Scientists discovered an underwater city of 20,000 octopus nests deep inside California coast; largest known nursery on the planet

Researchers have located the planet's largest known octopus nursery deep off the Californian coast, documenting 5,718 animals in 2.5 hectares and estimating about 20,000 around Davidson Seamount. Known as the Octopus Garden, the nursery is located about 10,500 feet below the surface off Central California. According to a statement released by the Monterey Bay Aquarium Research Institute (MBARI) on August 23, 2023, monitoring confirmed that pearl octopuses migrate to the site for mating and nesting.The Octopus GardenThe nursery is situated on a small hill near the base of the Davidson Seamount, which is an extinct volcano located 130 kilometres southwest of Monterey, California. The large concentration first drew attention in October 2018. Researchers from the Monterey Bay National Marine Sanctuary, NOAA, Ocean Exploration Trust, and collaborators, working aboard the E/V Nautilus, observed thousands of octopuses gathered on a rocky rise near the seamount.The position of the animals helped researchers understand exactly what was happening in the section of the ocean. Many of the octopuses were upside down, with their arms positioned around their bodies, a posture that is associated with females protecting and incubating their eggs.The species was identified as Muusoctopus robustus, known to researchers as the pearl octopus. They exhibit a pale purple colouration. According to MBARI, the animal is approximately the size of a grapefruit and occurs in the northeastern Pacific Ocean, from Oregon down to Baja California. The institute has observed the species at depths between 2,300 and 3,600 metres.Research on the nurseryAfter the initial discovery, researchers began a prolonged investigation to determine why so many animals were concentrated at that point. For three years, the institute and its partner institutions employed cameras, sensors, remotely operated vehicles and other equipment to observe the behaviour of octopuses and the environmental conditions of the nursery.Doc Ricketts, a remotely operated vehicle from MBARI, played a central role in the research. The team conducted 14 dives with the ROV during which scientists found adult males and females, developing eggs and hatchlings. No individuals of intermediate size or evidence of feeding were observed at the site. These observations indicated that the octopuses gather there specifically to mate and nest. The size of the concentration was measured using a combination of photographs and detailed mapping of the seabed. The LASS system, installed on the Doc Ricketts ROV, allowed high-resolution images to be produced and a photomosaic of the region to be assembled. In this survey, researchers documented 5,718 octopuses in just 2.5 hectares at the centre of the Octopus Garden. The slight elevation where the nursery is located occupies approximately 333 hectares, or 823 acres. Based on the surveys conducted, the team calculated that the total population could easily exceed 20,000 individuals.It is the thermal conditions of the area that explain why this small area attracts so many animals. At approximately 3,200 meters, the normal water temperature is around 1.6°C. However, in the crevices and fissures used by the octopuses, water warmed by the hydrothermal vents reaches nearly 11°C. The initial researchers observed a "sparkling" appearance in the water, caused by the mixture of hot and cold water. Subsequent investigations confirmed the existence of hydrothermal vents and showed that the nests are clustered precisely in the fissures bathed by the warmer water emerging from the seabed.Why this region?Octopuses are ectothermic animals, and the cold deep waters reduce their metabolism and slow embryonic development. At temperatures close to zero found in the abyssal environment, researchers expected that the pearl octopus's eggs would need five to eight years or even longer to hatch.But the temperature in these locations revealed a different outcome. Using 4K imaging and features like scars to recognise the animals, scientists tracked their clutches and found that the eggs hatched in less than two years. This acceleration was attributed to the heat from he hydrothermal vents, which increases the mothers' and embryos' metabolism. The longer the incubation, the greater the period during which the embryos are subject to predation, injury, infections, and other sources of mortality.Females migrate to the Octopus Garden in search of warm locations where they can nest. Each one lays a clutch of approximately 60 elongated eggs and covers the embryos with her body while they develop.Interestingly, the nursery also serves as a food source for other species. As is typically the case with cephalopods, both male and female pearl octopuses die after reproduction. Most mother remain alive until the eggs hatch, though some exhaust their reserves before development is complete. Their carcasses are consumed by various organisms from the area. Ratfish, cusk eels, sea slugs, anemones, and other scavengers take advantage of the remains left after reproduction. Researchers estimated that this biomass provides 72% more food to the local marine community than would be available outside the Octopus Garden.The Octopus Garden is described by MBARI as the largest known concentration of octopuses on the planet to date.