A field guide to the drifting light of the deep

Animals made
almost entirely
of water & light

Ctenophores — comb jellies — drift through the ocean, propelled by rows of shimmering, hair-like cilia. The rainbow that ripples down their bodies isn't a glow. It's sunlight, split apart.

Bathocyroe fosteri, a transparent deep-sea comb jelly with a red-pigmented gut, against the black of the mesopelagic ocean.

Not a jellyfish

They look like jellyfish. They really aren't.

Comb jellies were once lumped in with true jellyfish, but they belong to a phylum all their own. The tell is right there in the name: Ctenophora, from the Greek for “comb-bearer.”

Instead of stinging cells, ctenophores capture prey with colloblasts — cells that burst and glue tiny animals to a tentacle like flypaper. So while a jellyfish can leave a welt, a comb jelly can't sting you at all. And unlike a jellyfish, which pulses its bell to swim, a ctenophore rows itself along with eight strips of hair-like cilia — making it the largest animal on Earth to swim using cilia alone.

~186recognised living species, drifting in every sea from the poles to the tropics
1 mm – 1.5 mfrom near-invisible specks to the ribbon-shaped Venus's girdle, over a metre long
97–99%of a comb jelly's body is simply seawater held in jelly
7,000 mthe depth some species reach — from sunlit shallows to the crushing deep. Descend through the zones →

The signature trick

The rainbow is diffraction, not a glow

Watch a comb jelly and colour seems to run along its flanks in shimmering bands. It's tempting to call it bioluminescence — but it isn't. Each of the eight comb rows is packed with thousands of unusually long cilia, and as they beat in waves, they scatter passing light the way a CD or a soap bubble does. The colour is ordinary daylight, split into its spectrum by moving hairs.

These cilia are strange in their own right: most animal cilia are built on a “9+2” pattern of filaments, but a ctenophore's are “9+3,” with an extra strut thought to stiffen hairs that can grow up to 2 mm long.

Diffraction · by daylight

The travelling rainbow you see in photos and aquariums. No light is produced — moving combs simply bend the light already there. The genus Pleurobrachia, oddly, shimmers like this yet can't make its own light at all.

Bioluminescence · in the dark

A separate ability. Most species really can glow — usually blue or green, only visible in darkness — using calcium-triggered proteins called photoproteins. A few even release luminous “ink” when startled.

Try it yourself

Give the water a poke

When a comb jelly is jostled in the dark, it answers with a flash of blue-green light. Tap or click anywhere in the tank below to disturb this one and watch it fire back. Take the controls in the light lab →

Tap the water to disturb it
Flashes provoked0

Twelve things worth knowing

A pile of genuinely strange facts

01

No brain, no problem

Ctenophores have no brain and no central nervous system — just a net of nerves. In one species, some of those neurons fuse into a single continuous mesh with no synapses at all.

02

An alien nervous system

They lack the genes to make serotonin, dopamine and most neurotransmitters every other animal uses. They run mainly on glutamate — hinting their nerves may have evolved entirely separately.

03

Maybe the oldest branch

Many genome studies place comb jellies as the sister lineage to all other animals — the earliest offshoot of the animal tree. It's one of the liveliest debates in zoology.

04

They swim mouth-first

Where a jellyfish trails its bell behind it, a comb jelly usually rows forward with its mouth leading the way — eating in the direction it travels.

05

A part-time anus

The sea walnut's rear opening appears only when it needs to defecate, then vanishes without a trace. A “transient anus” found nowhere else in the animal kingdom.

06

Aging in reverse

Stressed adult sea walnuts can revert to an earlier larval form and grow up again — a Benjamin-Button trick shared with the famous “immortal” jellyfish.

07

Two can become one

Cut and placed together, two comb jellies can fuse into a single animal, merging their nervous and digestive systems — even when they're genetically different.

08

Steady on the statolith

Balance comes from a grain of mineral resting on four bundles of cilia. How the animal reacts depends on its overall “mood” — the state of its whole nerve net.

