A team of researchers from various institutions in the US has unraveled the mystery behind a spectacular phenomenon observed in the deep sea: the Octopus Garden, a massive aggregation of thousands of octopuses gathered at thermal springs near an extinct underwater volcano off the coast of Central California.
The study, published in the journal Science Advances, reveals how these octopuses use the warm water from the springs to accelerate the development of their eggs and increase their chances of survival.
The Octopus Garden: a unique deep-sea nursery
The Octopus Garden was first discovered in 2018 by researchers from NOAA's Monterey Bay National Marine Sanctuary and Nautilus Live, who observed thousands of octopus nesting on the seafloor at a depth of about 3,200 meters (10,500 feet) near the base of Davidson Seamount, an extinct underwater volcano 130 kilometers (80 miles) southwest of Monterey, California.
The site was named after the Beatles song "Octopus's Garden" and captured the curiosity of millions of people around the world, including scientists from Monterey Bay Aquarium Research Institute (MBARI).
For three years, MBARI and collaborators used high-tech tools such as remotely operated vehicles (ROVs), autonomous underwater vehicles (AUVs), and benthic observatories to monitor the Octopus Garden and learn more about its inhabitants.
They identified the species as Muusoctopus robustus, a deep-sea octopus that they nicknamed the pearl octopus because, from a distance, nesting individuals look like opalescent pearls on the seafloor.
They also counted more than 6,000 octopuses in a portion of the site and estimated that there may be 20,000 or more at this nursery, making it the largest known aggregation of octopuses on the planet.
The role of thermal springs in octopus reproduction
The researchers confirmed that pearl octopus migrate to the Octopus Garden to mate and nest. They observed males and females copulating and females laying eggs on rocks or shells near hydrothermal springs that emit warm water from cracks in the seafloor.
The warm water creates a temperature gradient that ranges from 2°C to 15°C (35°F to 59°F) within a few centimeters.
The females position themselves and their eggs within this gradient to optimize their thermal exposure.
The researchers found that the warm water from the springs has a significant effect on the development of octopus embryos.
They measured the development rate of embryos at different temperatures and found that it increased exponentially with temperature.
They calculated that embryos exposed to 15°C would hatch in about two months, while embryos exposed to 2°C would take more than four years to hatch.
This means that by nesting near thermal springs, pearl octopus can shorten their brooding period by more than 20 times.
The researchers believed that this strategy gives young octopus a better chance of survival. They explained that pearl octopus are semelparous, meaning that they reproduce only once in their lifetime and die shortly after hatching their offspring.
Therefore, reducing the time spent brooding can increase their reproductive success and reduce their exposure to predators and environmental stressors.
Moreover, faster development can result in larger and more competent hatchlings that can cope better with the challenges of the deep sea.
Implications for deep-sea conservation
The study highlighted the importance of understanding and protecting the hotspots of life on the deep seafloor from threats such as climate change and seabed mining.
The researchers noted that climate change can alter the temperature and chemistry of seawater, affecting the availability and distribution of thermal springs and other resources for deep-sea organisms.
They also warned that seabed mining can damage or destroy habitats such as hydrothermal vents and seamounts that host diverse and unique communities of life.
The researchers suggested that adaptive management strategies are needed to conserve these ecosystems and their services.
They concluded that discoveries such as the Octopus Garden can inspire people to appreciate and care for the wonders of the deep ocean.
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