By Lucy Bryan Research Intern
The ocean covers more than 70% of the Earths surface, however only 2-3% of the ocean is penetrated by sunlight. In general sunlight only penetrates around 200m deep, this layer is known as the euphotic zone, and is the area in which there is enough sunlight for organisms to photosynthesise. Below 200m there is very limited sun/star light, if any, however this doesn’t mean that there isn’t any light present at these depths. Many deep-sea organisms have evolved to use the natural phenomenon of bioluminescence, the production and emission of light by a living organism as a result of a chemical reaction. The general equation for this reaction is:
Luciferine + O2 (oxygen) + ATP (energy source) Luciferase Oxyluciferin + Light+ Byproducts
Bioluminescence has been observed as far back as 1500BC (Roda, 2011) and even though there are still a lot of questions about its evolutionary history it is believed that it has evolved independently several times from single-celled bacteria and dinoflagellates to fungi, jellyfish and vertebrates (Widder, 2010). It is a prevalent ecological trait in the deep sea with approximately 76% of observed marine organisms possessing this capability (Martini & Haddock, 2017) and occurs in approximately 80% of eukaryotic life that inhabits the deep sea (Davis et al., 2014). It has several different roles including speciation, predator deterrence and camoflage (Menlinik et al., 2022) (Davis et al., 2014).
Speciation is when a new species is formed when a group within a species separates from other members of its species and develops its own characteristics. In this case the new characteristic species-specific bioluminescent structures which have been linked to higher rate of diversification in deep-sea fishes (Davis et al., 2014), an example of this is lanternfish and dragonfish. It has also been hypothesized that unique bioluminescent signals are used for species recognition, and this promotes reproductive isolation aiding in speciation. (Branham and Greenfield, 1996). The bioluminescent animals emit distinct light patterns allowing others to know their identity (Davis, 2014).
Bioluminescence is also used to aid in camoflage, this is done through counter-illumination (Braken-Grissom, et al., 2020). Animals that use counter illumination have photophores on there bodies which can produce light that matches the background intensity allowing them to cancel out any shadow they cast.
Another role bioluminescence is suspected to play is predator defence, it has been suggested that the sudden flash of bright light emitted by dinoflagellates startles their predators, allowing enough time for them to escape (Abrahams and Townsend, 1993), the light can also attract a secondary predator to threaten the primary predator.
Despite there not being any predators, dinoflagellates still use bioluminescence as seen above. This is because when they are threatened by predators, they experience a mechanical stress which triggers the chemical reaction, however, a strong enough mechanical stress can be imparted on the dinoflagellates by waves, surges and swimming animals.
Bibliography:
Roda, A., 2011. A history of bioluminescence and chemiluminescence from ancient times to the present. Chemiluminescence and Bioluminescence, pp.1-50.
Greenaway, S.F., Sullivan, K.D., Umfress, S.H., Beittel, A.B. and Wagner, K.D., 2021. Revised depth of the Challenger Deep from submersible transects; including a general method for precise, pressure-derived depths in the ocean. Deep Sea Research Part I: Oceanographic Research Papers, 178, p.103644.
Martini, S. and Haddock, S.H., 2017. Quantification of bioluminescence from the surface to the deep sea demonstrates its predominance as an ecological trait. Scientific reports, 7(1), p.45750.
Davis, M.P., Holcroft, N.I., Wiley, E.O., Sparks, J.S. and Leo Smith, W., 2014. Species-specific bioluminescence facilitates speciation in the deep sea. Marine biology, 161, pp.1139-1148.
Collins, S.B. and Bracken‐Grissom, H.D., 2024. The language of light: a review of bioluminescence in deep‐sea decapod shrimps. Biological Reviews.
Haddock, S.H., Dunn, C.W., Pugh, P.R. and Schnitzler, C.E., 2005. Bioluminescent and red-fluorescent lures in a deep-sea siphonophore. Science, 309(5732), pp.263-263.
Haddock SHD, Moline MA, Case JF. Bioluminescence in the sea. Annu Rev Mar Sci. 2010;2:443–493. doi: 10.1146/annurev-marine-120308-081028.
Abrahams, M.V. and Townsend, L.D., 1993. Bioluminescence in dinoflagellates: A test of the burgular alarm hypothesis. Ecology, 74(1), pp.258-260.
Widder, E.A., 2010. Bioluminescence in the ocean: origins of biological, chemical, and ecological diversity. Science, 328(5979), pp.704-708.
Bevan, N., 2024. Bioluminescent plankton light up the Welsh coast. BBC News, 23 June.
(Bevan, 2024)
