In 2018, scientists exploring the base of an inactive undersea volcano off California were startled to discover a massive breeding ground for deep-sea octopuses. As many as 20,000 pearl octopuses (Muusoctopus robustus) were found at this “Octopus Garden” 3 kilometers underwater, in the largest known octopus nursery. As researchers moved in for a close-up view of nesting mothers, they were surprised again by shimmering waters amongst the nests: an indicator of warm water percolating from the seabed, bathing developing eggs in warmth from unseen hydrothermal springs. This fascinating discovery prompted several questions: Why nest at this site? How do the thermal springs influence the octopuses? My lab group at MBARI and collaborators from the Monterey Bay National Marine Sanctuary set about to shed some light on these mysteries.

 

Life is tough in the deep sea. High pressure, darkness, little food, and particularly the frigid metabolism: growth, reproduction, and other life processes are sluggish in the cold, challenging species to find ways to survive and reproduce, ensuring the future for the next generation. We know that the time required for brooding by octopuses worldwide is coupled with ocean temperature. Species in warm seas lay smaller eggs that hatch in days to weeks, while octopuses in cold waters have larger eggs and longer, even years-long, brood periods. The longest incubation known for any animal is that for “Octomom”, a deep-water octopus (Graneledone pacifica) that tended her nest in cold (3 ^oC) waters for about 4.5 years before her eggs hatched. The pearl octopus we studied lives in deeper, colder (about 1.6 ^oC) waters where brooding would be expected to take 8 to 10 years or more, a seemingly impossible feat for octopus moms who do not eat while brooding. 

 

The simplest explanation for this breeding aggregation is that pearl octopuses are attracted to the rocky habitat at the Octopus Garden. All octopuses living on the seafloor need clean rock surfaces to attach their eggs. Rocky nesting areas are abundant in this area, unlike most deep-sea environments, where a veneer of mud or sediment covers most of the seabed. However, after checking the temperatures of waters surrounding eggs in dozens of nests, we found that all nests were 1 to 10 ^oC warmer than the surrounding ambient waters. Despite searching, we could not find a single nest in cold waters, making it clear that nesting octopus moms chose locations warmed by thermal springs – but why? 

 

We expected that warmth would speed the metabolism of the mothers and their brood, shortening the time required for hatching and perhaps increasing their reproductive success. However, brooding in thermal springs is also risky; once laid, eggs are immovable, and episodes of hot temperatures, even for seconds, could wipe out the entire brood – there is no guarantee of reproductive success in thermal springs. A higher metabolism also burns energy stored in a mother’s body more rapidly and may influence how long a brooding mom can survive to tend her eggs. 

 

We solved this mystery during several expeditions over 3 years where we repeatedly visited the same group of nesting mothers to measure how quickly eggs were hatching. Nests were indeed warmer, averaging 5.1 ^oC, and the time required for hatching averaged 1.8 years, far shorter than the decade-long period expected for cold waters. How could a shorter brood period increase the reproductive success of pearl octopuses? Ideally, all eggs an octopus mom lays could hatch and survive. But even for a doting mom, some eggs may die before hatching due to injury, infection, or predation. For octopuses in warm seas, eggs hatch quickly, and exposure to deadly risks is brief: thus, most eggs may hatch unscathed. On the other hand, for the very long brood periods required for frigid waters, eggs can be exposed to the risk of injury or death for years – the longer they are at risk, the fewer eggs are expected to survive and hatch. Shortening the brooding period by exploiting warm springs would also shorten the time eggs are exposed to predation risk, presumably increasing the hatching success of the brood. Rocky habitats, ideal for egg-laying, may have initially attracted pearl octopuses to the Octopus Garden; however, the boost in reproductive success that appears linked to warm waters provides embryos with a better chance for hatching and early life, ensuring the continued success of the local pearl octopus populations. How many more fascinating natural wonders await discovery in the vast, yet unexplored areas in the deep ocean?