Lights in the Deep

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Eerie Light of Bioluminescence Reveals Secrets of The Deep 

Essay by Cheryl Lyn Dybas : photographs by Edie Widder PhD

SUDDEN BLUE FLASHES.  Shooting beams of red light.  An eerie green glow.  All are surreal displays put on by deep-sea animals that are bioluminescent.  The sparkle of marine bioluminescence occurs in species from fish in the deep ocean to jellyfish and dinoflagellates in the shallows. They create light through the interaction of the enzyme luciferase and its substrate, luciferin (the terms are derived from the Latin word lucifer – lightbringer), or by hosting light-emitting bacteria.

Bioluminescence is found in only a few species on land, such as fireflies, but is common in the world’s oceans.  Some 75% of the animals in the sea are bioluminescent.

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Among that 75% are countless creatures still unknown to science. In 2019, on a research cruise to the deep waters of the Gulf of Mexico, scientists reviewed videos that showed a luminescent tentacle rising out of the inky blackness. They were stunned. The videos, obtained last June, revealed a giant squid in its natural habitat -- for the second time in history. The squid, approximately 3 – 3.5 metres long, was filmed at a depth of 759 metres by members of an expedition called “Journey into Midnight.”

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Researchers aboard ship sent the footage to Michael Vecchione, a cephalopod biologist at the Smithsonian National Museum of Natural History, to ask if he could identify the creature. Vecchione confirmed that it was likely a squid of the genus Architeuthis — a giant squid. “Giant squid” is a term that is sometimes used to describe a range of large squid, but according to Vecchione, only one squid in the genus Architeuthis can be considered a true giant squid. The benchmark is taxonomy rather than size. It is genetically a giant squid or not.

A remotely operated underwater camera that gives scientists a look into the deep ocean without disturbing the creatures that live there filmed the giant squid. The camera has an appendage: a lure modelled on a bioluminescent jellyfish. “We call it an electronic jelly, or e-jelly,” quips Edie Widder, chief executive officer and senior scientist at the Ocean Research & Conservation Association (ORCA) in Fort Pierce, Florida.

Some jellyfish create bioluminescent displays when attacked, so large predators in the deep sea look for this display and show up to feed on whatever is disturbing the jellyfish. It is called “burglar alarm” behaviour, and is the jellyfish’s scream for help, Widder says. “It’s the last-ditch effort of an animal caught in the clutches of a predator, that has no hope of survival other than to attract the attention of a larger predator that may then attack its attacker -- giving it an opportunity to escape.”

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For example, the midwater jellyfish Atolla sets off flashing swirling rings of light when threatened. That display alerts other predators more likely to eat the attacker than the jellyfish itself. “We hoped that this kind of display would be of interest to a visual predator like the giant squid,” says Widder.

Imagining what life is like for animals in the deep ocean is “an enormous challenge,” Widder says. “Because so many have fantastic eyes, vision must play a major role in their existence. The light they have evolved to see – bioluminescence – is equally fantastic.”  How can scientists find out how this living light is used? “The trick is to observe unobtrusively,” Widder says.

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Even at depths of more than a mile, below where the last traces of sunlight disappear at about 1,000 metres, the “Journey to Midnight” scientists saw evidence of the importance of vision in the large-eyed shrimp and fish attracted to the bioluminescent e-jelly.  “With each research cruise, we anticipate the next opportunity to view a new, previously unseen aspect of what life is like for the creatures that inhabit the largest living space on our planet,” says Widder.


A tale of two squids : The first giant squid video was captured in 2012 off the coast of Japan. That video and the one recorded in 2019 can teach scientists a lot about the squid, says Widder. Both were filmed at 759 metres below the surface -- the squids therefore live in a world that is very dimly lit.  The videos show that giant squids are active hunters  — they do not wait for food to drift by, which at one time was a hypothesis for how they obtained meals. They also have huge eyes (the largest eyes of any animal on the planet) and their attraction to the jellyfish lure means they are visual predators. 

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