Mystery of California’s octopus garden in ocean’s midnight zone solved by scientists

Deep-sea octopuses are typically solitary creatures that inhabit frigid waters in one of Earth’s most challenging environments.

The 2018 discovery of thousands of the eight-legged cephalopods about 2 miles below the ocean’s surface flummoxed and fascinated marine scientists in equal measure. The consortium of octopuses clustered around a hydrothermal vent — an opening in the seafloor where warm, chemical-rich fluids flow out — about 10,500 feet (3,200 meters) down in what’s known as the midnight zone, a place of perpetual darkness.

The octopus garden — found on a small hill near the base of Davidson Seamount, an extinct underwater volcano 80 miles (128.7 kilometers) southwest of Monterey, California — was full of a species called Muusoctopus robustus, nicknamed the pearl octopus by the research team because of the way they look while upside down protecting their eggs.

The find is the largest known aggregation of octopuses on the planet — researchers counted more than 6,000 octopuses in just one segment of the site.

“We think there may be 20,000 octopus there. And the question is, well, why are they there? And why are they aggregating? It looks like the warm waters that are emanating from these springs is a key to why these animals are breeding there,” said Jim Barry, a senior scientist at the Monterey Bay Aquarium Research Institute.

Researchers believe the octopuses migrate to the deep-sea thermal springs in such numbers to mate and nest, according to a new study published Wednesday in the journal Science Advances. After laying eggs, expectant octopus moms keep the eggs clean and guard them from predators. The warmer temperatures speed up the creatures’ embryonic development.

“Very long brooding periods increase the likelihood that a mother’s eggs won’t survive. By nesting at hydrothermal springs, octopus moms give their offspring a leg up,” explained Barry, who was the lead author of the study.

The ambient water temperature at 10,500 feet is 35 degrees Fahrenheit (1.6 degrees Celsius). However, the water temperature in cracks and crevices at the octopus garden reaches about 51 degrees Fahrenheit (11 degrees Celsius).

Underwater technology

The researchers at Monterey Bay Aquarium Research Institute and their colleagues at other institutions used state-of-the-art underwater technology to understand the octopus garden.

The institute’s ROV Don Ricketts went on 14 day-long follow-up dives to the site between 2019 and 2022, taking high-definition video of the octopuses and mapping the octopus garden at a meter-scale resolution.

The remotely operated submersible also left a time-lapse camera and sensors to measure temperature and oxygen levels for long-term observations of the octopuses’ behavior. The camera recorded an image every 20 minutes, taking about 12,200 images from March 2022 to August 2022.

Faster brood times in the octopus garden

With this information, scientists pieced together why the octopus are attracted to the site. Scars and other distinguishing features allowed the scientists to monitor individual octopuses and the development of their broods.

The presence of adult male and female octopuses, developing eggs and octopus hatchlings indicated that the site is used exclusively as a breeding ground and nursery. The team did not observe any intermediate-size individuals or any evidence of feeding.

“We see hatchlings swimming away. We’ve never seen any small animals that suggests that they live right here. So they swim off somewhere and they start their life,” Barry said.

The study found that the eggs hatched in less than two years — much more quickly than the team had expected. One deep-sea octopus species broods its eggs for four and a half years. Away from the hydrothermal vents in the near-freezing temperatures of the deep sea, egg-brooding periods are thought to last for several years.

“Although it’s risky to brood in this warm water — you may be cooking your eggs, you may have abnormalities — the shorter brood period is where the advantage comes in and that seems to play out perfectly for them because we see what looks to be fairly high hatch rates,” he said.

Unraveling mysteries of the octopus life cycle

The researchers believe warmth from thermal springs increased the metabolism of female octopuses and their eggs, reducing the time required for incubation and making it less likely that the eggs are eaten by predators. However, it’s not clear whether this warmth is absolutely necessary for this species to reproduce and nest or something they simply like to seek out.

The researchers will continue to study the site at Davidson Mount and want to look for places with similar geology that could host other octopus gardens, although deep-sea exploration is very expensive, Barry said.

Octopuses are famously self-sacrificial parents — after laying a clutch of eggs, they quit eating and waste away, typically dying by the time the eggs hatch. Dead octopus and vulnerable hatchlings provide a feast for invertebrates such as sea anemones and sea stars that live side by side with the brooding octopuses.

Researchers have documented a total of four deep-sea octopus gardens to date — two off the coast of Central California on the Davidson Mount and two off the coast of Costa Rica.

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