- Light and Life Below the Twilight Zone
- Mission Logs
- June 16
Trying to imagine what life is like for animals inhabiting the deep ocean is an enormous challenge. Because so many have fantastic eyes, it’s evident that vision plays a major role in their existence. And the light they have evolved to see – living light, called bioluminescence – is apparently equally fantastic. There are built-in flashlights for finding food, glowing lures for attracting prey, sexually dimorphic (i.e., different in males and females) light organs for attracting mates, nozzles that can squirt light into the eyes of an attacking predator, exquisite optical structures on the bellies of many fish, squid, and shrimp that can function as camouflage and a plethora of light organs with no known function. How can we ever hope to find out how all these extraordinary lights are used?
These are just a few examples of the remarkable diversity of light-emitting structures fish have evolved to help them survive in the dark. The dragonfish, top left, sports a lure that dangles from its chin and an array of light organs called photophores along its flanks. The bearded netdevil anglerfish, top right, has actually evolved two different kinds of bioluminescence – symbiotic bacteria in a light organ called an esca that protrudes above its nose and a highly branched beard that hangs from its chin and emits intrinsic (i.e., internally produced) bioluminescence. Proceeding clockwise, the hatchetfish has optically elaborate photophores (shown in inset) that direct light downward to mask its silhouette. Below it, a different kind of dragonfish has a lure that protrudes from its chin directly in front of its mouth. It also has a built in flashlight under each eye and photophores along its belly. At bottom center, the shining tubeshoulder fish is so named because it actually has a tube on it “shoulder” that can squirt out liquid light. The lures of some anglerfish like that bottom left and shown in detail in inset seem like they have evolved to resemble something else – but what? Above it, a flashlight fish displays the brilliant light organ for which it’s named. Because its luminescence originates from symbiotic bacteria that glow all the time, it has evolved a shutter that is essentially like an upside down eyelid, which it can use to shut off the light. Staring out at you from the center of the image are the amazing headlights found between the eyes of the common lanternfish, Diaphus. Most of what we think we know about how these fish use their phenomenal light producing equipment is based on speculation rather than direct observation. Image ©2019 Dante Fenolio. Download larger version (jpg, 217 KB).
The trick is to be able to observe unobtrusively. That’s what the Medusa is designed to do. By illuminating with red light that most deep-sea inhabitants can’t see and using an optical lure that imitates a common bioluminescent jellyfish, it aims to attract and see the previously unseen.
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A slickhead (Alepocephalidae) tentatively identified as Bathytroctes macrolepis inspects the e-jelly. These fish are rarely caught in trawls. Video courtesy of Edie Widder and Nathan Robinson. Download larger version (mp4, 4.1 MB).
For the first two deployments of the Medusa during this expedition, we put out more than a mile of line (1,830 meters;~6,000 feet), making these the deepest such observations ever made. Even at such an extreme depth, far below where the last traces of sunlight disappear around 1,000 meters (3,280 feet), we saw evidence of the importance of vision as we observed very large shrimp and fish attracted to the e-jelly. And on the third deployment, to 1,464 meters (0.9 miles), we recorded a shrimp spewing bioluminescence. As far as we know, this is the first time that behavior has ever been recorded in situ.
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A shrimp bumps into the Medusa and executes an escape response that includes spewing bioluminescence. Video courtesy of Edie Widder and Nathan Robinson. Download larger version (mp4, 5.4 MB).
With each recovery of the Medusa, we haul the line in anticipating the opportunity to see some new, previously unseen aspect of what life is like for the creatures that inhabit the largest living space on our planet.