More than 90% of deep-sea animals make their own light

Introduction: a claim that sounds too spectacular to be true

The total darkness of the ocean depths

Beyond 1,000 meters of depth, sunlight no longer penetrates the ocean. This is what's known as the aphotic zone, a world plunged into total, permanent darkness where pressure crushes everything, where temperature hovers near freezing, and where life nonetheless thrives in often bizarre forms. One claim keeps resurfacing in scientific publications: more than 90% of animal species living at these depths supposedly produce their own light.

The figure is striking, almost too round to be credible at first glance. It's exactly the kind of statistic that deserves methodical verification rather than acceptance on the strength of its virality alone. Let's check together where this number comes from and what it actually reflects in the scientific literature.

What the scientific research actually says

The research institute MBARI, the Monterey Bay Aquarium Research Institute, ranks among the most active organizations studying marine bioluminescence. Its researchers use remotely operated underwater vehicles to directly observe, in their natural habitat, creatures that could never be properly studied at the surface. Their systematic surveys spanning decades of exploring deep water columns form one of the most solid databases on how common this phenomenon really is.

What strikes me most about this claim isn't so much the number itself as what it reveals about our collective ignorance: we know the surface of the Moon better than the depths of our own ocean.

What bioluminescence actually is

A chemical reaction, not an external energy source

Bioluminescence is a living organism's ability to produce light through an internal chemical reaction, without relying on a heat source the way a conventional light bulb would. The mechanism generally relies on a molecule called luciferin, which reacts with oxygen in the presence of an enzyme, luciferase, producing light with remarkable energy efficiency and virtually no heat loss.

This process is not unique to a single group of animals: it is found in jellyfish, fish, crustaceans, squid, marine worms, and even certain bacteria that live in symbiosis with larger animals. This taxonomic diversity suggests that bioluminescence emerged independently many, many times over the course of evolution, a phenomenon biologists call convergent evolution.

A phenomenon that also exists at the surface, but differently

Contrary to popular belief, bioluminescence is not exclusive to the deep sea. It is also found at the surface, in certain algae and plankton that sometimes light up nighttime waves with a spectacular bluish glow, a phenomenon visible on certain beaches around the world under specific conditions. However, it is in the depths that this mechanism reaches its greatest diversity and its greatest functional usefulness.

The key difference lies in the total absence of ambient light in the deep sea: at the surface, bioluminescence supplements daylight or moonlight, while in the depths, it often constitutes the only source of light available within a radius of several kilometers, which explains why it plays a far more central biological role there.

Why so many species produce their own light

Luring prey in total darkness

The best-documented function of bioluminescence among deep-sea predators is attracting prey. The most iconic case remains the deep-sea anglerfish, whose luminous appendage dangling above its mouth acts as an irresistible lure for curious small fish, drawn to the light in a darkness where any light source potentially signals food or, conversely, a social cue.

Other species use similar but subtler strategies, projecting bursts of light from their mouths or lighting up specialized filaments to catch the attention of potential prey before striking. This diversity of predatory strategies shows just how thoroughly evolution has exploited every possibility offered by this chemical mechanism in an environment devoid of any other form of visual communication.

Camouflage through counter-illumination

Paradoxically, some species use light to make themselves invisible rather than visible. This technique, called counter-illumination, involves emitting light from the animal's belly that exactly mimics the intensity of the faint residual light coming from the surface, erasing the animal's silhouette as seen from below by a potential predator looking to spot it against the light.

This extremely sophisticated mechanism requires precise control of the light intensity emitted, capable of adjusting in real time to variations in ambient brightness depending on depth and time of day. Several species of squid and small mid-water fish master this technique with a precision that continues to astonish the marine biologists who study it.

Communication, an underrated use of light

Recognizing members of one's own species

In the total darkness of the deep sea, recognizing individuals of the same species poses a considerable biological challenge. Bioluminescence offers an elegant solution: many species have specific light patterns, unique to their species and sometimes even to their sex, which function as a genuine visual signature allowing them to recognize each other and choose a potential mate in the most complete darkness.

These coded light signals play a crucial role in reproductive behavior among many species of deep-sea fish, where finding a compatible partner in an environment this vast and this dark is already, in itself, a major challenge for the species' long-term survival.

A language more widespread than conventional sound or visual communication

According to data compiled by organizations such as NOAA, the American oceanic and atmospheric observation agency, bioluminescence may represent, in terms of the number of species that practice it, one of the most widespread forms of biological communication on Earth, far more common than the sound-based communication systems developed by marine mammals or the color-based visual communication systems used by land animals.

There is something deeply humbling about this finding: the most widespread mode of communication on our planet is neither sound nor speech, but a form of light-based language that we ourselves almost never perceive. It puts our anthropocentric view of what counts as "normal" communication in the animal kingdom into sharp perspective.

Scaring off or confusing an approaching predator

Some deep-sea species use bioluminescence defensively, emitting a sudden, intense flash of light meant to startle or confuse a predator about to attack. This technique, comparable to the effect of an unexpected camera flash in the dark, can be enough to create a split-second of confusion that allows the prey to escape to safer waters.

