Two miles below the ocean's surface near Monterey, California, warm water seeps from the seafloor at the base of an underwater mountain. It's a magical place, especially if you're an octopus.
In 2018, one of us, Amanda Kahn, was aboard the research vessel E/V Nautilus when scientists discovered the "Octopus Garden." Thousands of pearl octopuses ( Mouse octopus robustus) were rolled into individual balls in lines and clumps. As Nautilus Live streamed the expedition online, the world was able to share the excitement of the discovery.
We now know why these amazing creatures gather in these and other warm underwater springs.
In a new study involving scientists from different fields, we explain why octopuses migrate to the Octopus Garden. It is both a mating site and a nursery where newborn octopuses develop faster than expected, giving them the best chance of survival in the deep, cold sea.
Life in the Octopus Garden
Female octopuses seek out rocky cracks and crevices where warm water seeps from the rocks. There they vigilantly guard their offspring. These mothers live solely on their energy reserves and will never eat again. Like most cephalopods, they make the ultimate sacrifice for their offspring, dying after their eggs hatch.
The Octopus Garden, at the base of Davidson Seamount, about 80 miles southwest of Monterey, California, is the largest of a handful of octopus farms recently discovered in the eastern Pacific Ocean. Many are found near hydrothermal vents where warm water seeps from the seabed.
We wanted to know what makes these environments so attractive to nesting octopuses.
To solve this mystery, we brought together geologists, biologists and engineers. Using the Monterey Bay Aquarium Research Institute's deep-sea robots and sensors, we studied and mapped the Octopus Garden on several visits over three years to investigate the link between thermal springs and pearl octopus breeding success. We found almost 6,000 nests in an area of 2.5 hectares, which suggests that there are more than 20,000 octopuses living here.
A time-lapse camera that kept watch over a group of nesting mothers for six months opened a window into the dynamic life in the Octopus Garden.
We witnessed male octopuses approaching and mating with females. We cheered for the successful emergence of the youngsters, who looked like translucent miniatures of their parents. And we mourned the deaths of mothers and their offspring.
If a nest became empty, it was quickly filled by another octopus mother. We saw that nothing was lost in the Octopus Garden. Dead octopuses provided an essential food source for a wide range of scavengers, such as sea anemones and snails.
Warmer water accelerates embryo development
A new generation of octopuses must overcome at least two hurdles before hatching.
First they have to develop from egg to boy. They start out as opaque, sausage-shaped eggs glued to rocks. Over time, small black eyes and eight tiny arms become visible through the egg capsule. Second, crucially, they must not succumb to external threats, including predators, injuries and infections. The longer the incubation period, the greater the risk that an embryo will not survive before hatching.
For octopus species that live in warm, shallow waters, breeding periods last only a few days to weeks. But in the abyss a completely different scenario plays out. Temperatures around freezing dramatically slow metabolic processes in cold-blooded animals such as octopuses. The longest known breeding period for any animal actually comes from another deep-sea octopus species, Graneledone pacifica where a mother nursed her nest for no less than 4½ years. An octopus farm for this species was recently discovered off the west coast of Canada.
At Davidson Seamount, where the ambient water temperature is 35 degrees Fahrenheit (1.6 degrees Celsius), we would expect pearl octopus embryos to take five to 10 years, or possibly longer, to develop. Such an extended incubation period would be the longest known for any animal, exposing an embryo to exceptional risks.
Instead, temperature and oxygen sensors we were able to place in octopus nests documented a much warmer microenvironment around the eggs. On average, the temperature in octopus nests was about 41°F (5.1°C), considerably warmer than the surrounding waters. We predicted that octopus embryos would develop faster in this warmer water.
Distinctive features and scars allowed us to identify individual mothers. We followed the development of their offspring through repeated visits. Although we expected faster growth in the warm water, we were astonished to find that the eggs hatched in less than two years. Nesting in thermal springs clearly gives pearl octopuses a boost.
But nesting in thermal springs is a potentially risky strategy. Once the eggs are laid, they are cemented onto the rock. We know little about the thermal tolerance of pearl octopuses or their embryos, but even brief exposure to water that is too hot can be fatal to developing embryos, destroying any hope of successful reproduction for that mother. One of the first recorded deep-sea octopus farms may have experienced unpredictable fluid flow.
Nurseries highlight the risks to seabed habitat
The thermal springs in the Octopus Garden are part of a hydrothermal system on the flank of the ridge. Here, water seeping beneath the seabed absorbs heat from the Earth's mantle before dissipating through volcanic rock outcrops such as Davidson Seamount. These systems have become an emerging focus in seafloor geology, although only a few have been discovered to date.
Unlike hydrothermal vents, which form on the tops of ridges and belch plumes of hot water visible hundreds of meters above the bottom, thermal springs on the flanks of ridges are cryptic. These springs seep in warm water that disappears only a few feet above the bottom, making them extremely difficult to find and visible only by a slight sheen in the water.
Our years of recording thermal springs in the Octopus Garden show that these can be stable environments, with the potential to release warm fluids for thousands of years. Such stability benefits not only the pearl octopus, but also the community that thrives alongside the nesting mothers.
The recent discoveries of octopus farms off the Pacific coast of Costa Rica, including near hydrothermal vents, suggest that these areas may be more common than previously thought. It also highlights that hydrothermal vents can be vital biological hotspots.
The deep sea is the largest living space on Earth, and its vast size can mask the importance of these kinds of local hotspots. Davidson Seamount and its Octopus Garden are protected as part of Monterey Bay National Marine Sanctuary, but many more biological treasures such as thermal springs may be at risk, especially as deep-sea mining proposes to scrape off large, little-explored areas of the seabed. We hope that the octopus mothers we met at this farm inspire everyone to rethink how to manage the undiscovered hidden gems that may have been lost.
This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand our complex world. It was written by: Amanda Kahn, San Jose State University and Jim Barry, San Jose State University Read more: Jim Barry receives funding from NOAA. Amanda Kahn does not work for, consult with, own shares in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
