Octopus makes own quicksand to build burrow on

Species: The southern sand octopus (Octopus kaurna)
Habitat: The seafloor on the south-eastern coast of Australia

The southern sand octopus has taken hide-and-seek to a whole new level. It shoots jets of water into the seafloor creating quicksand that allows it to vanish.

A skilled architect, the octopus can build a mucus-lined home – complete with a chimney –20 centimetres down into the seabed, where it holes up during the day. It only emerges from its underground burrow at night to crawl over the seafloor and snack on small crustaceans.

Now, its unique burrowing technique has been revealed for the first time.

Other octopuses, including Octopus berrima bury themselves under a thin layer of sediment by digging into it with sweeping arm movements. But because they need direct access to the water column to breath, they remain close to the sediment surface, with their funnel sticking out.

The sand octopus has a rather different technique – it actively digs deeper though the soft sand and constructs an actual subsurface burrow, says Jasper Montana of the University of Melbourne. “This is the first known cephalopod to burrow,” he says.

Montana and his team first caught the octopus in the act of burrowing in 2008 when they were scuba diving at night in Port Philip Bay, south of Melbourne, Australia. When they shone a light on the octopus, the startled animal spread out its arms and repeatedly injected high-powered jets of water into the sediment using its funnel. This caused grains of sand to be temporarily suspended in water, making it like sandy water.

“The sediment became fluid like quicksand,” Montana says. The octopus put its arms into the sand while still pumping out water and eventually dived down into the sediment. The liquefied sand is likely to reduce drag and so allow the animal to burrow more quickly, using less energy, Montana’s team speculates.

The team later collected some specimens from the wild and put five of them in an aquarium designed to show what happened once the octopus was underneath the sediment (see diagram).

They found that the animal used its arms and mantle to push the sand away and form a burrow. It also extended two arms to the surface to create a narrow chimney to breathe through. Finally, it secured the walls of its new home with a layer of mucus that kept the grains of sand together so the entire thing maintained its shape.

The underground home was a true gem of cephalopod architecture – with the walls staying clear of the animal’s body.

“It actually was a pocket – like a space that the octopus could sit within,” Montana says. “So it was a true burrow.”

The team suspect the octopus may have evolved its building acumen to avoid predators. Unlike many other octopus species, it lacks camouflage skills. When they excavated much of the burrow under the sea, they found that the octopus could reconstruct its burrow and chimney within minutes by pushing the tips of two arms up to the surface.

With its thin arms and elongated body, it may also have evolved to access and feed on worms and crustaceans that live inside narrow burrows underneath the sand, Montana says. This is potentially an abundant yet unexploited food source.

In support of this theory, the team has spotted the octopuses probing down pre-existing worm holes with their arms and pushing their entire bodies inside.