Light in marine organisms.

Light in marine organisms.

Bioluminescence is the product of the internal reaction known as chemiluminescence; an adaptation taken on by a diversity of species. The reasons for producing light across all kingdoms is independently varied; spanning across courtship, defensive and offensive mechanisms and emotional responses. The ability to naturally produce light has been observed in about 10,000 species from 800 genera (Haddock et al. 2010). Emitting light in darkness is found in abundance in marine habitats for example: invertebrates, fish, bacteria, dinoflagellates. In contrast to its terrestrial counterparts where bioluminescence is reduced and mostly found in some Arthropods and Annelids. In this paper, the different adaptations and uses of bioluminescence will be explored in 5 phyla, along with the fundamental chemistry behind the biological reaction that enables organisms to produce light.

The two unique chemicals that cause the chemical reaction that results in bioluminescence is luciferin and either luciferase or photoprotein. Luciferin is the substrate during the reaction, producing the light. The arrangement of luciferin molecules determines the observed colour. In some cases, like certain species of midshipman fish, intake of luciferin has been through consuming Ostracods or ‘seed shrimp’. This is because some species cannot synthesize luciferin. This is termed a ‘predatory’ or sometimes a ‘symbiotic’ relationship.

Habitat and species also influence the colour produced. Deep-dwelling species can detect significantly shorter wavelengths of light than shallow species.  For example, it is seen in both ctenophores and medusae (Haddock, S. H. D.  Case, J. F. 1999). In the deep ocean, blue-green light waves are more visible, so organisms are subsequently more sensitive to the green-blue spectrum (Langley, L. (2019). Figure 2 is a visual representation of how many meters down, coloured light can travel through water. With ultraviolet being the least penetrating at less than 100 meters, and blue and green being most penetrating at over 250 meters. Therefore, most bioluminescence is seen as blue to green. However, there are a few exceptions of red/purple light in specialized genus, like Malacosteus. Which can initially produce blue light but then reabsorbs the blue light into a protein, which then emits a red glow. This ensures the Malacosteus goes undetected by prey whilst maintaining vision (kathrine, J. Wu. (2018).

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Although most of the enzymes and substrates behind chemiluminescence remains the same, bioluminescence and its uses within different phyla is hugely varied.  In the following paragraphs, bioluminescence in Dinoflagellates, Arthropoda, Mollusca, Cnidarians and Annelids will be explored. Figure 3 is a phylogenetic tree showing the ancestry relationships between nonluminous and luminous phylum, including the phyla mentioned above.

Figure 3. Haddock. Moline. Case. (2010)   Phylogenic tree of Eukaryotes in relation to luminous and nonluminous species.

Dinoflagellate are protists that can be heterotrophic or autotrophic.

Dinoflagellates seen in figure 3 branching from a common ancestor with plants, is one of the most prominent luminous phyla. Dinoflagellates are single-celled Protista and where chemiluminescence takes place within the organism varies between species. In contrast to the intracellular bioluminescence of dinoflagellates (Figure 4), many copepods release their bioluminescent chemicals from luminous glands located on appendages, such as the tail or swimming legs. This characteristic has been proposed to act as defense against predation (recently reviewed by Marcinko et al. 2013). Marine zooplankton predate on dinoflagellates and it has been theorised in several reports that zooplankton have photophobic responses (E. Buskey, L. Mills. 1983) (E. Buskey, C.G Mann, E Swift 1987) (E.J Buskey, E. Swift 1985). This would indicate that bioluminescence is an evolutionary trait taken on by dinoflagellates; which would reduce predation pressure by deterring zooplankton with flashes of light. 

Figure 4 Bioluminescence in the dinoflagellate from microsources in the cytoplasm. Cell length ¼ 1 mm.

 (Widder and Case, 1981).

The more complex Arthropoda classify organisms with chitin exoskeletons, segmented bodies and paired jointed appendages. In the subphylum crustacea where bioluminescent species are present, photophores can be found along the lower surface of their body, which they use for counterillumination. Some species also have two small light organs on their eyestalks. They most likely act as feedback mechanisms for determining if their ventral photophores are matching background light (Kathrine, J. W. (2018).

(Figure 5) (Watasenia scintillans). Fenolio, D.  (2016)

The most prominent members of the marine molluscs are the cephalopods. However, some genera in the families Sepiolidae and Loliginidae, have bacterial symbionts that produce luminescence as the cephalopods are unable to do so themselves (Nyholm et al. 2009).  Intrinsic bioluminescence (luciferin with their specialized luciferase.) is found in most other cephalopod species. Photophores are found to cover the ventral mantle with bright photophores located near the eyes and on the tips of the tentacles. Typical for various species of squid. Cephalopods often generate an array of luminescent displays with their photophores. Figure 5 shows a firefly squid (Watasenia scintillans) revealing a bright blue, speckled mantle and tentacles. These members of the Oegopsida group also have three visual pigments and a double-layered retina, in comparison to other cephalopods which only have one colour pigment. It is suggested that this adaptation enables the firefly quid to distinguish between ambient light and bioluminescence so that communication is more obvious to same species individuals.                                                                Another behavior exhibited by cephalopods is flashing. A common technique used to express emotions or confusing prey/predator. It ranges from less than a second to around 10 seconds (Wood, J., et al. 2008).

In the cnidarian phylum, the following groups:  corals, anemones, hydroids and medusae, all luminesce. As far as is known, the luminous species all use coelenterazine as their light-emitting substrate. The cause for bioluminescence in most hydrozoans is likely for warning or threatening purposes, but it may also be used to attract prey directly to stinging tentacles (Morin, J., 1976).

Marine polychaetas have multiple light emitting sub-groups. The tube-dwelling chaetopterid Chaetopterus and the terebellid Polycirrus ooze glowing particles from their tubes when the worms are disturbed. Although, bioluminescence has also been documented to be used during spawning by many polychaetas. Females produce luminescent secretions that attract the males during courtship. (Figure 6)

Figure 6. Bioluminescent display of Odontosyllis enopla. During courtship, the females secrete a bright bluish-green luminous mucus while releasing gametes. Wood, J, B. (2018).

Bioluminescence, a natural phenomenon that is still being researched to the present day. Within the natural realm, one of the main objectives behind bioluminescence is to evade or deter predators; Seen in dinoflagellates, terebellid Polycirrus and hydrozoans. Another central explanation to why organisms luminesce is intraspecies communication. Seen in many cephalopods like Watasenia scintillans. The complexity of self-emitted light has spanned across courtship and helping females attract males to mass events where millions of dinoflagellates are able to light up the surface of the ocean when disturbed.

Overall, bioluminescence is a differentiated adaptation within marine organisms, with many of the reasons behind it unknown or undiscovered. However, it is apparent that self-emitted light can be useful for both the organism presenting it, and the organisms in the same habitat.

References.

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