Bioluminescence: Lighting up the Depths of the Ocean
Rose Boardman
In the darkness of the ocean lies an underwater display of flashing colours and bright lights. But where does this light come from and why is it used?
Descending into the depths of the ocean. Photo: Caitlin Gilmour @_caitlingilmour
Despite the absence of sunlight, the deep sea happens to be the location of a dazzling display of colour and light through a process known as bioluminescence. Bioluminescence occurs through a chemical reaction that produces light energy within the body of an organism. For a reaction to occur, a species must contain luciferin (a molecule that produces light when it reacts with oxygen).
Bioluminescence is found in many marine organisms from bacteria, algae, jellyfish, worms, crustaceans, sea stars, fish, and even sharks! Usually, the animal contains the chemicals necessary for the reaction that produces bioluminescence. However, some animals ingest bacteria or other bioluminescent creatures to gain the ability to light up. For example, the Hawaiian bobtail squid (Euprymna scolopes) has a special light organ that is colonised by bioluminescent bacteria during birth.
So why do these animals light up?
Defence & Protection
Animals can produce their light as bright flashes, which may stun a predator. This gives the prey an opportunity to escape. In larger animals, a flash may be used to highlight size to intimidate predators. For example, the Black dragon fish (Idiacanthus atlanticus) will flash along its tail and fin. Other organisms will secrete the light in the form of a cloud to create a distraction while they disappear.
Bioluminescence can also be used to provide camouflage. Photophores on the bottom side of an animal can match the brightness and colour of the surface above. This makes it harder for predators searching for prey from below to see what they are looking for.
Sacrificial Tag
In this situation, an organism may choose to lose part of its glowing body to a predator. These lost body parts can continue to glow for hours in the predator’s stomach. The glowing tissue can draw attention to the predator, making it risky to consume bioluminescent prey. Some organisms then have the ability to regrow the missing body part resulting in a win-win situation for them whilst their predator suffers.
Feeding
Deep-sea species can use their light to lure prey towards their mouths, or light up the area nearby so they can detect their prey. Studies have shown that the octopus species; Stauroteuthis has luminous suckers to attract planktonic prey during feeding.
Communication
Bioluminescence is an extremely useful way for species to communicate. Depending on the conditions, a bioluminescent flash can be seen from tens to hundreds of meters away. Bioluminescence can also play a part in attracting a mate. The male Caribbean ostracod, (a tiny crustacean), uses bioluminescent signals on its upper lips to attract females.
These are just a few examples of the ways deep-sea species use bioluminescence (check out this documentary for more)