Gloomy Octopus: Facts, Octopolis and Diving

Sweep a torch along the base of a Sydney reef at dusk and sooner or later it will catch a pair of pale eyes staring back from under a ledge. Then the arms unfurl, mottled grey above and rust red beneath, sliding out of a den ringed with rubble and empty shells. The Australian Museum notes that those white eyes and orange-rust arms are often the first thing you see as this octopus emerges from its lair.

This is the gloomy octopus (Octopus tetricus), also called the common Sydney octopus, and the researchers who study it describe it as the most common local octopus in NSW waters. For east coast divers it is the octopus: an easy find on shore dives, a regular star of [night dives](https://scubadownunder.com/blog/the-enchantment-of-night-diving-in-australia) and, thanks to two extraordinary sites in Jervis Bay, the animal that challenged the idea of the solitary octopus.

## Identifying the gloomy octopus

The name comes from its large eyes, which can give the animal a doleful look. Its skin is typically grey to mottled brown, and the undersides of its long, unequal arms are a rich rust red. It can raise papillae across its body to produce a spiky outline, a trick it often uses when imitating seaweed.

It is a big octopus. The Australian Museum gives an arm span of up to 2 m, and researchers have reported animals of more than 3 kg from New South Wales waters.

Its traditional range runs from southern Queensland to southern New South Wales, plus [Lord Howe Island](https://scubadownunder.com/dive-sites/lord-howe-island) and northern New Zealand. It has since pushed south, reaching Victoria after about 2000 and eastern Tasmania in 2006, a shift thought to be associated with the southerly extension of the warm East Australian Current.

## Octopolis and Octlantis: Jervis Bay's octopus "cities"

Octopuses are usually thought of as loners, and the gloomy octopus was no exception. Then in 2012, Peter Godfrey-Smith and Matthew Lawrence described a site in Jervis Bay where gloomy octopuses had lived together at high density over the long term. Their dens were dug into a bed of scallop shells that had built up around a small human-made object. The site became known as Octopolis.

In 2016 a second site turned up elsewhere in Jervis Bay. Nicknamed Octlantis, it sits around three rocky outcrops and has no human object at its centre. On the day it was found, 15 octopuses were present along with several empty dens.

Both sites appear to grow through positive feedback. Octopuses bring scallops home to eat, and the discarded shells make better building material than the soft sediment around them, so more octopuses can dig in. Dens can be narrow vertical shafts more than 40 cm deep, lined with shells and sometimes as little as 20 cm apart. Around the rims, the octopuses pile barricades that have included sponge fragments, beer bottles, fishing gear and even the researchers' cameras.

The scientists involved are clear that the "city" label is misleading: no buildings, fences or artworks have been made. What they found is arguably more interesting, a place where a supposedly solitary animal has to live with the neighbours.

## Signals, scuffles and thrown shells

That crowding has opened a rare window into octopus social life. A 2016 study in Current Biology, based on field observation of the octopuses' encounters, showed that gloomy octopuses signal their intentions. Darker colours went with aggression, often combined with standing tall with the mantle raised high. When a dark octopus approached another dark one, the encounter was more likely to escalate to grappling; when it approached a paler one, the paler animal more often retreated.

In 2022 the same group reported something stranger. Octopuses at the site gather silt, shells or algae in their arms and propel them away with a jet from the siphon. In about 21 hours of footage filmed in 2015, the team logged 102 throws, most of them by two females. Some hit other octopuses, and one male who was hit repeatedly began ducking as the thrower got ready. The authors describe targeted throws as a form of tool use directed at other octopuses.

## A master of camouflage

The gloomy octopus changes both the colour and the shape of its skin to blend in, including to imitate seaweed. If you have watched a [giant cuttlefish](https://scubadownunder.com/blog/the-giant-cuttlefish-masters-of-disguise-and-the-shallow-sea) flicker through its patterns, you have seen the same family talent at work.

## A short, intense life

Gloomy octopuses live fast. A study of the newly established population in Tasmania estimated a lifespan of about 11 months. Females lay numerous small eggs, 1-2 mm long, in strings attached to the roof of rock crevices, and these hatch into planktonic young. Octopus mothers care for their eggs without feeding until they die, so a female guarding eggs is in the final chapter of her life. Sexual cannibalism has been recorded too, with females eating males after mating.

## Where divers meet gloomy octopuses

Reef Life Survey divers record the species frequently, at 11.6 percent of the reef sites they survey within its range, from the shallows to about 30 m, with photo records from Jervis Bay, Port Stephens, Narooma and Sydney. It lives on intertidal rocky shores as well as subtidal reefs, and the Australian Museum notes it may spend much of its life on soft sediment, moving onto rocky reef more during the breeding season.

