Get Your Weekly Digest

Receive the best podcast summaries and insights delivered to your inbox every week.

We respect your privacy. Unsubscribe anytime.

Radiolab

The Glow Below

with Edie Witter

Published October 31, 2025
View Show Notes

About This Episode

Host Molly Webster talks with deep-sea explorer and oceanographer Edie Witter about her decades studying bioluminescence in the deep ocean. Witter describes her first encounters with glowing deep-sea creatures, the many survival functions of bioluminescence, and the surprising evolutionary origin of light-producing bacteria. The conversation explores how light operates as camouflage, weapon, and communication system in the deep sea, and how interacting with bioluminescent life can profoundly affect human perception and awe.

Topics Covered

Bioluminescence Deep sea exploration Marine biology Ocean ecosystems Science communication Marine microbiology Evolution of bioluminescence Personal narrative

Disclaimer: We provide independent summaries of podcasts and are not affiliated with or endorsed in any way by any podcast or creator. All podcast names and content are the property of their respective owners. The views and opinions expressed within the podcasts belong solely to the original hosts and guests and do not reflect the views or positions of Summapod.

Quick Takeaways

  • Edie Witter's fascination with bioluminescence began when she handled a bright red deep-sea shrimp that squirted neon blue light into her numb hand.
  • Bioluminescence is energetically expensive, so deep-sea organisms use it for critical life-or-death functions like hunting, mating, defense, and camouflage.
  • Some fish use bioluminescent bacteria in specialized light organs and can control the light either mechanically, with eyelid-like shutters, or physiologically, by regulating oxygen supply.
  • Experiments suggest light-producing bacteria first gained an evolutionary advantage by activating DNA-repair enzymes to protect against UV damage, long before vision-based uses evolved.
  • In a darkened submersible, Witter discovered the "flashback" phenomenon, where a brief flash of light triggers surrounding marine snow and organisms to glow in unison.
  • Marine snow-falling organic particles seeded with bioluminescent bacteria and cyanobacteria-likely underlies much of the mysterious flashback glow in the deep ocean.
  • Many humans describe encounters with bioluminescence as some of their most meaningful life experiences because it feels like a direct, interactive contact with living systems.
  • Witter has even experimented with "talking" to bioluminescent shrimp using light flashes, eliciting courtship-like displays without knowing what message she is actually sending.

Podcast Notes

Introduction and setup of the deep-sea bioluminescence story

Host introduces Radiolab and today's focus

Molly Webster opens the episode and names deep-sea explorer and oceanographer Edie Witter as the central voice for the story[1:51]
• Molly greets listeners, identifies the show as Radiolab from WNYC, and frames the episode around a conversation with Edie Witter
Initial intention to discuss anglerfish immune systems and mating[2:11]
• Molly says she called Edie to talk about anglerfish and a weird thing she had heard about their immune systems and how they mate
• She quickly abandons that original anglerfish topic after hearing a different story from Edie

Edie's early deep-sea research context

Working on ships with a scientist who kept deep-sea animals alive[2:34]
• As a grad student in the 1980s, Edie went to sea on ships with a scientist who had developed a way to bring deep-sea animals up alive
• Normally, animals brought up in nets from the deep arrive dead because temperature changes effectively cook them alive
Thermally insulated closing cod end innovation[2:46]
• The scientist had built a "thermally insulated closing cod end" that attached to the end of a net, allowing animals to be brought up while staying cold and alive
• On deck, animals would be swirling around in an icy cold tub of water into which Edie plunged her hand until it went completely numb

First encounter with a glowing deep-sea shrimp

Description of the bioluminescent shrimp[3:03]
• One of the first animals she pulled out was a bright red shrimp about the size of a hamster
• The shrimp was squirting neon blue light out of tubes on either side of its mouth
• The light pooled in her hand and then dripped between her fingers back into the tub
Sensory impression of holding "dripping light"[3:24]
• Edie calls it "cold light," brilliantly blue when you are fully dark-adapted
• Molly remarks she usually thinks of light as contained (like in a light bulb) and not as something that can literally drip or pool
• Edie says the experience was revelatory and she could hardly believe such phenomena existed

