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Deep-sea Hot Springs May Be Vital to Nesting Octopus

In 2018, researchers from NOAA’s Monterey Bay National Marine Sanctuary and Nautilus Live first observed thousands of pearl octopus nesting on the deep seafloor near Davidson Seamount. | © 2023 MBARI

More than 70 miles off the coast of central California, almost 6,000 octopus (Muusoctopus robustus) have been found nesting at a depth of about 10,500 feet. The "Octopus Garden," as it was termed by the scientists and engineers who first discovered it in 2018, rests on warm, underwater hot springs along the foothills of an extinct submarine volcano called the Davidson Seamount.

A new study in Science Advances suggests that these rare, deep-sea heat sources could be vital to M. robustus nurseries, and in turn to other deep-sea species.

The study was the result of many return trips to the Octopus Garden since its discovery by the crew of the Exploration Vessel Nautilus in 2018. During these return trips, James Barry and colleagues used the remotely operated vehicle (ROV) Doc Ricketts, operated by the Monterey Bay Aquarium Research Institute, to glean insights about M. robustus and the environment in which they aggregate. The ROV, equipped with high-definition cameras, sensors, and manipulator arms, captured a dynamic, year-round breeding ground.

Warmth in the Frigid Abyss

Frigid deep-sea temperatures — ranging from around -2°C (28.4°F) to 3°C (37.4°F) — slow the metabolism and embryonic development of the creatures who live there. Some deep-sea octopus species have been thought to nest for years in these temperatures, which could increase their chances of being injured or preyed upon. But added warmth from these hydrothermal springs, with temperatures measured at up to 10.8°C (51.4°F), could help to speed this process.

"We think that one of the reasons there's a huge reproductive benefit for this particular octopus using these springs is because it can shorten that brood period and avoid a lot of external risk," said Barry, senior scientist at MBARI, benthic ecologist, and first author of the study. Nesting times at the Octopus Garden averaged about 1.8 years — a duration that would be expected for nesting temperatures of about 5.1°C (41°F), the researchers estimated.

"Are they only breeding at these warm spots, or are they breeding everywhere, and they just happen to be here as well?" Barry pondered. "I'd love to know." Every nest for which temperatures were measured had at least some level of warming above ambient temperatures (~1.6°C, or 34.9°F), he said.

A "Big, Robust Octopus"

M. robustus are quite elusive aside from their activities at the Octopus Garden — which primarily consist of mating, nesting, and ultimately dying. Octopus start to deteriorate shortly after reproducing, a phenomenon known as senescence.

"We observe them when they are with their eggs, at a point in their lives when instinct tells the female to focus on her eggs exclusively," said Janet Voight, associate curator of invertebrate zoology at the Field Museum and coauthor. She described this species, from the family Enteroctopodidae, as particularly bulky — "well-muscled, with a broad head and mantle." When asked what they do when they're not at the Octopus Garden, Voight wasn't certain.

"We have to assume that most hatchlings disperse away from the aggregation in a random direction, and look for prey," Voight said. However, it's still unclear whether these octopuses stay close to the warm spot, whether they leave and come back, or if they leave forever in search of prey at other distant corners of the deep sea.

Life at the Octopus Garden

The Octopus Garden is the largest collective of octopus ever encountered. The researchers counted 5,718 octopus, including 4,707 nesting females, within just a 2.5-hectare area (about 6.2 acres, or 270,000 square feet), using imagery from a high-resolution subsea mapping system. They suspect that the entire area could easily have surpassed 20,000 octopus.

"The mass aggregation of octopuses is an oasis of food in the otherwise food desert of the deep ocean," said Amanda Kahn, assistant professor at Moss Landing Marine Laboratories at San José State University and coauthor. "Small shrimp and predatory snails scavenge abandoned eggs and capture hatchlings. Anemones nestled among the nesting mothers feed on hatchlings and on the octopus mothers after they pass."

An aggregation of female pearl octopus (Muusoctopus robustus) nesting at the Octopus Garden, located near Davidson Seamount off the Central California at a depth of approximately 3,200 meters.
Aggregation of female pearl octopus nesting near Davidson Seamount off central California at a depth of approximately 3,200 meters. | © 2022 MBARI

Typically, deep-sea organisms rely on particulate organic matter, or "marine snow," that rains down from the surface ocean and is sparse by the time it reaches the abyssal zone, encompassing the seafloor (benthic zone) and the water column at depths anywhere from about 3,000 to 6,500 meters (9,800 to 21,300 feet).

"The turnover of octopus represents over a 70% increase in the availability of food around the nursery, which supports a much richer and more abundant community of invertebrates and fish than surrounding habitats," Steven Litvin, senior research specialist at MBARI and coauthor, explained.

Warm, But Not Too Warm

Octopus nurseries are rare finds in the deep sea; the Octopus Garden is one of only three documented. In 2019, another was found about six miles away near a volcanic cone, and aptly named Octocone. Another was only recently discovered near Costa Rica at the Dorado Outcrop.

"To date, these hydrothermal spring habitats have generally been considered novel, or rare," explained Andrew DeVogelaere, a research ecologist at NOAA's Monterey Bay National Marine Sanctuary and coauthor. "We now believe that the 'foothills' of seamounts may be fertile exploration grounds to find more hydrothermal springs, and that they could be quite common."

Hydrothermal springs are different from hydrothermal vents. The latter is typically found along volcanically active mid-ocean ridge crests. Vents spew hot, mineral-rich fluids that are easily detectable by nearby sensors. Springs, meanwhile, are only moderately warm. The only visible clue that the Octopus Garden was situated on a spring was the faint, mirage-like shimmer of water above the seafloor captured up close, which signaled density differences between warm and ambient water.

Searching for More Warm Spots

Because they're so difficult to detect, deep-sea hydrothermal springs remain largely undiscovered. The study findings, however, led the researchers to suspect that they're much more common — and more vital to deep-sea habitats — than previously understood. They've started searching for more warm spots by targeting geologically similar settings.

map of MBARI Octopus Garden
Location of the Octopus Garden off the coast of central California. | © 2023 MBARI

"In order to search more effectively for warm venting sites, we need to understand much more about the processes involved," said David Caress, principal engineer at MBARI and coauthor. "We are starting a project to study the nature and formation of the fluid circulation resulting in the Octopus Garden venting."

The Octopus Garden has been protected since being added to the Monterey Bay National Marine Sanctuary in 2008. However, the study findings highlight the need to find and protect more of these cryptic environments.

"Until now, hydrothermal springs have been a missing piece of the marine protected area puzzle," DeVogelaere noted. "We need to determine where and how common other deep-sea hydrothermal springs are, for future ocean conservation efforts."

[Credit for related image: © 2023 MBARI]


Nyla Husain

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