Other organisms go further, releasing outright luminous clouds into the water, an escape technique reminiscent of an octopus's ink cloud but with an added luminous component meant to further disorient the attacker while its prey quietly slips away into the surrounding darkness.

A distress signal that attracts bigger predators

A particularly ingenious strategy, documented in certain deep-sea jellyfish, involves emitting a light signal that does not directly scare off the threatening predator, but instead attracts an even bigger predator likely to turn on the original attacker. This strategy, sometimes nicknamed the marine world's "burglar alarm," illustrates the incredible sophistication of the ecological interactions unfolding in an environment often wrongly imagined as relatively simple and devoid of complex life.

This kind of behavior requires precise synchronization between triggering the light signal and perceiving immediate danger, suggesting sensory and nervous capabilities far more developed than what was traditionally attributed to organisms as seemingly simple as certain gelatinous deep-sea jellyfish.

Defending and fact-checking the science

The methodological challenges of deep-sea exploration

Establishing a precise statistic like the 90% figure requires observing a representative sample of species living at great depths, which remains extraordinarily difficult given the technical and financial constraints of deep-sea exploration. The remotely operated underwater vehicles used by institutes like MBARI allow for direct observation, but their field of view remains limited compared with the sheer size of the ocean volume involved.

Researchers therefore combine direct observations with genetic analyses, the study of specimens captured by specialized nets, and cautious statistical extrapolations to estimate the actual proportion of bioluminescent species at different depths. This multi-method approach, while rigorous, necessarily carries a margin of uncertainty that should be kept in mind when evaluating this kind of impressive statistic.

Verdict: a claim broadly corroborated by research

Whenever a spectacular number circulates on social media, I always prefer tracing it back to the primary source before repeating it: that discipline, more than anything else, is what separates a serious column from an amplified rumor.

Based on available scientific publications, notably those covered by the journal Nature in its thematic coverage of bioluminescence, the claim that more than 90% of animal species living deeper than 1,000 meters are bioluminescent appears broadly corroborated, although the exact proportion may vary depending on the geographic area studied and the precise depth range considered.

As a columnist, I always prefer to flag the margin of uncertainty rather than present a number as an absolute, set-in-stone truth: deep-sea science moves fast, and this percentage could very well be revised as new exploration technologies emerge.

What this discovery changes about our understanding of life

Challenging our intuitions about animal life

This biological reality deeply challenges the intuition that light would be a marginal phenomenon in the animal kingdom, reserved for a few curiosities like fireflies or glow-worms occasionally spotted at the surface. In the deep sea, it is actually the absence of self-generated light that is the exception rather than the rule among species catalogued to date.

This reversal of perspective shows just how much our understanding of living things remains skewed by our everyday surface experience, where sunlight dominates every other visual process by far. Exploring the deep sea means literally exploring a world governed by physical and biological rules different from those we intuitively know.

Potential applications for biomedical research

Beyond pure scientific curiosity, understanding bioluminescence mechanisms has already led to concrete applications in biomedical research, notably the use of fluorescent proteins derived from marine organisms as markers to visualize cellular processes in the lab. This discovery, originally derived from studying bioluminescent jellyfish, in fact earned a Nobel Prize in chemistry for the researchers who isolated and characterized it.

This kind of payoff illustrates why funding fundamental oceanographic exploration remains worthwhile even when its practical applications aren't immediately obvious: the most useful discoveries sometimes come from the most improbable and unexplored corners of our planet.

Conclusion: a phenomenon as verified as it is fascinating

What this fact-check reveals

At the end of this verification, the claim that more than 90% of deep-sea animals produce their own light broadly holds up against the available scientific sources, with the usual methodological caveats inherent to any estimate concerning an environment as hard to access as the ocean depths. Institutions like MBARI and NOAA continue refining these estimates as new exploration technologies emerge.

Above all, this finding is a reminder of just how much the ocean depths remain a largely unknown territory, where remarkably sophisticated biological phenomena unfold continuously, far from any human gaze, in a silence and darkness that are anything but empty or inert.

Why this should push us to explore even more

This kind of fact-check also illustrates why it remains essential to invest in scientific ocean exploration, a field that remains underfunded compared with other research priorities, even though most of our planet's habitable volume lies precisely within these still largely unexplored depths.

Every new oceanographic expedition led by institutions like MBARI or NOAA has a reasonable chance of revealing never-before-seen biological mechanisms, since the diversity of deep-sea life continues to surprise even the most experienced specialists in the field, year after year.

By Maxime Marquette, columnist

Sources

Primary sources

MBARI — Monterey Bay Aquarium Research Institute, marine bioluminescence research

NOAA — American oceanic and atmospheric observation agency

Nature — Thematic coverage of bioluminescence

Secondary sources

National Geographic France — Animals

Futura Sciences — Planet

Sciences et Avenir — Science news