Normally, octopuses live either in the open sea or on the sea floor. But the relatively recently discovered dumbo octopus Cirrotheutis muelleri combines both worlds, as scientists have now observed. According to this, the cephalopod floats most of the time in the open water of the ocean to seek protection from predators close to the bottom, but briefly shoots down to the seabed of the deep sea to feed. Until now, this behavior was only known from certain fish and sea cucumbers.
One of the largest animal migrations in the world takes place in the oceans every night. Fish, zooplankton and other creatures that have been hiding from predators at depth during the day then rise to the surface and hunt there themselves. But by no means all sea creatures participate in the hustle and bustle. In cephalopods such as octopuses, for example, no such migration behavior had previously been observed.
Picnic in the depths
However, this could also be related to the fact that nobody has been able to look closely enough so far. Because it is only now that researchers working with Alexey Golikov from the GEOMAR Helmholtz Center for Ocean Research in Kiel have collected all kinds of video material from a hitherto little researched octopus species. The video footage comes from the Arctic and was recorded there by remotely operated underwater vehicles and towed in situ observation systems. The "Dumbo" octopus Cirrotheutis muelleri can be seen on the videos. The first representatives of this group of octopuses, which have large lateral lobes on their heads, were only discovered in the deep sea a few years ago. They are among the deepest living octopuses ever. Its eight arms are connected by a thin skin and resemble a brownish-reddish umbrella.
"Most observations to date report individuals near the sea floor," said Golikov and his colleagues. "But we also repeatedly saw the animals 500 to 2600 meters above the sea floor." The team observed that Cirrotheutis only sank to the depths to search for food, but otherwise stayed in the open water zone above. This makes the Dumbo octopus the first cephalopod with proven vertical migratory behavior. However, it is a kind of “ghost diver” because it migrates in the opposite direction to most fish and other sea creatures. While they go up to feed, Cirrotheutis enjoys his picnic below. Previously, this reverse migration was only known from certain fish and sea cucumbers, according to Golikov's team.
Hike combines the best of both worlds
Depending on the depth of the water in which the Dumbo octopuses were, their behavior also changed, as the research team reports. In the open water, the Cirroteuthis behave extremely calmly. They float there almost motionless with the "open umbrella" and only occasionally flap their short fins, presumably to maintain or change their height, according to the researchers. According to Golikov, this type of locomotion is very energy efficient and at the same time unobtrusive, so the octopuses do not unnecessarily attract the attention of predators.
Video recordings showing the animals searching for food in the depths reveal a completely different behavior. "Using their wing-like fins, they swim slowly across the sea floor, land suddenly, enclose their food, and take off with their meal, flapping violently," Golikov describes. There are numerous characteristic imprints in the sediment of the sea floor, which the researchers interpret as traces of this landing and the enveloping of the prey. Overall, however, the visits to the seabed buffet never lasted longer than two and a half minutes.
For Golikov and his team, this behavior is a clear indication that Cirroteuthis combines the best of both worlds with its migratory movement. It makes use of the rich food supply on the seabed, where it captures crustaceans and annelids, among other things. At the same time, however, it also protects itself from the big predators of the deep such as bowhead sharks and pilot whales by quickly resurfacing in higher water layers. In doing so, Cirroteuthis not only saves its own skin, but also creates a connection between the upper water layers and the sea floor. Golikov and his colleagues suspect that in this way he contributes to the circulation of nutrients.
Source: GEOMAR Helmholtz Center for Ocean Research Kiel; Specialist article: Proceedings of the Royal Society B, doi: 10.1098/rspb.2023.0640