Bioluminescence overlooked in marine biology at the time

Lack of coverage in textbooks[4:01]
• Edie recalls that at the time you could pick up a marine biology textbook and find no mention of bioluminescence
• She clarifies that scientists knew about bioluminescence but did not consider it important enough to include
Transition to a life's work chasing light[4:25]
• After that shrimp encounter, Edie spent the next 40 years and counting diving repeatedly in search of glowing creatures
• She says she stopped counting after about 300 dives to depths of 3,000 feet and is definitely in the hundreds
• She notes she has seen many things from submersibles that she is pretty sure nobody has ever seen before

Experiencing the bioluminescent deep in a WASP diving suit

Description of the WASP suit and first open-ocean dive

WASP suit design and origin[5:23]
• Edie's first open-ocean dive was in the Santa Barbara Channel testing a diving suit called WASP
• WASP was developed by the offshore oil industry for working on oil rigs down to 2,000 feet
• The suit is like an astronaut suit with a plexiglass bubble for the head and Michelin man-style arms
• It has no legs; instead, there is a pod with thrusters controlled by foot switches at the bottom of the suit
Edie's mindset before the first WASP dive[6:01]
• Her first big dive in WASP was only to 800 feet to ensure she wouldn't have a claustrophobic meltdown
• Molly comments that Edie is "so gutsy" and asks if she was nervous; Edie says she was
Being lowered and entering the dark[6:24]
• It was an early evening dive, and Edie was lowered off the back of the ship with an ungainly splash
• At 800 feet, she turned out the lights in the suit

First-hand view of the fireworks of bioluminescence

Visual spectacle when lights go off[6:43]
• As soon as she turned off the lights, she found herself in the center of a fireworks display of bioluminescence
• She saw sparkles, glows, and squirts of light all around her
• All the light she saw was in brilliant shades of blue, not yellow
Why blue dominates in the deep sea[7:21]
• Edie explains that blue light travels farthest through seawater, so most bioluminescence in the ocean is blue
Variety of bioluminescent forms[7:30]
• Some light appears like small smoke clouds, while others appear as little clouds of particles
Paradox of seeing nothing when the lights are on[8:26]
• When she turned on the suit lights, there was almost nothing visible in the water column that she could identify as the sources of all this luminescence
• You have to turn the lights off to see what you want to see, because the bright light overwhelms the faint glow

Why bioluminescence must matter for survival

Energy cost of producing light implies importance[8:36]
• Edie emphasizes that it takes a lot of energy to produce light, a resource that is critical to life
• Given the cost, she concludes that bioluminescence must be used for life-or-death functions
Starved bioluminescent copepod experiments[8:56]
• Experiments show that starved bioluminescent copepods give up the ability to make eggs before they give up the ability to make light
• In that copepod species, bioluminescence is used for defense, and they cannot live without it

Bioluminescence functions: feeding, mating, defense, and camouflage

Using bioluminescence to find food and mates

Built-in light organs as flashlights for hunting[9:25]
• Many fish, squid, and shrimp have built-in light organs next to their eyes that they use like flashlights to see in the dark and find food
Light organs as mate-finding beacons[9:30]
• Some animals have oddly shaped light organs that allow males to find females of their species to mate with

Bioluminescent burglar alarms

Flashing to attract a predator's predator[9:58]
• Some animals flash every light organ they have when caught by a predator, in what is called a bioluminescent burglar alarm
• The goal is to attract larger predators that might attack the attacker, giving the flashing animal a chance to escape

Counter-illumination and camouflage from below

Matching downwelling sunlight[10:25]
• Many deep-sea animals produce light from their bellies that matches the color and intensity of sunlight filtering down from above
• This counter-illumination camouflages their silhouette when seen from below
Quick adjustment to changing light conditions[11:03]
• If a cloud goes over the sun and dims the light, these animals can rapidly dim their bioluminescence to keep matching the background light
Cookie cutter shark as perfect cloaking example[10:31]
• The cookie cutter shark produces bioluminescence from its belly with very small light organs, creating an almost perfect counter-illumination pattern
• Edie describes this as the perfect cloaking device because the shark completely disappears when viewed from below

How do animals sense and match ambient light?

