A Bombardier worm (Swima bombiviridis). The Green “bombs” identified by the arrow is found near the head of the worm and is dropped when the organism is distressed or threatened. Photo courtesy of Casey Dunn (2007).

The deep ocean has long been regarded as a dark, desolate environment where few to no species existed, however in 1977, this was proved to not be the case with many new deep sea organisms discovered , unfortunately even today, most of the deep oceans remain fairly unexplored. As visible light disappears below 1000m, the deep sea is not only oppressively dark, but the pressures are immense and the temperatures are low. Many species have adapted to the minimal light conditions whereas other species have evolved the process of bioluminescence. Bioluminescence is the emission of light by organisms as a result of the biochemical reactions within cells known as photophores. It has been found that almost 80% of luminescent organisms exist within the marine environment, emitting a range of wavelengths; the most common being between 420 – 500nm corresponding with violet and blue light.

Bioluminescence is a fascinating area of study due to the wide range of possible ecological implications, especially within the deep sea. The evolution of bioluminescence has resulted in a large number of deep sea organisms retaining functional, if rudimentary, eyes to detect bioluminescence. This ability to receive and recognise light emissions and the corresponding colours that species use, can allow for an inference about predator-prey relationships as well as the communication between members of the same species.

A “spewing shrimp” discharging a blue bioluminescent cloud to distract predators.
Photo: Bioluminescence 2009 Expedition, NOAA/OER

There are two main forms of bioluminescence; the first is where luminescent chemicals are released directly in to the water column, serving to blind or distract predators. An example of this is in the Bombardier worms (Swima bombiviridis), a deep sea swimming annelid that releases green light bombs when disturbed. A similar strategy is used by the Spewing shrimp (Acanthephyra purpurea) where a blue bioluminescent cloud is ejected from the shrimp to confuse and distract potential predators. The other form of bioluminescence is where the chemicals are retained within the photocytes in the skin; this is more commonly seen and serves multiple functions, sometimes within a single individual.

 

Locating prey

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A deep sea Anglerfish with its bioluminescent lure on the tip of the extended dorsal spine. Photo courtesy of Wikicommons; Masaki Miya et al. (2010)

The deep sea Dragonfish (Grammatostomias flagellibarba) is thought to use the flashing lights from its photophores to attract potential prey, the barbel on its chin is tipped with these cells and act as a lure, flashing on and off whilst waving back and forth, attracting unsuspecting small fish or crustaceans that can then be snapped up in to its large mouth. The lanternfish (Symbolophorus barnardi) have photophores on their head, underbody and tail which are used to attract small fish for food and this species is able to see blue-green bioluminescent light from over 100ft away. A similar strategy is used by the infamous deep sea Anglerfish (Melanocetus johnsoni); an elongated dorsal spine is tipped with photophores and waved as a lure while the fish lies in wait. These are classed as ambush predators, bringing food to them rather than expending energy to search out prey items.

The alternative to waiting for food to come to them is to actively search out prey, like the Stoplight loosejaw fishes (Malacosteus niger), who use a bioluminescent beam of light to illuminate their prey. Red light cannot be perceived by most organisms in the deep sea thus the Stoplight loosejaw is able to use a red bioluminescent beam of light to hunt whilst remaining undetected by predators and prey alike. However as red light is rapidly attenuated, this species has a much lower visual range than alternative species that use wavelengths of blue light.

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A Sketch of a Stoplight loosejaw, (Malacosteus niger). The bioluminescent organs are found below the eyes. From plate 35 of “Oceanic Ichthyology” by G. Brown Goode and Tarleton H. Bean, published 1896. From [http://www.photolib.noaa.gov/historic/nmfs/images/big/figb0612.jpg]
Attracting mates 

Bioluminescence is thought to be used as a method of communication between species and is important in the process of courtship; that is to attract mates. Plainfin midshipman (Porichthys notatus) has over 700 photophores organised in to four rows that are used in luminous courtship displays. This species is only luminous during this period of time leading to the conclusion that its bioluminescence is only used during courtship. The deep sea Finned Octopus (Stauroteuthis syrtensis) is another species that developed light organs, instead of classic suckers because of the selective advantage of visual displays to attract a mate superseded the advantage provided by their adhesive properties. Unfortunately for many deep sea species the process of reproduction and courtship is largely unknown.

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The concept of counter-illumination portrayed with a Firefly Squid. The blue dots on the lower diagram represent the photophores where illumination hides the silhouette of the animal from predators below. Photo courtesy if Wikicommons; Chiswick Chap (2014)

Defence against predation

This is thought to be the most common function of bioluminescence. It is extensively used in a process called counter-illumination. As most predators in the deep ocean have upward facing eyes to identify the silhouettes of prey, many pelagic organisms like the deep sea Hatchetfish (Argyropelecus gigas) use their photophores to replace their opaque silhouette with bioluminescence of comparable colour, intensity and angular distribution to down welling in ambient light. This is a strategy employed by many different species and has evolved separately over 40 times. This repeated evolution suggests that bioluminescence may directly or indirectly provide its producer with a selective advantage over its non-luminescent counterparts.

The function of bioluminescence reflects the environment in which each organism has evolved. The elaborate and colourful displays created by species can not only provide information about possible courtship displays but information about the predators can be inferred as well. The numbers of bioluminescent organisms in the deep ocean creates an elaborate, brightly lit world where one species lights up to find food, whilst others light up to prevent becoming food, but even now little has been illuminated about bioluminescence in many species.

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