SANTA CRUZ, Calif. (KRON) — A deep-sea mystery involving thousands of octopuses found off the coast of Central California was recently solved by a team of ocean researchers.

The mystery appeared three years ago, when the largest known “Octopus Garden,” in the world was discovered two miles underwater on the seafloor.

In 2018, researchers discovered thousands of octopuses near the base of an extinct volcano, Davidson Seamount, 80 miles off the coast of Monterey.

“It is the biggest aggregation of octopuses ever discovered, and the only one off the coast of the U.S.,” a San Jose State University spokesperson wrote.

Octopus mothers protect and brood their eggs. (Image courtesy MBARI)

A team of biologists, ecologists, geologists, and engineers monitored the Octopus Garden using high-tech tools to learn why octopuses are attracted to the deep-sea location. The team included members of SJSU’s Moss Landing Marine Laboratories, Monterey Bay Aquarium Research Institute, and NOAA.

In a new study published Wednesday in Science Advances, researchers confirmed that octopus migrate to the Octopus Garden to mate and nest their eggs. The presence of adult males and females, developing eggs, and octopus hatchlings indicated that the nursery is used exclusively for reproduction.

A male pearl octopus waives a tentacle. (Image courtesy MBARI)

Most of the octopus mothers observed at the volcano were upside down, inverting their arms and folding them around their bodies. This posture indicated pearl octopus (Muusoctopus robustus) mothers were protecting or brooding their eggs. The mothers waved their arms to circulate water among the eggs and swatted away scavengers trying to invade nests in their nursery.

Upon further investigation, researchers confirmed the octopus nests were clustered in crevices bathed by hydrothermal springs where warmer waters flow from the seafloor.

The ambient water temperature was 35 degrees. However, the water temperature within the cracks and crevices at the Octopus Garden reached nearly 41 degrees.

This location was confirmed to be an octopus nursery, and the largest known one in the world. (Map courtesy MBARI)

Researchers counted more than 6,000 octopuses in a portion of the site, and believe there may be 20,000 or more at this nursery.

The females of this species are devoted super-moms. They brood their eggs for five years, and when the eggs finally hatch, the mother dies.

Researchers were surprised to find that eggs near the volcano hatched in less than two years.

“We tracked individuals from among the thousands at this nursery and discovered that eggs, which at those temperatures would be expected to take five years or longer to hatch, surprisingly hatched in less than two years,” said Moss Landing Marine Laboratories and San Jose State University professor Amanda Kahn, who co-authored the study.

(Image courtesy MBARI)

The shorter brooding period in warmer waters greatly reduces the risk of developing octopus embryos from being eaten by predators.

“Mothers must equip each egg with sufficient energy (yolk) to support the costs of embryonic development and early juvenile success … but must also retain enough energy to sustain maternal care throughout incubation,” the study states.  

All of the octopuses migrated through cold dark waters to reach the volcano’s hydrothermal springs. The study states, “Of all the hurdles faced by animals in the deep sea, cold may be the most challenging.”

Mixed amongst the nesting octopus are several unidentified orange sea anemones. (Image courtesy MBARI)

“Thanks to MBARI’s advanced marine technology and our partnership with other researchers, we were able to observe this unique hotspot of life on the deep seafloor in tremendous detail, which helped us discover why so many deep-sea octopus gather there. These findings can help us understand and protect other unique deep-sea habitats from climate impacts and other threats,” said MBARI Senior Scientist Jim Barry, lead author of the study.

MBARI’s remotely operated vehicle Doc Ricketts and Low-Altitude Survey System sensor suite developed by engineers in MBARI’s Seafloor Mapping Lab were instrumental in studying the Octopus Garden. (Photo by Dave Caress / MBARI)

Marine researchers have discovered a handful of Octopus Gardens off the coast of Costa Rica. California’s only Octopus Garden and nursery are protected as part of Monterey Bay National Marine Sanctuary.