One of the most famous characteristics of the deep is the beautiful display of light some organisms are able to emit known as bioluminescence. It has grasped the attention of many scientists, including Charles Darwin who claimed to see ‘milky seas‘ (similar to Figure 1) on his voyage on the HMS Beagle (1832-1836) as well as flashes of light from jellyfish, and the glow from the glow-worms in South Africa. In the film Mission Blue, Sylvia Earle, currently one of the world’s most famous and influential marine biologists, dived down to 380 m to what was expected to be complete darkness. Yet it was not darkness she found rather “thousands of sparkles and flashes everywhere” claiming it to “amazing” and she was “as excited a little child”.

Bioluminescent phytoplankton glowing blue in the darkness of night on a beach on Vaadhoo Island, Maldives. Darwin encountered similar sightings on his voyage of the HMS Beagle and documented such phytoplanktonic bioluminescence as 'Milky Seas'
Figure 1 Bioluminescent phytoplankton giving off a blue coloured glow in the darkness of night on a beach on Vaadhoo Island, Maldives. Darwin encountered similar sightings on his voyage of the HMS Beagle and documented such bioluminescence as ‘seeing Milky Seas’ (photographed by Doug Perrine)

A recent study suggested up to 90% of the organisms of the deep are able to produce light. It is thought to be so common because the deep sea is a very stable environment with temperature, light intensity and salinity and all other conditions remaining unchanged for decades.

The Deep Dark Sea

The deepest part of the ocean is so deep it could cover 25 Empire State Buildings standing tall (over 4000 m). ‘Deep sea’ begins at the mesopelagic zone (also known as the ‘twilight’ zone) between a depth of 200-1000 m. Below this are the bathypelagic, abyssopelagic and hadopelagic zones which are all in complete darkness as no light can penetrate to these depths. This zonation is shown in Figure 2.

There are 5 layers of the ocean. Mesopelagic begins at 200m deep and is classed as 'deep sea'. Diagram taken from Sea and Sky
Figure 2 The image shows the layers of the ocean in relation to depth. Mesopelagic begins at 200m deep and is classed as ‘deep sea’ (Image taken from Sea and Sky).

How do the organisms emit the light?

Bioluminescence is a result of a chemical reaction in an organism that results in the production of photons. It requires two molecules: Luciferin and either Luciferase or Photoprotein. In one reaction the enzyme Luciferase interacts with the compound Luciferin. When oxygen is added to this reaction, it creates a byproduct called oxyluciferin which  creates the light emitted by these organisms. However, some organisms cannot produce the enzyme Luciferase, instead they use a chemical known as a Photoprotein. In this reaction, Luciferin combines with oxygen and with the help of another agent, such as a calcium ions or a magnesium ions, light is produced. Most marine organisms at such depths are limited to only seeing blue-green wavelengths (410 nm – 720 nm); most marine bioluminescence is the green-blue part of the visible light spectrum. Most species are unable to process other wavelengths such as the yellow, red or violet parts of the spectrum.

The ‘Green Bombers’ of the deep sea

In 2001, a novel group of worms, Swima, were discovered in the deep-sea (2732-3600 m) off the Californian and Oregon Coast. The most well-known species of the Acrocirridae genus is Swima bombiviridis who were given the nickname ‘green bombers’ due to their highly adapted ability to release a green bioluminescent ‘bomb’. Figure 3 shows Swima bombiviridis seconds after its ‘bombs’ have been released .

Swima bombiviridis is able to drop up to 8 green bombs which is filled with a bioluminescent liquid. These worms are up to 30mm long and their bombs are 1-2mm diameter
Figure 3 Swima bombiviridis is able to drop up to eight ‘green bombs’ which are all filled with a bioluminescent liquid. These worms are up to 30mm long and their ‘bombs’ are 1-2 mm diameter. Credit: BBC

The pelagic Swima bombiviridis are an exceptionally unique species who lack eyes, have over 30 long spinous chaetae per parapodium used for swimming, have a transparent foregut and a thick gelatinous sheath which is penetrated by long, club-shaped papillae. Yet the most valuable asset they have are their branchiae. They have 3 forms of branchiae;

  1. A long, transparent white subulate branchia
  2. A row of up to over 30 finger-like branchiae
  3. 4 pairs of elliptical, lobe like segmental branchiae

This last set of branchiae are the most important as these are the so called ‘green bombs’ they release. They have eight (4 pairs) of these bombs growing on the basal part of the nephridiopore papillae (shown in the video below). The most recent research suggests the bombs are filled with a simple, fluid or mucus which contain unorganised clusters of bioluminating particles. These bombs are approximately 1-2mm diameter consisting of 2 small central chambers and 2 very small hemolymph– filled chambers.

Video: Summary of the discovery of Swima bombiviridis with images showing the location of the ‘green bombs’ with an appearance from Chrissy Piotrowski (Curatorial Assistant at the California Academy of Sciences). Credit: Monterey Bay Aquarium Research Institute

As explained in the video, it is assumed these bombs are detached when used as a defence mechanism when the worm feels threatened. A similar adaptation is used in the Vampire Squid, Vampyroteuthis infernalis, when it releases bioluminescent ink to scare predators, allowing it to escape. When a predator approaches, the ‘green bombers’ detach the bombs which intensely glow for several seconds before diminishing. This glow is believed to periodically startle the predators whilst the worm escapes.

The research on the newly discovered clade of annelid worms is quite limited as they are difficult to study in situ as it requires very technical machinery to get to such depths. Furthermore, fact that their discovery is so recent stresses how little knowledge we have of the deep sea and how much more opportunity we have to develop our understanding of the deep sea.


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