Light sensors above the eye[11:41]
• Some species have a light organ above the eye that serves as a reference so they can see the light organ against the background light and adjust their own output
Unknown sensing mechanisms in other species[12:06]
• Edie notes that for other animals, scientists do not know how they sense and match light because they lack obvious sensor systems

Complex light organs and anglerfish adaptations

Bearded sea devil anglerfish with dual light systems

Forehead lure vs. chin barbel[11:56]
• One type of anglerfish, the bearded sea devil, has a luminescent lure on its forehead used to attract food and a separate chin barbel
• The chin barbel's light is produced by intrinsic chemistry (luciferin and luciferase), while the forehead lure uses bioluminescent bacteria

Origin and control of bioluminescent bacteria in fish

Environmental acquisition of bacteria[13:26]
• Edie says many bioluminescent bacteria species are present in ocean water, and it is thought that anglerfish acquire them from the environment
Bacterial glow linked to respiration[13:26]
• Bioluminescent bacteria glow continuously because their light output is linked to their respiratory chain and continues as long as they have oxygen
Fish control bacterial light via oxygen or shutters[13:19]
• Some anglerfish control light output by regulating the amount of oxygen reaching the bacteria
• Other fish have mechanical shutters, such as a flashlight fish with an eyelid that closes over the light organ to block the glow
• Some species roll the whole light organ back into their head, like pop-up headlights on a Lamborghini

Bioluminescence as an indicator of life and ecosystem processes

Using light to infer life and carbon dynamics[14:04]
• Edie says she has always taken bioluminescence as an indicator of life and believes it has much to tell us about life in the ocean
• She suggests bioluminescence can help show where life is and how the ocean's carbon pump is functioning
Molly's sense of bioluminescence as "super intelligence"[14:33]
• Molly imagines having another eye on her body reading the environment and changing her appearance to match it, calling it "super intelligence" and "super evolution"

Evolutionary origins of bioluminescent bacteria

The original evolutionary puzzle of bacterial light

Why early bioluminescence seemed non-adaptive[19:58]
• Edie recalls that during her graduate years, scientists debated how bioluminescent bacteria could have evolved because a single bacterium does not produce enough light to be seen by any known organism
• Without visibility, it was hard to see how producing light could provide a selective advantage, especially given its energetic cost

Experiments mixing light and dark bacterial strains

Dark strains outcompete light strains under normal conditions[19:29]
• Polish scientists mixed a light-producing strain of bioluminescent bacteria with a dark (non-luminous) strain and grew them together in a dish
• Under normal conditions, the dark strain always overgrew the light strain because it did not spend energy on light production
UV irradiation reverses the advantage[20:03]
• When the mixed culture dish was irradiated with UV light, the outcome flipped and the light-producing cells gained the advantage
• Under UV exposure, the light-producing strain overran the dish

Role of photolyase and UV-damage repair

Light-activated DNA repair enzyme[20:38]
• The advantage came from an enzyme called photolyase that repairs DNA damage caused by UV light
• The light produced by a single dinoflagellate was sufficient to activate this light-activated enzyme
Original selective advantage unrelated to vision[20:42]
• Edie explains that the earliest selective advantage of bioluminescence had nothing to do with vision but with protection against UV damage in the upper ocean
• Hundreds of millions of years ago, tiny cells glowed invisibly as far as vision was concerned, yet the glow helped them heal UV-damaged DNA

Transition of bioluminescence function as life moves deeper

From UV protection to ecological interactions[20:47]
• As organisms moved deeper into the ocean where sunlight mattered less, the role of bioluminescence shifted
• Deeper-dwelling creatures began using glow to lure prey, deceive, hunt, mate, and survive in darkness

Flashback phenomenon and marine snow in the deep sea

Piloting Deep Rover and dark-adapted observation

Setting up deep dives to see natural light[22:00]
• In 1985, Edie piloted a single-person submersible called Deep Rover
• She trimmed the sub to neutral buoyancy and tried to make it as dark as possible, even blacking out tiny indicator lights with tape
• Initially, after waiting, she saw nothing but total blackness
Thrusters reveal vortices of light[22:41]
• When she activated the thrusters, vortices of light swirled up out of the thrusters and particles streamed back over the sphere
• The movement of the sub disturbed organisms and particles, triggering bioluminescent responses

Discovery of the "flashback" phenomenon

Flashing the lights on and off[23:15]
• Later dives showed that simply flicking the sub's lights on and off could stimulate bioluminescent responses
• With a pattern of on-off, on-off light, on the second flash everything surrounding the sub seemed to light up in unison and then fade out together
• Edie and colleagues call this synchronous response the "flashback phenomenon"