In Sydney, [PADI's guide to the city's shore dives](https://blog.padi.com/8-great-shore-dives-in-sydney-australia/) lists octopuses among the critters hiding in the overhangs and crevices at Freshwater, on the northern beaches.

Two habits improve your odds. This octopus mostly emerges at night to feed on crabs, snails and bivalves, so dusk and night dives are prime time. By day, look for its lair: a hole under a ledge surrounded by collected rocks and rubble, with a scatter of freshly drilled prey shells nearby.

## Other octopuses divers see in Australia

- **Blue-ringed octopuses** (Hapalochlaena) are small and the one group of octopuses known to be dangerous to people. SDU's [blue ring octopus guide](https://scubadownunder.com/blog/blue-ring-octopus-the-oceans-tiny-assassin) covers them in detail. - **Maori octopus** (Macroctopus maorum): a large, smooth-skinned, dark red octopus of southern temperate waters, recorded to 198 cm and a major predator of rock lobsters. - **Pale octopus** (Octopus pallidus): a southern species that sits on sand among sponges and other attached invertebrates, often raising its skin into long, spiky protuberances. - **Day octopus** (Octopus cyanea): Reef Life Survey divers have photographed it in Torres Strait, at Lord Howe Island and in NSW, as well as in Indonesia.

## Are gloomy octopuses dangerous to divers?

Not really, but the beak deserves respect. Divers Alert Network notes that scientists now understand all octopuses may be venomous to some degree, though most use it on their prey. Of the octopuses Australian divers meet, only the blue-ringed group is known to be dangerous to people.

A bite is still possible if an octopus is grabbed or cornered. Divers Alert Network (DAN) says bites from most octopus species are generally not problematic, yet its incident files include a diver who picked up a juvenile giant Pacific octopus bare-handed and was bitten. The hand swelled, became infected and took about seven weeks to heal. DAN's advice: do not handle marine life, wash any marine bite straight away with soap and clean water, and get it checked by a doctor.

## Diving responsibly with gloomy octopuses

- **Let the octopus set the distance.** Some will watch from the den or even edge closer. Reaching in or pulling one out stresses the animal and invites a bite. - **Leave the den alone.** The rocks and shells around the entrance are its defences, not loose debris. - **Know the rules.** In NSW the recreational bag limit is 10 octopus, and octopus cannot be taken from ocean rock platforms anywhere in the state or from rock platforms in Sydney Harbour.

The gloomy octopus has not been evaluated for the IUCN Red List and remains common through much of its range: a familiar neighbour that still has plenty to teach anyone patient enough to watch.

## Sources

- [Australian Museum: Common Sydney Octopus](https://australian.museum/learn/animals/molluscs/common-sydney-octopus-octopus-tetricus/) - [Reef Life Survey: Octopus tetricus, gloomy octopus](https://reeflifesurvey.com/species/octopus-tetricus/) - [Scheel et al.: Octopus engineering, intentional and inadvertent](https://pmc.ncbi.nlm.nih.gov/articles/PMC5824970/) - [Scheel, Godfrey-Smith & Lawrence 2016: Signal use by octopuses in agonistic interactions (Current Biology)](https://doi.org/10.1016/j.cub.2015.12.033) - [Godfrey-Smith et al. 2022: In the line of fire, debris throwing by wild octopuses (PLOS ONE)](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0276482) - [The Conversation: Accident and intent in an octopuses' garden](https://theconversation.com/did-they-mean-to-do-that-accident-and-intent-in-an-octopuses-garden-85462) - [Ramos et al. 2014: Body size, growth and life span of Octopus tetricus (PLOS ONE)](https://pmc.ncbi.nlm.nih.gov/articles/PMC4121162/) - [Reef Life Survey: Maori octopus](https://reeflifesurvey.com/species/macroctopus-maorum/), [pale octopus](https://reeflifesurvey.com/species/octopus-pallidus/) and [day octopus](https://reeflifesurvey.com/species/octopus-cyanea/) - [NSW DPIRD: Saltwater bag and size limits](https://www.dpird.nsw.gov.au/fishing/recreational/fishing-rules-and-regs/saltwater-bag-and-size-limits) - [Divernet and DAN: How to deal with an octopus bite](https://divernet.com/medical/dan-column-octopus-bite/)

Header photo: A gloomy octopus (Octopus tetricus) tucked under a rock in Clovelly Pool, Sydney, showing its pale eyes and rust-red arm undersides. by Sylke Rohrlach, [CC BY-SA 4.0, via Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Gloomy_Octopus-Octopus_tetricus.JPG).