Human mothers spend nine long months watching what they eat and lugging around a growing belly as they wait for babies to be born, but that's nothing compared to what a mother deep-sea octopus endures.

At 4½ years, an octopus has claimed the record for the longest known egg-brooding period or pregnancy in the animal kingdom.

A deep-sea octopus observed by researchers from the Monterey Bay Aquarium Research Institute in California sat with her arms wrapped around her eggs for 53 months, apparently without eating the entire time.

How the mother survives that long is still the biggest mystery. - Brad Seibel, University of Rhode Island

"We were just amazed," said Brad Seibel, an associate professor of biology at the University of Rhode Island who co-authored the study, published today in the journal PLOS ONE.

"How the mother survives that long is still the biggest mystery."

The mother octopus was curled over a clutch of about 160 teardrop-shaped, olive-sized eggs, visible on the upper left, that were attached to a rock from May 2007 to September 2011. (Robison et al./PLOS ONE)

It's particularly amazing, because most octopuses rarely live more than about two years.

The mother octopus, who could be easily identified by her unique scars, was first spotted in April 2007 by a remote-controlled sub run by the Monterey Bay Aquarium Research Institute, where lead author Bruce Robison is chair of the research division. The octopus was in the Monterey Canyon off the coast of central California, about 1,400 metres below the surface, and at that time, she didn't have any eggs.

She was identified as belonging to a species called Graneledone boreopacitfica, which lives as far north as 50 degrees latitude, reaching B.C.'s southernmost waters, said Seibel. Fully grown, it has an arm span of about half a metre, although it looks about "football sized" when curled up.

When the sub next returned in May 2007, the octopus was curled over a clutch of about 160 teardrop-shaped, olive-sized eggs that were attached to a rock.

Protection from predators

Octopuses are cold-blooded animals, so they can't keep their eggs warm the way birds do, but they do protect their eggs from predators such as crabs and fish — and sometimes even scarier threats. Seibel said if a remote controlled sub tries to pick up a rock with octopus eggs attached, the mother octopus "definitely puts up a fight."

When the remote-controlled sub returned in October 2011, the octopus was gone and empty egg cases were left behind on the rock. (© 2011 MBARI)

The mother also prevents the eggs from getting buried by sediment and suffocated, and may also clean them and keep parasitic organisms from growing on them, Seibel added.

There was no evidence that the mother octopus ate anything while caring for her eggs — the researchers noted that there were crabs and shrimp nearby, but the octopus simply pushed them away if they got too close. She ignored pieces of crab offered by the robot sub.

"We could see that her condition deteriorated over the four and a half years or so — she was clearly wasting away, using her own body as energy," Seibel said.

The sub visited the octopus 18 times over the course of the 4.5 years. The mother octopus was still caring for her eggs in September 2011, but she was gone and the egg cases were empty when the sub returned a month later.

"The ultimate fate of the brooding female is death," the paper said. But it noted that the extremely long brooding period also appeared to extend the life of the mother far beyond what might be expected for an octopus.

The baby octopuses of this species hatch as well-developed, four centimetre-long mini versions of the adults.

Cold water, slower growth

That was one reason why they take so long to develop, the researchers wrote. The other reason is that development of cold-blooded animals is drastically slowed down by cold temperatures, and it was a nippy 2.8 to 3.4 C in the part of the deep sea where the eggs were laid.

The remotely operated vehicle Doc Ricketts, run by the Monterey Bay Aquarium Research Institute, was used to visit the octopus 1400 metres below the surface of the ocean. (© MBARI)

Scientists had previously studied the parental behaviour of octopuses and squids that live in warmer, shallower waters, which generally take months rather than years to hatch their eggs.

Based on that data, they had predicted that deep-sea octopus eggs might take years to hatch, Seibel said, but "didn't have a lot of confidence" in the prediction, since most octopuses didn't live that long.

"While the discovery of a 53-month brooding period is remarkable, this reproductive strategy is not unusual, it is common and is clearly successful because G. boreopacifica is one of the most abundant deep-living octopods in the eastern North Pacific," the researchers wrote. "The results only seem extraordinary when compared with well-studied shallow-water species; which indicates how little we really know about the deep sea."

The study was funded by the Monterey Bay Aquarium Research Institute, which is in turn funded by the David and Lucile Packard Foundation.

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Extreme prenatal care

Here are some other records held by devoted parents of the animal kingdom, as cited in the study: