The long and lonely vigil of an octopus mother
Scientists believe four and a half years guarding one clutch of eggs is a record. Yi-Di Ng reports.
A dedicated deep-sea octopus mother off the coast of California has spent four and a half years tending to a single clutch of eggs – by far the longest brood time of any known animal, reports marine biologist Bruce Robison in the journal PLOS ONE.
“These deep-sea species have always been at the fringes of our knowledge,” says Mark Norman, cephalopod researcher and head of science at Museum Victoria. “I’m not surprised they brood for a long time but I’m surprised it was this long.”
In April 2007, while surveying a site some 1,400 metres underwater, Robison and his team from the Monterey Bay Aquarium Research Institute noticed a pale purple Graneledone boreopacifica octopus moving towards the surface of a rocky outcrop. A month later they returned to the spot and were greeted by the sight of the same octopus – easily identified by the scars on its arms – carefully guarding a clutch of eggs.
Here was a very rare opportunity – for the first time researchers could follow the progress of a deep-sea octopus nest from the moment the eggs were laid until they hatched. The team returned to the site 18 times over the next four and a half years. Not once did they find it unattended. The devoted octo-mum was always there, guarding the same clutch of eggs – a fact the team confirmed by measuring the eggs’ steady growth.
The last time Robison saw the octopus was in September 2011 by which time she was visibly shrunken, her skin loose and pale. By the following month the octopus and her babies were gone. She had spent 53 months tending to the clutch of eggs – much longer than the 14-month record previously held by an Arctic octopus.
Why such a long brooding time? One likely explanation is the temperature. The waters where Robison and his team found the octopus never run warmer than 3.4°C. “It’s like living in a fridge,” explains Norman. “At those temperatures everything slows down.”
In this deep, cold water there is also a survival advantage to brooding longer and emerging more mature, Norman adds. Unlike the shallow-dwelling octopus babies that hatch into an environment rich with food and plankton, deep-sea octopus hatchlings enter a nutrient-poor, pitch-dark world where food is scarce. The longer the babies spend developing in the egg the better equipped they are to survive when they hatch. The hatchlings of the Graneledone boreopacifica emerge much more mature than shallow-water octopus hatchlings – they are fully formed miniature adults, fully capable of fending for themselves.
But it’s a reproductive strategy that comes at the cost to the mother. Octopus eggs risk being suffocated by silt and debris in stagnant water, so the mother constantly keeps water moving around her eggs. Robison’s brooding octopus dedicated herself single-mindedly to this task, refusing to eat despite the abundance of prey animals around the nest. Even when the researchers offered her a crab she would not take it.
The octopus survives for so long without eating in a similar way a hibernating bear does, by using its fat reserves. But the octopus breaks down and digests her own body as she tends to her nest, which explains why Robison’s octopus gradually weakened and shrank, growing pale and translucent.
“It really demonstrates the extremes these creatures have to go to to make a living,” says Norman. But it’s clearly a strategy that works – Graneledone boreopacifica is one of the most abundant octopus species in the eastern North Pacific Ocean.
While an octopus that spends more than four years brooding a single nest may sound shocking, Robison notes that in the ocean’s cold, dark depths a long, slow life is probably the norm. “It indicates how little we really know about the deep sea,” he says.