THE ANIMAL FILES

Greater blue-ringed octopus

Hapalochlaena lunulata

Bright blue can be revealed by opening folds in the skin.

The greater blue-ringed octopus lives around mixed shallow seabeds in the Indo-West Pacific. Its rings are an iridescent warning display, made conspicuous through coordinated changes in skin folds and surrounding pigment.[1]

Follow the evidence ↓
Greater blue-ringed octopus
Image: Jens Petersen · Commons · CC BY 2.5
Arms
Eight[1]
Display
Iridescent blue rings[1]
Food
Small crustaceans and molluscs[1]

Reflective structures behind the blue

Reflecting layers create blue iridescence. Muscles alter folds around the rings to conceal or expose them, and nearby dark pigment increases contrast. A visible flash therefore involves more than adding blue pigment.[1]

Shelter among varied seabeds

Rubble, reef and sand form parts of its shallow-water habitat. The octopus shelters in cavities and comes out to feed or find a mate. Meal remains near a shelter can provide evidence of prey use, though not every shelter has the same surroundings.[1]

Arms bring prey to the beak

Small crabs, shrimps and molluscs are seized with the arms and brought toward the mouth. The beak can penetrate hard coverings. Females also hold and guard developing eggs beneath the arms.[1]

A COMMON MIX-UP

Are blue rings simply patches of ordinary blue pigment?

No. Reflective structures and the exposure of skin folds contribute to the display.[1]

TRY A LITTLE REASONING

Why would a dark border make a blue ring more conspicuous?

Reveal the explanation

It raises the contrast between the reflective ring and its surrounding skin.[1]

Sources & editorial notes

Adapted and translated from the cited Wikipedia revisions, with source-linked reasoning. Image credit and licence are listed separately above.

Adapted and translated from Wikipedia contributors; exact revisions are linked below. CC BY-SA 4.0

  1. Wikipedia contributors — Greater blue-ringed octopus (fixed revision; adapted and translated)
  2. NCBI Taxonomy — Hapalochlaena lunulata (identity and classification)

Last fact check:

One discovery leads to another

Giant Pacific octopusEight arms. A hidden beak. A very different way to be a mollusc.Enteroctopus dofleini ↗Vampire squidThe “vampire” collects sinking scraps instead of drinking blood.Vampyroteuthis infernalis ↗Common cuttlefishEight holding arms, two reaching tentacles.Sepia officinalis ↗