The Secret Life of Cuttlefish

Not a Fish

Despite the name, the cuttlefish is not a fish. It is a cephalopod — a class of molluscs that also includes octopuses and squid. The 'bone' you find on beaches (the cuttlebone) is an internal shell: a porous, gas-filled structure that the cuttlefish uses to regulate buoyancy, adjusting the ratio of gas to liquid in the chambers to rise or sink without expending energy.

Cuttlefish belong to the order Sepiida, with over 120 species ranging from the giant Australian cuttlefish (Sepia apama, reaching 50 cm mantle length) to the flamboyant cuttlefish (Metasepia pfefferi), a small Indo-Pacific species that produces some of the most dramatic colour displays in the ocean — and whose muscles are highly toxic, making it one of the few toxic cephalopods known.

Chromatophores and the Speed of Colour

Cuttlefish skin contains three layers of specialized cells:

• Chromatophores: Sacs of pigment (yellow, orange, red, brown, black) controlled by direct muscle attachment. A single cuttlefish has up to 10 million chromatophores. When a muscle contracts, the sac expands and colour becomes visible. The speed of individual chromatophore response is measured in milliseconds.
• Iridophores: Structural colour cells that reflect light through thin-film interference — creating iridescent blues, greens, and silvers.
• Leucophores: Reflective cells that produce white or near-white colouring by broad-spectrum reflection.

The combination allows a cuttlefish to produce what is functionally a high-definition moving image on its skin — patterns, textures, gradients, and moving waves of colour — in real time.

The Paradox: Colourblind in Colour

Cuttlefish are colourblind. Their eyes contain a single type of photoreceptor — unlike human eyes, which have three (red, green, blue). Yet cuttlefish produce extraordinarily accurate colour matches to their background.

The proposed explanation, confirmed in research from the Woods Hole Marine Biological Laboratory: cuttlefish use chromatic aberration — the way different wavelengths of light focus at slightly different depths — sampling different wavelengths by rapidly adjusting their distinctive W-shaped pupil, effectively creating colour information from a single receptor type.

Hunting and Intelligence

Cuttlefish are ambush predators, using camouflage to approach prey (typically crustaceans and small fish) before striking with their two long tentacles. The strike is ballistic, extended in milliseconds from a retracted position inside the eight shorter arms.

Cognitive testing has demonstrated that cuttlefish possess impulse control, working memory, and individual learning transfer. They are likely the most cognitively complex invertebrate regularly encountered on reef dives.

Where to Find Them

Cuttlefish are found in shallow coastal waters across the Indo-Pacific, Mediterranean, and Atlantic. The largest reliable aggregation is the Spencer Gulf, South Australia — during the Australian winter (May-August), giant cuttlefish (Sepia apama) aggregate in their thousands at Whyalla for breeding. Elsewhere: Lembeh Strait (flamboyant cuttlefish in muck diving), Komodo (broadclub cuttlefish common on reef tops), and Mediterranean (common cuttlefish in rocky subtidal zones).