I used to think bioluminescence was just nature showing off.
But here’s the thing—down in the deep sea, where sunlight hasn’t reached for roughly a hundred million years give or take, those glowing bodies aren’t just pretty. They’re sentences. They’re arguments. They’re sometimes even lies. Steven Haddock, a marine biologist at the Monterey Bay Aquarium Research Institute, once told me that we’ve probably seen less than one percent of the bioluminescent conversations happening below a thousand meters, and honestly, that statistic still keeps me up some nights. Because what we have seen is already strange enough: flashlight fish coordinating hunting patterns with synchronized pulses from light organs beneath their eyes, dragonfish sending species-specific flicker codes that work like encrypted military channels, and—this one still gets me—ostracods releasing glowing mucus trails in sequences so precise they might as well be Morse code.
Anyway, the firefly squid off the coast of Japan do something I can barely wrap my head around. They cover themselves in thousands of photophores—tiny light-producing organs—and they don’t just turn them on and off. They modulate intensity, color temperature, even the angle of emission.
When Light Becomes Language in the Aphotic Zone
Wait—maybe I should back up. The aphotic zone starts around 200 meters down, where photosynthesis becomes impossible and the pressure starts doing weird things to your equipment. That’s where roughly 76 percent of bioluminescent species live, according to a 2021 survey published in the Annual Review of Marine Science. And they’re not all using the same chemical pathway, either. Some use luciferase enzymes oxidizing luciferin substrates—the classic setup. Others have co-opted bacterial symbionts that glow on command. A few species we’ve catalogued, like certain siphonophores, seem to have evolved bioluminescence independently at least three separate times, which suggests the evolutionary pressure to communicate in darkness is almost unbearable.
Edith Widder, who designed the electronic jellyfish lure that first captured footage of a giant squid in its natural habitat, once described bioluminescent signaling as “the most common form of communication on Earth that humans almost never witness.” She’s right, and it’s frustrating.
The dinoflagellates do something especially clever—they flash when disturbed, which sounds counterproductive until you realize they’re not trying to scare off the predator. They’re trying to attract the predator’s predator. It’s called a “burglar alarm” display, and I guess it makes sense: if something’s about to eat you, your best bet might be lighting up the whole neighborhood so something bigger notices the commotion. Dolphins have definately figured this out, because researchers have observed them intentionally agitating bioluminescent plankton to illuminate prey fish silhouettes from below.
The Biochemical Vocabulary Hiding in Wavelength Variations
Turns out wavelength matters more than I initially thought.
Blue-green light travels farthest in seawater—it penetrates deeper and scatters less than other wavelengths. So most deep-sea bioluminescence sits in that 470-490 nanometer range, which is not coincidentally right where most deep-sea fish eyes have peak sensitivity. But here’s where it gets interesting: some species have evolved red bioluminescence, which is essentially invisible to most other creatures down there. Stoplight loosejaws, for instance, have red photophores under their eyes and are among the only fish that can actually see red light. They’ve basically created a private communication channel, a flashlight that only they can detect, letting them hunt and signal without eavesdroppers. That’s not just adaptation—that’s encryption.
Sönke Johnsen at Duke University has spent years measuring the specific flash patterns different species use, and the diversity is staggering. Some are simple blinks. Others are complex sequences with specific intervals, intensity ramps, even color shifts in species with multiple photophore types. Cookie-cutter sharks produce a ventral glow that mimics surface light patterns, essentially creating an invisibility cloak—except they leave their collar dark, which creates a small-fish silhouette that attracts predators they then bite.
What We Still Don’t Know About Chemical Conversations in the Abyss
I’ll admit I find the symbiotic relationships most unsettling. Hawaiian bobtail squid harbor Vibrio fischeri bacteria in specialized light organs, and they’ve evolved such precise control over bacterial populations—culling 95 percent of them each morning, then allowing regrowth—that the partnership works like a dimmer switch. The squid provides nutrients and a safe home; the bacteria provide camouflage glow. But do the bacteria benefit, or are they just captives with good accommodations? Nobody’s been able to answer that definitively.
And then there’s the question of grammar. If flash patterns are communication—and the evidence strongly suggests they are—then presumably there’s syntax, maybe even dialects. Researchers have documented regional variations in the bioluminescent displays of certain jellyfish populations separated by ocean basins, which implies either genetic drift or something closer to cultural transmission. We don’t know which yet. We might never know, because studying communication in an environment that hostile to humans and our equipment means we’re always working with incomplete data, fragmented observations, and more questions than when we started.
Honestly, sometimes I think the deep sea keeps its secrets on purpose.
