Light and Darkness

One of the mysteries of science is the question of bioluminescence [1]. Why do some ocean organisms light up? As Sönke Johnsen describes in his book “Into the Great Wide Ocean” [2] the open ocean far from the shore and miles above the seafloor is a vast habitat that is the home of peculiar and alien life forms like giant squids, transparent fish and bioluminescent anglerfish.

So why do some ocean organisms have the ability to create their own light? Well, because there is total absence of light in the deep sea at the bottom of the ocean. Sunlight does not penetrate the ocean beyond 1,000 meters. The majority of the ocean, below that depth of 1,000 meters, is characterized by complete darkness and tremendous pressure. The deeper you go, the darker it beomes, and below 1 km there is no light anymore.

One way to adapt to the darkness which does not allow any vision is the use of sound to locate objects based on their echoes. Sperm whales are well adapted to deep-sea diving and use echolocation to locate prey in the perpetual darkness. Another way is bioluminescence, which is also an adaptation to the perpetual darkness of the deep sea.

Just like sunglasses are an adaptation to too much light, creating your own light is an adaptation to too little light. Light helps to find food and mates in the darkness. Remember if anything is happening in nature it is usually either supper or pairing time. The bioluminescent octopus, the firework jellyfish, the angler fish and other animals living in the deep see use bioluminescence for this purpose [3].

The firework jellyfish which is shown in the video above has a spectacular and stunning appearance that resembles a burst of fireworks. It lives at depths of 1,200 to 1,500 meter where the eternal darkness begins. Some jellyfish who are able to generate light live near the surface (where it can be useful in the night), for example the crystal jellyfish, but bioluminescence is more common in the deep sea where light begins to fade. Already Charles Darwin observed that the ocean at some places can glow in the dark:

While sailing in these latitudes on one very dark night, the sea presented a wonderful and most beautiful spectacle. There was a fresh breeze, and every part of the surface, which during the day is seen as foam, now glowed with a pale light. The vessel drove before her bows two billows of liquid phosphorus, and in her wake she was followed by a milky train. As far as the eye reached, the crest of every wave was bright, and the sky above the horizon, from the reflected glare of these livid flames, was not so utterly obscure, as over the rest of the heavens.

[1] The Where, the Why, and the How: 75 Artists Illustrate Wondrous Mysteries of Science, Jenny Volvovski, Matt Lamothe, Julia Rothman

[2] Into the Great Wide Ocean: Life in the Least Known Habitat on Earth, Sönke Johnsen, Princeton University Press, 2024

[3] The Radiant Sea, Steven Haddock, Sönke Johnsen, Abrams Books, 2025