Giant octopus wears jellyfish cape after it devours its owner | New Scientist
An elusive deep-sea giant has been filmed with its prey for the first time. It turns out it eats jellyfish and other gelatinous animals.
The octopus, Haliphron atlanticus, was filmed swimming docked on top of a medusa jellyfish, with its beak devouring its innards, while the medusa’s sticky tentacles were still hanging out of its mouth. The researchers think it might even be using the jellyfish tentacles as a handy feeding implement.
Little is known about H. atlanticus, and the researchers who filmed it using remotely operated vehicles have only seen it three times in as many decades. Most other octopuses eat more substantial prey such as fish and crustaceans, so it is a surprise to see this large species eating jellyfish.
What’s on the menu?
Most of what we know about the seven-armed octopus comes from specimens caught in trawl nets. It lives in deep open waters, growing up to 4 metres long and a weight of 75 kilograms. It is known to be eaten by sperm whales, swordfish and blue sharks. Males of the species have one of their eight arms permanently folded away, giving them the common name of seven-armed octopus.
Steven Haddock from the Monterey Bay Aquarium Research Institute in California and his colleagues have now filmed three of these octopuses in the wild. The team saw them feeding on jellyfish and also analysed the stomachs of five previously caught specimens, all of which contained gelatinous zooplankton and three of them contained jellyfish. This makes sense, because the open ocean is rich in such creatures, so the octopus makes use of what’s on the menu.
Haddock says that the discovery shows us just how complex the ocean food web is. It is a rare example of a marine animal that can grow large feeding primarily on gelatinous fauna, such as jellyfish, others being sea turtles and ocean sunfish, for example.
The way the octopus had the jellyfish arms freely hanging out, while keeping the bell in its mouth provides evidence for the idea that the octopus uses jellyfish as living tools, says the team.
The theory is that they use the jellyfish tentacles to ensnare more prey and then feed off whatever makes its way to the body of the medusa, with octopus beak docked inside it.
Plastic not fantastic
Haddock thinks the importance of jellyfish in marine food webs has been unappreciated and underestimated. Given that top predators feed on giant octopuses, this may be an important way of channelling energy from the bottom to the top of the oceanic food chain, his team argues.
A diet that relies on jellyfish might put the octopuses at risk from our plastic rubbish, says Ken Collins of the University of Southampton, UK. Plastic pollution is a massive problem on the sea surface where sea birds and turtles mistake plastic bags for jellyfish.
“We have leatherback turtles migrating from the Caribbean to the Irish Sea to feed on massive barrel jellyfish off the Welsh coast,” he says. “One of the sad consequences of turtles’ love of jellyfish is that they mistakenly eat plastic litter – plastic bags, balloons from mass publicity releases, which quite simply block their gut and they eventually die.”
When the bags eventually sink, it is supposedly the end of the story – but Collins wonders if the same could be happening to this octopus away from our sight.
Mike Webster from the University of St Andrews, UK, says that this study provides information about the basic biology and ecology of an underexplored ecosystem and will hopefully allow us to monitor and perhaps slow the impacts we have on this ecosystem.
“These trips have a real feeling of exploring the frontier in a way that research expeditions on land perhaps no longer provide, in part because the terrain and the animals that are encountered are so often unfamiliar and surprising,” says Webster.