Scientists Solve Mystery of Octopus Garden in Ocean’s Midnight Zone

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

The 2018 discovery of thousands of eight-legged cephalopods some 2 miles below the ocean’s surface has baffled and fascinated marine scientists.

The consortium of octopuses huddled around a hydrothermal vent – ​​an opening in the sea floor through which hot, chemical-rich fluids come out – about 3,200 meters deep, in what is known as the midnight zone, a place of perpetual darkness.

The octopus garden — found on a small hill near the base of Mount Davidson, an extinct underwater volcano 80 miles southwest of Monterey, California — was full of a species called Muusoctopus robustus, which the team has dubbed the pearl octopus. of research because of their appearance while protecting their eggs upside down.

The discovery 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 might be 20,000 octopuses there. And the question is: why are they there? And why are they aggregating? It appears that the warm water emanating from these springs is the key to why these animals breed there,” said Jim Barry, Senior Scientist at the Monterey Bay Aquarium Research Institute.

Researchers believe that octopuses migrate to deep-sea hot springs in large numbers to mate and nest, according to a new study published Wednesday in the journal Science Advances.

After laying eggs, mother octopuses keep the eggs clean and protect them from predators. Higher temperatures accelerate the embryonic development of creatures.

“Very long incubation periods increase the likelihood that the mother’s eggs will not survive. By nesting in hydrothermal vents, mother octopuses give their young an advantage,” explained Barry, who was the lead author of the study.

The ambient water temperature at 3,200 meters is 1.6 degrees Celsius. However, the water temperature in the cracks and crevices of the octopus garden reaches around 11 degrees Celsius.

underwater technology

Researchers at the 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 performed 14-day follow-up dives at the site between 2019 and 2022, recording high-definition video of the octopuses and mapping the octopus garden at meter-scale resolution.

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

Faster broods in the octopus garden

With this information, scientists discovered why octopuses are attracted to the site. Markings and other distinguishing features have allowed scientists to monitor individual octopuses and the development of their clutches.

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

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

The study found that the eggs hatched in less than two years – much faster than the team expected. One species of deep-sea octopus broods its eggs for four and a half years. Away from hydrothermal vents, in the near-freezing temperatures of the deep sea, incubation periods for eggs are believed to last for several years.

“While it is risky to hatch in this hot water – you could be cooking your eggs, you could have abnormalities – the shorter brooding period is where the advantage comes in and this seems to work perfectly for them because we see what appears to be quite high hatch rates. “, he said.

Unraveling mysteries of the octopus life cycle

The researchers believe that the heat from the hot springs increased the metabolism of female octopuses and their eggs, reducing the time needed for hatching and making the eggs less likely to be eaten by predators. However, it is not clear if this heat is absolutely necessary for this species to reproduce and nest or if it is something it simply enjoys.

Researchers will continue to study the Davidson Mount site and want to look for sites with similar geology that could support other octopus gardens, although exploration offshore is very expensive, Barry said.

Octopuses are famous sacrificial parents – after laying a clutch of eggs, they stop eating and wither away, usually dying by the time the eggs hatch. Dead octopuses and vulnerable hatchlings provide a feast for invertebrates like sea anemones and starfish that live side by side with the octopuses that hatch their eggs.

The researchers have documented a total of four deepwater octopus gardens to date – two off the coast of Central California at Mount Davidson and two off the coast of Costa Rica.