Giant Australian Cuttlefish: Masters of Disguise

The giant Australian cuttlefish (*Ascarosepion apama*, long known as *Sepia apama*) is the largest cuttlefish in the world. It reaches 50 centimetres in mantle length and up to a metre in total length with its tentacles outstretched, and can weigh more than 10.5 kilograms. It lives in temperate and subtropical Australian waters from Brisbane to Shark Bay, and around Tasmania, on rocky reefs, seagrass beds and sand and mud bottoms down to 100 metres, so divers can meet it from Sydney to Perth. In one place, Point Lowly in upper Spencer Gulf, it gathers to breed in numbers seen nowhere else, and in 2026 that gathering collapsed.

Built for Disguise

Cuttlefish are cephalopods, the class of molluscs that also includes squid, octopuses and nautiluses. Like all cuttlefish, the giant cuttlefish has eight arms and two feeding tentacles, three hearts and blue blood, and smooth, W-shaped pupils. Inside it carries the cuttlebone, a porous, chambered shell of aragonite: the animal controls its buoyancy by changing the ratio of gas to liquid in the chambers.

Its skin holds three layers of colour cells under nervous control: chromatophores (red to yellow), iridophores (iridescent) and leucophores (white). Together they let it change colour and pattern in a fraction of a second, and by raising papillae it can change the texture of its skin to imitate rock, sand or seaweed. Cuttlefish cannot see colour, but they can see the polarisation of light, and their skin can produce polarised patterns, which they may use to communicate.

Feeding

Giant cuttlefish are opportunistic, voracious carnivores that feed mainly on crustaceans and fish. A bioenergetics study found that they are mostly active by day but spend about 95 per cent of the day resting and hiding in crevices, foraging for only a few per cent of the time, which lets them put most of their energy into growth.

Breed Once, Then Die

Giant cuttlefish live for one to two years. Breeding begins with the southern winter: males drop their camouflage for bright, rapidly changing displays, and females lay their eggs one at a time, fixing each to the underside of a rock in a cave or crevice, where it hatches three to five months later. The species is semelparous: adults breed in a single season and die soon after, fasting and losing condition as the season goes on. In upper Spencer Gulf there are two life paths: some animals grow fast and spawn at seven to eight months old, while others grow slowly and return to spawn at two years.

The Upper Spencer Gulf Aggregation

Across most of their range giant cuttlefish breed in pairs or small groups, and loose spawning groups rarely exceed ten animals. The exception is the rocky shore around Point Lowly near Whyalla, the only known site where the species forms large breeding groups. In a normal year tens of thousands gather on the boulder reef there between May and August, and their eggs hatch from September to late October. SARDI's Mike Steer has explained that it is the only place in northern Spencer Gulf with enough rock for the eggs; everywhere else is sand and mangroves. In the aggregation males outnumber females by up to 11 to one. Big males guard females and egg sites, while smaller "sneaker" males mimic the colours and shape of females to slip past them and mate.

The aggregation was fished hard in the 1990s: commercial catches of around 200 tonnes a year in 1995 and 1996, and 245 tonnes in 1997, led to closures from 1998. A ban on taking cuttlefish in northern Spencer Gulf was reinstated in 2022 and became law on 22 December 2025, with fines of up to $20,000.

The 2026 Collapse

SARDI has estimated the aggregation since 1998, and numbers swing widely, from 13,500 in 2013 to 247,000 in 2020. They have fallen since, to 81,420 in 2024 and 63,734 in 2025. In 2026 SARDI's standard surveys in June and July found no cuttlefish at all, which it called unprecedented, and the Department for Environment and Water's estimate was 1,811, the lowest on record. SARDI found no evidence that water temperatures had been unusual, and given the algal bloom's known impact on calamari and abalone it considers the bloom the most likely explanation; the bloom pushed into the upper gulf around late December 2025 and January 2026. A 90-day exclusion zone from 1 July 2026 banned swimming, snorkelling, diving and anchoring at the breeding site between Black Point and Stony Point; check DEW's latest advice for its current status.

The population has recovered before. After the 2013 low it climbed past 100,000 by 2015, and in 2026 adults were still returning to the site, spawning and laying eggs. The state and Adelaide University have started a research project to understand the decline and the bloom's part in it.

Predators

Indo-Pacific bottlenose dolphins in upper Spencer Gulf have a specialised way of eating them. Researchers filmed a dolphin herding a cuttlefish onto sand, pinning and killing it, beating it with its snout until the ink drained, then pushing it along the sand to strip the skin and release the cuttlebone before eating it. Long-nosed fur seals and yellowtail kingfish also eat giant cuttlefish. Their defences are camouflage and, when that fails, ink: a cuttlefish can squirt a cloud to cover its escape, or a cuttlefish-sized blob of ink that acts as a decoy.

Conservation Status

The IUCN listed the giant cuttlefish as Near Threatened in 2009 because of a declining trend. In 2011 the federal Threatened Species Scientific Committee ruled the upper Spencer Gulf population ineligible for listing as a threatened species because it was not taxonomically distinct, although later work has found it genetically distinct (unpublished). The Cuttlefish Coast Sanctuary Zone at Point Lowly was added to the National Heritage List in February 2023.

Quick Facts

- Scientific name: *Ascarosepion apama* (formerly *Sepia apama*) - Size: mantle to 50 cm, up to 1 m including outstretched tentacles, more than 10.5 kg - Range: Brisbane to Shark Bay, including Tasmania, to 100 m deep - Lifespan: one to two years; breeds once, then dies - Status: IUCN Near Threatened (2009); taking cuttlefish banned in northern Spencer Gulf

Site Access and Logistics

Outside the breeding season you are most likely to meet a giant cuttlefish on its own, resting in a crevice or hovering over the reef, anywhere in its range. For the aggregation, see our Point Lowly guide: in a normal year it is a shallow shore dive or snorkel to about 5 metres between May and August, best in June and July, but check the exclusion zone and the latest survey news before you plan a trip. With so few animals left in 2026, local operators asked visitors to leave them undisturbed.

Sources

- Wikipedia: Giant cuttlefish - Wikipedia: Cuttlefish - Wikipedia: Cuttlebone - PIRSA: Giant Australian Cuttlefish management (pir.sa.gov.au, recreational fishing) - SARDI: 2026 Giant Australian Cuttlefish population estimate, advice note to PIRSA, 7 August 2026, PDF (pir.sa.gov.au) - SA Department for Environment and Water: More monitoring and research for Giant Australian Cuttlefish as numbers reach historic lows (21 August 2026) - SA Department for Environment and Water: More protection for Giant Australian Cuttlefish (30 June 2026) - InDaily: Temporary restrictions as giant cuttlefish numbers plunge (30 June 2026) - Whyalla City Council: Permanent protection for Giant Australian Cuttlefish now in place (whyalla.sa.gov.au, latest news) - Finn, Tregenza and Norman (2009): Preparing the perfect cuttlefish meal, complex prey handling by dolphins, PLoS ONE - Diving Adelaide: Whyalla Giant Cuttlefish Migration - Scuba Divers Federation of SA: newsletter of July 2026, sdfsa.net

Header photo: Two giant Australian cuttlefish, one displaying with its arms raised, on the reef at Point Lowly, South Australia, in May 2022, by Peter Crowcroft, CC BY 4.0, via Wikimedia Commons.