Christmas lights in the sea: Bioluminescence
Oceans can be a bleak and dark environment, especially in the deeper depths. With sunlight being limited once past 200 metres in depth, darkness sets in. This depth range is known as the ‘twilight zone’. Below 1000 metres, light disappears completely. This zone of the ocean is known as the ‘midnight zone’. You would expect this region to be barren but several species inhabit this zone in the depths, with more being discovered every year. While you might think this environment to be gloomy, you would be mistaken. Imagine a large array of colourful flashes going off in the darkness, like flashing Christmas lights on a tree. This is all due to bioluminescence. This biological reaction allows for organisms to take advantage of what might seem a harsh, extreme environment. This stunning lightshow is not just limited to the deep, as it can be seen at night along beaches due to bioluminescent plankton.
What is Bioluminescence?
Bioluminescence is the emission of visible light by an organism created by a chemical reaction. To be able to produce the light, the organism must have luciferin within the body. Luciferin is a compound, which becomes oxyluciferin when in the presence of luciferase or photoproteins, through an oxidation reaction. Luciferase is an enzyme that acts as the catalyst, and the oxidation reaction also produces energy in the form of light, which is bioluminescence. There are four separate types of luciferin used by organisms in the ocean but all use oxidation reactions to produce light. The reaction is commonly produced in light-producing organs known as ‘photophores’. Photophores are found across various organisms such as fish and cephalopod species, and are distributed across the animal’s body in lines or in distinctive patterns.
Some species however are not able to produce bioluminescent light due to not having luciferin in their body, and as a result they rely on other organisms like bacteria to produce light for them. One species that does this is the Hawaiian bobtail squid (Euprymna scolopes). This is a nocturnal species which is vulnerable to attack from predators at night when it is most active, and therefore developed a symbiotic relationship with heterotroph bacteria (Aliivibrio fischeri). The squid acts as a host for the bacteria, with the bacteria being homed in a special organ in the mantle of the squid. In return for producing light for the squid, the bacterium gets supplied with carbon and nitrogen. The bacteria produce the light, however the squid is able to control the intensity using thick lens and yellow filters. The light is emitted downwards, which mimics the light levels of the moonlight and starlight in order to help hide the squids silhouette from predators lurking below; this essentially camouflages the squid to the environment and is known as counterillumination.
Why use bioluminescence?
Counterillumination is one form of defence used by prey species. Another method is known as the ‘burglar alarm’. This method of defence is used by dinoflagellate, crustaceans and more. Dinoflagellates will produce light if a predator is near and this is thought to act as an alarm to deter the predator. The light will mean that large predator might be attracted to the area, making the smaller predator vulnerable, causing it to swim away. This is similar in crustaceans such as ostracods, except they excrete colourful light for defence against predators. This light often sticks to the predator, putting the predator at risk. This is highlighted in the video below, showing ostracods reacting to cardinal fish attacks. In a more extreme form of defence known as a ‘sacrificial tag’, the prey will actively release a limb. The limb will continue to glow after release and If it is eaten by the predator, they will be at risk. In the deep-sea, organisms are often transparent and therefore If they consume the limb, they will be detectable to other predators as the limb will produce light hours after consumption. While this could be seen to hinder the prey by losing a limb, they are able to regrow the missing limb over time.
The most common use of bioluminescence by predators is called lures. The most popular example of lures is used by angler fish. Angler fish have an extended dorsal spine which has a photophore located at the end. By lighting the end of the dorsal spine and slowly waving its spine, it lures unsuspecting prey into thinking the lure is prey. When within range, the angler fish will use its large mouth to engulf its prey. Lures are also used in the cookiecutter shark (Isistius brasiliensis). Cookiecutter shark uses a flaw in counterillumination and prey attraction to its advantage. A
dark band under the mouth of the shark makes predators below think there is prey above. Once the predator approaches, the cookiecutter shark will take a large bite out of the predator. Predator such as sperm whales have been seen to have bites on their body caused by the cookiecutter shark.
Bioluminescence is also thought to be used in communication between intra-species, as well as for mating; for example, Caribbean ostracods have been studied for using bioluminescence in mating displays. The study showed that there are species-specific patterns of signalling to attract mates, and this pattern is produced on the upper lips of the males to attract the females. The lure on the anglerfish is used not only for hunting, but is also used to attract mates.
Why should we care about bioluminescence?
71% of the earth surface is covered in water, with 96.5% belonging to oceans. With only 5% of the total ocean explored, understanding bioluminescence is highly important. Understanding behaviour can allow for scientists to find new species using lures; Edith Widder for example used lures to get the first live recording of a giant squid. Understanding how bioluminescence works can also help humans, because a reduction in electrical lights due to the use of bio-lighting, can in turn reduce energy usage and costs globally.