Invertebrates
Cuttlefish
Sepia officinalis · also called common cuttlefish, European cuttlefish
The common cuttlefish is a cephalopod — a relative of the octopus and squid — and one of the most sophisticated communicators and camouflage artists in the ocean. It thinks with a large brain, sees with strange eyes, and talks with its skin.
Overview
The common cuttlefish lives over sand, mud, and seagrass across the eastern Atlantic and the Mediterranean, moving into shallow coastal water to breed in spring and summer and retreating deeper in winter. It spends much of the day partly buried, flicking sand over its back with its fins, and hunts around dawn and dusk. Like all cephalopods it is short-lived, active, and intelligent, with a life built around a single breeding season.
The skin as a screen
A cuttlefish’s defining ability is near-instant control of its own appearance. Three stacked layers of skin cells — pigment sacs, iridescent reflectors, and white scatterers — are driven directly by the brain, letting the animal repaint itself in a fraction of a second. It uses this to:
- Disappear, matching the colour, brightness, contrast, and even texture of the seabed.
- Hunt, rippling hypnotic dark bands across its body to disorient prey.
- Communicate, flashing courtship patterns at females and threat displays at rival males — occasionally both at once, on opposite flanks.
Remarkably, it does all of this while being, as far as researchers can tell, colour-blind.
Body and senses
Behind the skin, the cuttlefish is built like its cousins: a soft body supported by the internal cuttlebone, eight arms and two longer retractable tentacles around a parrot-like beak, a funnel for jet propulsion, an ink sac for escape, three hearts, and copper-based blue-green blood. The eyes are large and sharp, with the distinctive W-shaped pupil, and the brain is among the largest of any invertebrate relative to body size.
Hunting
A hunting cuttlefish stalks within range, aims with both eyes, and then shoots out its two feeding tentacles — tipped with sucker-covered clubs — to snatch a crab or shrimp in around 30 milliseconds, drawing it back to the beak. Venom in the saliva subdues the prey, and the beak and rasping tongue break it apart.
Reproduction and death
Cuttlefish breed once, at the end of a one- to two-year life. Males compete with displays and, when outmatched, sometimes with disguise — a small male may adopt female colouring and posture to slip past a guarding rival. Females lay clusters of black, grape-like eggs, stained with ink, attached to seaweed and debris, then die. The male dies soon after too. The young hatch as fully formed miniature cuttlefish, already able to hunt and camouflage.
Cuttlefish and people
Sepia officinalis is fished commercially across its range for food, and its ink (sepia) was historically used as a drawing pigment. It is currently listed as Least Concern, though catches in some regions have declined, and it is a popular subject in public aquariums and in laboratories studying camouflage, vision, and cephalopod cognition.
Cuttlefish: common questions
How does a cuttlefish change colour so fast?
Its skin has up to millions of tiny pigment sacs called chromatophores, each ringed by muscles under direct nerve control from the brain. Expanding or contracting them shifts the visible colour almost instantly. Beneath sit reflecting cells — iridophores and leucophores — that add blues, greens, and whites and produce shine. The cuttlefish also changes skin texture, raising fleshy bumps called papillae to mimic rock or weed, so it matches both the colour and the three-dimensional look of its background.
Is it true cuttlefish are colour-blind?
Almost certainly. Their eyes contain only one type of light-sensitive pigment, which should make colour vision impossible, yet they match the colour of their surroundings with great accuracy. One leading idea is that their unusual W-shaped pupil spreads different wavelengths out by distance (chromatic aberration), and the cuttlefish reads colour from how sharply each wavelength focuses as it changes eye shape and pupil position.
What is the "passing cloud" display?
When hunting, a cuttlefish can send dark bands rolling across its skin toward its head like a moving barcode. This "passing cloud" appears to mesmerise or startle small crabs and fish, freezing them or provoking a twitch that gives away their position, just before the cuttlefish fires its two feeding tentacles. Males also use dynamic stripe displays to court females and to intimidate rivals — sometimes showing courtship colours on the side facing a female and aggressive colours on the side facing a competitor at the same time.
What is a cuttlebone?
The white, brittle oval often found on beaches and given to pet birds is the cuttlefish's internal shell. It is a rigid stack of thin calcium-carbonate layers filled with gas and liquid in varying proportions. By adjusting that ratio the cuttlefish fine-tunes its buoyancy, hovering effortlessly without constant swimming. It is an ancient structure — cuttlefish are related to the extinct, shelled ammonites and belemnites.