Marine snow as likely source of much flashback light

Nature and role of marine snow[24:01]
• Marine snow consists of particulate organic matter that filters down through the ocean and forms the base of the deep-sea food web
• It is formed as plankton that photosynthesize at the surface die and sink as flocculent white particles that resemble snow
Bioluminescence from bacteria and cyanobacteria in particles[24:11]
• Some marine snow is bioluminescent because it is colonized by bacteria that glow
• When a particle is bumped, oxygen is introduced into the microenvironment and the bacteria emit light
• Cyanobacteria within marine snow may respond to light by photosynthesizing and producing oxygen, which can further stimulate bioluminescence
Other identifiable organisms contributing to flashback[25:33]
• Edie says that most of the flashback seems to come from marine snow, but occasionally she can identify specific organisms like tomopterid worms
• Tomopterid worms are recognizable because their bioluminescence is yellow, almost gold in appearance

Edie's interpretation: natural light exchanges and bacterial strategy

Flashback as a naturally occurring interaction[25:33]
• Edie believes the flashback phenomenon is not unique to the presence of a submersible but is something that naturally occurs between deep-sea creatures
• She imagines a fin brushing the water, or a light on a fish's belly flashing, triggering similar waves of light in surrounding particles
Bacteria glowing to be eaten[25:21]
• Edie asserts that the bacteria glow in order to be eaten, because being ingested reintroduces them into the food-rich environment of a fish gut
• She explicitly notes she is being anthropomorphic but says this strategy benefits the bacteria
Becoming part of the deep-sea dance[25:49]
• When she triggers flashback with her sub's thrusters or lights, Edie feels she has become part of the deep-sea system she is observing
• She likens her thruster to an animal's fin and her flashlight to a lure, making her part of the "dance" of light

Bioluminescent experiences, communication, and human awe

Edie's emotional response to deep-sea light

Awe and desire to stay in the deep[27:06]
• Edie says that although she is not a poet, she often feels a tremendous sense of awe during these dives
• She frequently feels, "I want to stay here; I want to understand this," underscoring her curiosity
Curiosity as a human survival trait[27:37]
• Edie reflects that humans are born as strangers in a strange land, knowing nothing, and survive by exploring and sharing information
• She believes something innate in humans responds strongly to this kind of exploration, and others she has taken down have had similar experiences

Molly's personal pond bioluminescence story

Glowing pond edge in rural Ohio[27:48]
• Molly grew up in rural Ohio with a pond on her family's property
• On her last night before her parents sold the property, she walked at night to the pond and saw that the entire edge where water met soil was glowing
• She had never seen that happen before and believes it must have been bioluminescence
Edie's reaction and speculation[28:38]
• Edie suggests that must have been bacteria associated with something organic in the water but says such visible shoreline glow is rare
• She calls Molly's pond story an interesting mystery that needs to be solved and says she cannot say for sure what it was
• Molly recalls waking her parents to come see, and they all stood together looking at the glowing edge of the pond

Meaningfulness of bioluminescent encounters

Bioluminescence as a peak life experience[28:16]
• Edie finds it interesting that people who experience bioluminescence often rank those moments among their most meaningful lifetime experiences
• She describes it as interacting with life itself, "life illuminated," with an interactive component where your movements make things happen
Merlin-like feeling in bioluminescent bays[29:47]
• In a bioluminescent bay, running your hand outside a kayak produces sparkles from each fingertip
• Edie says this feels like being Merlin, because your gestures appear to magically conjure light

Light as communication in the deep sea

Framing bioluminescence as communication[30:20]
• Molly remarks that these interactions feel like a deep communication, as if you and the glowing life are saying something to each other
• Edie calls bioluminescence a form of communication and provocatively suggests it may be the most common form of communication on the planet, depending on how one defines communication
What is being communicated remains unclear[30:59]
• Edie says she focuses most on what light is communicating back to her but admits she often has no idea what the message is

Attempting to "talk" to luminescent shrimp

Experimenting with light signals[30:16]
• Edie recounts a case with deep-sea shrimp where she got a strong bioluminescent response by flashing light at them
• She says she had no idea what she was saying but was pretty sure it was "something sexy" based on the nature of the shrimp's response
Shrimp courtship-like display[31:48]
• The shrimp responded by putting out a string of glowing dots in the water, which Edie describes as a string of glowing pearls
• She notes that this is the kind of display a crustacean would produce in a titillated, come-hither context
Continuing to probe an unknown light language[31:57]
• Molly observes that when Edie is down there flashing her light in patterns (like three flashes), she does not know what that might mean in shrimp language
• Edie confirms that she experiments by flashing and "we'll just see what happens," and calls it the best job in the world

