As reported by the German-American team of biologists in the current issue of the journal Current Biology
, the octopuses deposit their eggs onto sponges that only grow locally on manganese nodules. The researchers had observed the previously unknown octopus species during diving expeditions in the Pacific at depths of more than 4000 metres - new record depths for these octopuses. The results of the study show that it is key that the extraction of resources in the deep sea is preceded by scientific investigations into the ecological consequences to support policy in developing legal frameworks.
The appearance of the Hawaiian Casper octopus in front of the camera at a depth of 4290 metres during a NOAA expedition is the greatest depth at which such finless octopuses have to date been observed. Six months earlier, researchers of the Alfred Wegener Institute, the GEOMAR, the Max Planck Institute for Marine Microbiology, and the Centre for Marine Environmental Sciences (MARUM) had filmed and photographed more specimens of this or similar hitherto unknown octopus species at a depth of 4120 to 4197 metres in the Peru Basin in the south-eastern Pacific Ocean.
"Until we made these observations, we had assumed that these octopuses only occur at depths of up to 2600 metres. But the species discovered can now be seen to colonise much greater depths," says lead author Dr Autun Purser of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI).
“Without manganese nodules the octopuses cannot find a brooding ground”
Two of the octopuses were seen by the camera system to be guarding their eggs. "… the animals had deposited their eggs onto the stems of dead sponges, which in turn had grown on manganese nodules. The nodules served as the only anchoring point for the sponges on the otherwise very muddy seafloor. This means that without the manganese nodules the sponges would not have been able to live in this spot, and without sponges the octopuses would not have found a place to lay their eggs," the AWI researcher explains.
With the so-called DISCOL experiment from the late 1980s scientists found that many deep-sea animals need manganese nodules in their habitat. At the time, also in the Peru Basin, German researchers had removed manganese nodules by ploughing them into the seabed. In subsequent years, they observed the consequences of this human intervention on the deep-sea community. 26 years later, researchers of the JPI Oceans MiningImpact project returned to the place where the DISCOL experiment was carried out with the new German Research Vessel SONNE. Their conclusion: "The removal of the manganese nodules at that time caused the community of animals that are attached to the seafloor, which also includes sponges, to almost fully collapse. Even 26 years later many animal populations have not yet recovered," explain the authors in the new study.
"Our new observations show that we have to know about the behaviour of deep-sea animals and the specific way in which they adapt to their habitat in order to draw up sustainable protective and usage concepts," says AWI researcher Antje Boetius, head of the Sonne expedition to the Peru Basin.
The work on board the SONNE research vessel was made possible by the German Federal Ministry of Education and Research as part of the MiningImpact project of JPI Oceans.