the eggs of their young with bioluminescent bacteria (Freed et.al., 2019). However, being unable to find symbionts in the esca of juveniles, this hypothesis was disproved (Freed et.al., 2019). The most likely explanation is that the bacteria are found in the environment and enter the esca through the escal pore (Freed et.al., 2019). It is theorized that luminous bacteria are selected because the luciferase systems detoxify harmful species (Wilson and Hastings, 1998). These harmful reactive oxygen species (ROS) might be generated in large amounts due to the esca making it difficult for most species of bacteria to inhabit the light organ (Wilson and Hastings, 1998). In this symbiotic relationship, the bacteria receive free “room and board” which consists of housing inside the esca, in addition to the abundance of nutrients provided by the skin of the anglerfish and the shelter given by the micro papillae on the surface of their skin (Wilson and Hastings, 1998) (Munk, 1999). The anglerfish can manipulate a light source and, because of the symbionts, are allowed to fully develop, especially their esca.
The bacterial version of the bioluminescence reaction begins with luciferase catalyzing an oxidation reaction involving a long-chain aldehyde and a very light-sensitive FMNH₂ (Wilson and Hastings, 1998). Due to being bound to the luciferase, FMNH₂ is now protected against spontaneously oxidizing (Wilson and Hastings, 1998). The first step of this reaction is the formation of a very stable luciferase-bound flavin hydroperoxide followed by the aldehyde reacting to form a peroxy hemiacetal (Wilson and Hastings, 1998) These steps result in the emitter, enzyme-bound 4a-hydroxyflavin, and a quantum yield of roughly 0.3 hv per reacting FMNH₂ molecule (Wilson and Hastings, 1998). The bacterial symbionts within the esca can control the entire bioluminescent reaction and light.
Bacteria emit light constantly, but the emission can only occur under specific conditions for the bacteria (Wilson and Hastings, 1998). The cell density of the bacteria must be high enough for the luminescence-related DNA to be transcripted, which can be determined by bacteria using autoinducers (Wilson and Hastings, 1998). Autoinducers detect a pheromone that indicates to the bacteria that the cell density is enough to begin transcription; some species of symbionts such as the Vibrio bacteria can respond to multiple autoinducers (Wilson and Hastings, 1998). Although bacteria are the organisms in this symbiotic relationship that release light, both bacteria and anglerfish are capable of controlling the light.
Theories of Energy Transfer
The bioluminescent reaction that occurs because of the luminous symbionts is a multi-step process that finally emits light (Freed et.al., 2019). Fluorescence produced by this chemical reaction is said to be a product of an energy transfer (Wilson and Hastings, 1998). There are several proposals for how this energy transfer occurs (Wilson and Hastings, 1998). The first is a chemically induced electron-exchange luminescence (CIEEL) (Wilson and Hastings, 1998). CIEEL is a product molecule that is formed in an excited state that transfers energy to an added hydrocarbon (Wilson and Hastings, 1998). Another theory of energy transfer is the Forster resonance energy transfer (FRET) where an excited donor transfers energy to an acceptor group (Wilson and Hastings, 1998). This can happen over longer distances and depends on the overlap and positioning of molecules (Wilson and Hastings, 1998). The third theory is that an accessory photophore absorbs light emitted by the primary excited state and is reemitted as fluorescence (Wilson and Hastings, 1998).
Variations in Light Emission
There are variations in the bioluminescent reactions that are dependent upon the organism, therefore the range of wavelengths from the light emitted can differ. Similarities have been found between the light emissions coming from the esca and bioluminescent bacteria alone (Munk, 1999). Light from the esca falls within the 470 to 490 nm range while light from the bacteria falls within the 475 to 500 nm range (Munk, 1999).
Other differences in bioluminescence lie within the reaction itself. The bioluminescent reaction starts with luciferin and moves through several steps until yielding a photon of light. As seen in the bacterial reaction, one main difference is that this reaction tends to operate without the use of intermediates, skipping a middle step that many other bioluminescent processes go through (Wilson and Hastings, 1998). Another variation can be seen with the enzyme in the bioluminescence reaction, luciferase, which can be different in each organism but all share one commonality in bacteria (Wilson and Hastings, 1998). They are all heterodimers, protein structures formed by two proteins, of α and β subunits (Wilson and Hastings, 1998).
Multiple factors influence the way that light is displayed after the bioluminescent reaction. The light can vary in color depending on three factors: the interference of accessory fluorophores, the structure of luciferase, and luciferase-bound products (Wilson and Hastings, 1998). Accessory fluorophores such as green fluorescent protein (GFP) and yellow fluorescent protein (YFP) can alter emissions of the colors and speed up the process of the decay of luciferase (Wilson and Hastings, 1998). The luciferase structure can also alter the color of bioluminescence in addition to the emission matching the fluorescence of an excited luciferase-bound product. This is because the product of the bioluminescence reaction is extremely unstable after emission (Wilson and Hastings, 1998).
Anglerfish, specifically the escal light organ, can manipulate the light produced in the chemical reaction. It is theorized that blood flow relates to an anglerfish’s ability to control the light (Munk, 1999). Apart from this theory, the esca have been observed to luminesce spontaneously, which can be attributed to smooth muscle in the esca and the lightproof cup.³ Much of the control that anglerfish have over the luminescence comes from how the organism controls its symbionts (Munk, 1999). Luminescent bacteria can be expelled from the esca through the escal pore, changing the cell density inside the light organ. This is similar to the ability that some species of anglerfish have; to eject luminous materials from their esca to distract possible predators (Munk, 1999). The light can also be manipulated by moving the esca. Although the motility of the esca and illicium vary among species, anglerfish such as H. azurlucens, H. groenlandicus, and H. albinares can sweep the apparatus forward and backward (Munk, 1999). Using these various methods, the frequency, brightness, and location of the luminescence can be controlled for the anglerfish’s benefit.
Conclusion
Despite living in conditions normally uninhabitable to most life on this planet, bioluminescent bacteria and anglerfish have found a way to successfully coexist in the deep sea. Through utilizing the esca and illicium, anglerfish have created a habitable environment for luminescent bacteria to gather together in a nutrient-rich area. With a high cell density, the bacteria can undergo the reaction of bioluminescence and constantly emit light, which the anglerfish can manipulate using its many structures within the esca. Having the power to control light in the black waters enables female anglerfish to lure in prey and attract mates. The complicated reaction of bioluminescence in microscopic organisms produces many magnificent effects on larger organisms and the deep-sea environment as a whole.
References
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