Production notes, Edie's current role, and related content

Episode production and Edie's current position

Credits and Edie's organizational role[32:34]
• The episode was produced by Maria Paz Gutierrez with original reporting and production help from Molly Webster
• Molly thanks Dr. Edie Witter, describing her as the CEO and senior scientist at the Ocean Research and Conservation Association

Documentary about Edie and other deep-sea stories from Radiolab

Upcoming documentary featuring Edie[32:21]
• Molly notes there is a documentary called "A Life Illuminated" about Edie that is making the rounds at festivals, created by Sandbox Films and director Tasha Van Zandt
• She says Edie reports it contains some of the best underwater footage of bioluminescence ever recorded
Mention of commercial deep-sea dives and related Radiolab episodes[34:02]
• Molly mentions that commercial operations exist that can take people down about 2,000 feet, estimating costs around $50,000
• She points listeners to other Radiolab episodes involving the deep sea, including "Octomom" and "The Darkest Dark," which feature people connected to Edie
• Molly closes the episode by thanking listeners and signing off as Radiolab

Staff credits overview

Main hosts and leadership[34:00]
• A listener voice from Somerset, New Jersey reads the staff credits, naming Lulu Miller and Latif Nasser as hosts and Soren Wheeler as executive editor
• They mention Sarah Sandback (executive director), Pat Walters (managing editor), and Dylan Keefe (director of sound design)
Staff list and fact-checkers[33:46]
• The credits list the Radiolab staff producers and contributors by name, including Maria Paz Gutierrez and Molly Webster among others
• Fact-checkers named include Diane Kelly, Emily Krieger, Anna Pujol-Mazzini, and Natalie Middleton
Foundational and leadership support[34:00]
• Another listener voice from Frederick, Maryland reads acknowledgments of leadership support from the Simons Foundation and the John Templeton Foundation for Radiolab's science programming
• They note foundational support from the Alfred P. Sloan Foundation

Lessons Learned

Actionable insights and wisdom you can apply to your business, career, and personal life.

1

Phenomena that are energetically expensive, like bioluminescence, usually serve critical, non-obvious functions; when something persists despite high cost, look for the life-or-death advantage it confers.

Reflection Questions:

  • • What in your own work or life currently consumes a lot of energy or resources but might be providing a crucial benefit you haven't fully recognized?
  • • How could you more systematically investigate the real advantages and tradeoffs behind the most expensive processes in your business or daily routine?
  • • What is one costly habit or system you use that you could analyze this week to better understand whether its benefits truly justify its expense?
2

Turning off familiar sources of illumination-noise, distractions, or default perspectives-can reveal entire worlds of activity that were previously invisible.

Reflection Questions:

  • • Where in your life are you relying so heavily on one way of seeing (a metric, a tool, a habit) that you might be blinding yourself to subtle but important signals?
  • • How might deliberately creating periods of "darkness" (silence, offline time, or reduced inputs) change what you notice about your environment or relationships?
  • • What is one experiment you could run this month to temporarily remove a familiar source of information and see what new patterns or insights emerge?
3

Seemingly small or insignificant signals-like a faint glow from a single cell-can unlock powerful protective or adaptive mechanisms when they interact with the right environment.

Reflection Questions:

  • • What weak signals or early indicators in your life or organization are you currently ignoring because they seem too small to matter?
  • • How could you create conditions where these subtle signals are more likely to interact with supportive systems (people, tools, structures) and reveal their true value?
  • • What is one minor, easily overlooked pattern you've noticed recently that you could choose to investigate more deeply this week?
4

When you interact gently and curiously with complex systems, you stop being a detached observer and become part of the ongoing "conversation," which can change both what you see and what happens.

Reflection Questions:

  • • In which relationships or projects are you still behaving like an outside observer instead of an engaged participant?
  • • How might adopting a more exploratory, low-impact way of interacting with a system (team, market, community) reveal feedback and dynamics you currently miss?
  • • What is one complex system you're involved with where you could experiment with smaller, more sensitive interventions to learn how it really responds?
5

Profound awe and curiosity are not luxuries; they are core drivers of learning and innovation, motivating us to explore, persist, and share what we discover.

Reflection Questions:

  • • When was the last time you felt genuine awe, and how did it influence your motivation or creativity afterward?
  • • How could you intentionally design more opportunities for wonder and exploration into your work or personal routines?
  • • What is one area you're already curious about that you could commit to exploring more deeply over the next month, purely for the sake of learning?

Episode Summary - Notes by Charlie

The Glow Below
0:00 0:00