09

Spider-like hunters

Some hang still and fish with tentacle “webs,” some ambush like jumping spiders, and some dangle a single sticky droplet on a thread — much like a bolas spider.

10

Enormous appetites

When food is plentiful a comb jelly can eat ten times its own weight in a day, then simply shrink when times get lean and grow back later.

11

Half a billion years old

Despite bodies too soft to fossilise well, comb-jelly fossils reach back ~515 million years. Some ancient ones wore armoured skeletons with spines — long since vanished.

12

Weapon thieves

The genus Haeckelia eats jellyfish and keeps their stinging cells, re-arming its own tentacles with borrowed weapons instead of making colloblasts.

Real photographs

A gallery of comb jellies

Bathocyroe fosteri hanging in the dark deep sea, transparent with a red gut.
The deep-sea drifter — Bathocyroe fosteriPhoto: Marsh Youngbluth, NOAA / Ocean Explorer · public domain
A sea walnut, Mnemiopsis leidyi, showing bright iridescent comb rows.
The sea walnut, shimmering — Mnemiopsis leidyiPhoto: Bruno C. Vellutini · CC BY-SA 2.0
A warty comb jelly, Mnemiopsis leidyi, under aquarium light.
Up close at the aquarium — Mnemiopsis leidyiPhoto: Benoît Prieur · CC0 (public domain)

The many shapes of a comb jelly

Five types of comb jelly

Cydippids

Sea gooseberries · Pleurobrachia

The classic shape: a small, round, glassy body trailing two long, retractable tentacles used to fish for copepods. Many textbook comb jellies are built on this plan.

Lobates

Sea walnuts · Mnemiopsis

Two muscular oral lobes flank the mouth like cupped hands, sweeping in food. This group includes the famous sea walnut and the deep-sea Bathocyroe.

Beroids

Melon jellies · Beroe

Tentacle-less, sack-shaped predators that hunt other comb jellies — swallowing prey larger than themselves, with rows of stiff macrocilia acting like teeth in the throat.

Cestids

Venus's girdle · Cestum veneris

Flattened into a shimmering transparent ribbon that can stretch beyond a metre, rippling through the water with a slow, snake-like undulation.

Platyctenids

The odd ones out

Flat, bottom-dwelling and often mistaken for sea slugs or flatworms. Most lose their combs as adults and creep along other animals — some even reproduce by cloning off bits of themselves.

One shared blueprint

Whatever the shape

Different as they look, nearly all carry the same eight comb rows radiating from a balance organ at the top — a design half a billion years in the making.

Beyond the fun facts

A few real-world stories

Invasion

The comb jelly that broke a fishery

In the early 1980s, a sea walnut native to the American Atlantic coast turned up in the Black Sea, almost certainly pumped out of a cargo ship's ballast tanks. With few local predators, it bred fast and ate its way through fish eggs and the plankton that young anchovy rely on. Within about a decade, the anchovy catch had collapsed.

What eventually reined it in was another accident. A second comb jelly, Beroe ovata, arrived the same way — and Beroe eats other comb jellies. The sea walnut is now roughly kept in check by its own predator.

Origins

Did nerves get invented twice?

Comb jellies have a nervous system, but it runs on unusual chemistry — they're missing the genes for serotonin, dopamine and most of the signalling molecules other animals depend on. Several genome studies also place them at the very base of the animal tree, below even sponges.

If that's correct, it points to an odd conclusion: nerves and muscles may have evolved once in comb jellies and then again, separately, in everything else. Plenty of biologists aren't convinced, and the argument is still running.

Fieldwork

Known mostly from cameras

A lot of what we know about open-ocean comb jellies comes from photographs rather than specimens. The most delicate species tear apart in a collecting net and dissolve in preservative, so divers and deep-sea robots film them alive instead. Some have been recorded clearly enough to recognise as new species, yet still haven't been formally named.