Not completely sedentary: Researchers have discovered traces on the bottom of the Antarctic deep sea, which apparently sponges left behind in the sediment during their wandering. So far, the phenomenon remains a mystery: assumptions about how and why these primitive animals move around in their mysterious habitat must first be confirmed by further investigations.
They have specialized tissues, but no muscles, nerves or sensory cells: The sponges are therefore considered to be one of the most primitive forms of animal life. They have inhabited aquatic habitats on earth since prehistoric times and have produced great diversity: over 8,300 species of sponges have been able to adapt to many different conditions – from the depths of the sea to fresh waters. They sit there on surfaces and filter nutrient particles out of the water with their literally spongy tissue.
An ability to move does not seem to be necessary for this and so these plant-like beings usually always stay in place – they are considered sessile. However, studies have already shown that sponges are not quite as simple and passive as long thought: They can therefore react actively to mechanical stimuli or water currents. There have already been indications that some species can move at least a bit through wave-like contractions of the tissue. But real migrations, as they are now emerging, were not previously known.
View into the realm of the wandering sponges
The researchers led by Teresa Morganti from the Max Planck Institute for Marine Microbiology in Bremen came across the phenomenon while exploring a remote habitat that extends around 350 kilometers from the North Pole at a depth of around 1000 to 500 meters. The recordings on the so-called Karasik Seamount were made by a camera system that was moved across the sea floor by the research icebreaker Polarstern in 2016.
The exploration first showed that the seabed of this very nutrient-poor habitat is covered by an astonishingly dense community of Arctic sponge species, some of which are over a meter in diameter. But it was the detailed analyzes of the high-resolution images of the sea floor that caused the big surprise: sometimes meter-long path-like tracks emerged in the sediment, which ended exactly where living sponges were sitting.
At first, a possible explanation appeared that the rounded formations were passively moved by ocean currents or that they had slowly slid down slopes. But the tracks led in all directions and even uphill. “There are no such strong currents in the Arctic deep sea that could explain the structures found on the sea floor,” says co-author Autun Purser from the Alfred Wegener Institute, Helmholtz Center for Polar Marine Research. Morganti continues: “This is why we come to the conclusion that the sponges have actively moved across the sea floor and have left these traces as a result of their movement,” says the sponge expert.
How, why and how fast?
As she and her colleagues point out, the exact background of the phenomenon remains unclear: So far, there have only been guesses as to how, how quickly and why the sponges migrate in their bizarre habitat. They may move very slowly, contracting and expanding their bodies again, as previous observations on some species of sponges suggest. Reasons for locomotion, in addition to avoiding unfavorable environmental conditions and spreading offspring, could primarily be strategies of foraging, say the researchers.
It is possible that the animals move over the ground in order to make nutrients in the sediments more accessible. Perhaps the behavior is also linked to the fact that the sponges can form comparatively high populations in the very nutrient-poor habitat. More detailed information on how and why the sponges are in the depths of the Antarctic Sea could now provide further investigations and, above all, time-lapse recordings, say the scientists.
Source: Cell Press, Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, specialist article: Current Biology, doi: 10.1016 / j.cub.2021.03.014