Where to dive with octopus, cuttlefish and squid

What they are

Cephalopods are a class of marine molluscs that includes squid, cuttlefish, octopuses and nautiloids. Octopuses have eight arms, while squid and cuttlefish have eight arms and two tentacles. All can change colour quickly using tiny pigment-filled sacs in the skin.¹

Sizes vary widely. Male giant Australian cuttlefish can reach 50 cm and about 10 kg. The mimic octopus grows to about 24 inches (around 60 cm) and imitates animals such as lionfish, sea snakes and jellyfish. Blue-ringed octopuses are small, with the Australian Museum giving 4.5 cm for the blue-lined octopus; they are brown until disturbed, when their blue markings appear as a warning.^2,3,4

Where and when divers see them

The mimic octopus lives in shallow Indo-Pacific waters, foraging by day on open sand flats; it was first found in 1998 off Sulawesi, Indonesia. The blue-lined octopus lives on intertidal rocky shores and in coastal waters to 15 m, hiding under ledges and in crevices.^3,4

Giant Australian cuttlefish gather each winter to spawn at Point Lowly in northern Spencer Gulf, South Australia, the only known site where they form large breeding groups; in most years they number in the tens of thousands. Numbers vary a lot: surveys by the South Australian Research and Development Institute (SARDI) recorded a high of 247,000 in 2020 and a low of 13,500 in 2013, and in late June 2026 survey divers encountered fewer than 50.^2,5

Conservation status

The giant Australian cuttlefish, Sepia apama, is listed as Near Threatened on the IUCN Red List, in an assessment dated 11 March 2009 (as reproduced by SeaLifeBase). National Geographic's fact page lists the mimic octopus as not evaluated. South Australian law prohibits taking cuttlefish in northern Spencer Gulf. After numbers fell in 2026, a temporary 90-day exclusion zone from 1 July 2026 (to about late September 2026) banned recreational swimming, snorkelling, diving and anchoring in part of the Upper Spencer Gulf Marine Park, including the sanctuary zone between Black Point and Stony Point.^3,5,6,7

How to behave around them

There is no cephalopod-specific code of conduct here. The Coral Reef Alliance's general diving guidelines ask divers to stay neutrally buoyant at all times, not to touch or handle marine life, and never to chase or harass it. For the blue-lined octopus, a blue-ringed species, this is also a safety point: the Australian Museum describes it as shy and non-aggressive, and says it may bite only when picked up, injecting a paralysing venom, tetrodotoxin, which has caused several deaths in Australia.^4,8

• Never pick up or handle an octopus, especially a blue-ringed one.^4,8
• Do not chase or harass the animal.⁸
• Parts of the Upper Spencer Gulf Marine Park were closed to diving and snorkelling for 90 days from 1 July 2026; check South Australia's current rules before diving the cuttlefish aggregation.⁵