Mysterious ocean-floor trails show Arctic sponges on the move
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VIDEO: THIS VIDEO SHOWS SPONGE SPICULE TRAILS ON THE SEAFLOOR. view more
CREDIT: AWI OFOBS TEAM, PS101
The aquatic animal known as the sponge is often described as entirely sessile: once they've settled in a spot and matured, they aren't generally thought of as moving around. But, according to a new study in the journal Current Biology on April 26--in which researchers describe mysterious trails of light brown sponge spicules (spike-like support elements in sponges) across the Arctic seafloor--that isn't always so.
"We observed trails of densely interwoven spicules connected directly to the underside or lower flanks of sponge individuals, suggesting these trails are traces of motility of the sponges," the researchers, led by Teresa Morganti of the Max Planck Institute of Marine Microbiology and Autun Purser of the Alfred Wegener Helmholtz Centre for Polar and Marine Research, write. "This is the first time abundant sponge trails have been observed in situ and attributed to sponge mobility."
It looked as though the sponges had "crawled" into their current positions. In fact, sponges do have a motile larval stage. But most species are thought to become sessile as adults. Sponges, after all, have no muscles or specialized organs for moving around. They can react to external stimulation and move a little by contracting or expanding their bodies. There also has been some evidence of movement in sponges raised in the lab. In some cases, that movement involved remodeling their whole bodies.
Nevertheless, the new findings took the research team by surprise. The discovery was made by studying video captured in 2016 by the research icebreaker Polarstern as it surveyed the submerged peaks of the permanently ice-covered Langseth Ridge.
A towed marine camera sled and a hybrid remotely operated vehicle (HROV) showed that the peaks of the ridge were covered by one of the densest communities of sponges that's ever been seen. The researchers determined that the impressive sponge populations were primarily comprised of large numbers of Geodia parva, G. hentscheli, and Stelletta rhaphidiophora individuals.
They say it's not clear, given the challenging environment, how the area supports such a vast community of sponges. But, even more intriguing were the numerous trails of sponge spicules. Far from a rarity, the researchers saw trails in nearly 70% of seafloor images that contained living sponges.
Those trails were several centimeters in height and up to many meters long. They often connected directly to living sponges. The trails were seen in areas with lots of sponges, as well as in more sparsely populated areas. The researchers report that they also often seemed to be in areas with smaller, juvenile sponges.
The researchers generated 3D models from the images and video to show the way the trails were interwoven with each other. They say that the findings suggest that the moving sponges sometimes change direction. They don't think the movement is simply a matter of gravity. In fact, the images suggest that the sponges frequently traveled uphill. It may be that the sponges move in order to get food, perhaps driven by the scarce Arctic resources.
"These features are all indicative of feeding and population density behavioral trends previously observed in encrusting sponges," the researchers write. "The extremely low primary productivity, sedimentation, and particle advection rates of the Langseth Ridge region overall result in some of the lowest standing stocks of benthic life; so potentially, this Arctic Geodia community relies on particulate and dissolved fractions from the degradation of old organic debris trapped within the spicule mat as additional food sources. We suggest that the mobility indicated here may be related to sponges searching for and feeding directly on the accumulated detrital matter trapped within the sponge spicule mat underlying the living sponges."
It's also possible that the movement has something to do with reproduction or the dispersal of young sponges. To learn more about how fast and why the sponges make these unexpected moves, they say that further time-lapse imagery and other studies are needed.
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This work was supported by DFG Cluster of Excellence "The Ocean in the Earth System" at the University of Bremen from the ERC Adv Grant ABYSS, the European Union's Horizon 2020 research and innovation program, the Helmholtz Association, the Max Planck Society, and NASA.
Current Biology, Morganti et al.: "In situ observation of sponge trails suggests common sponge locomotion in the deep central Arctic" https://www.cell.com/current-biology/fulltext/S0960-9822(21)00353-5
Current Biology (@CurrentBiology), published by Cell Press, is a bimonthly journal that features papers across all areas of biology. Current Biology strives to foster communication across fields of biology, both by publishing important findings of general interest and through highly accessible front matter for non-specialists. Visit http://www.
CAPTION
This figure shows typical sponge spicule trails.
CREDIT
AWI OFOBS team, PS101; Morganti et al./Current B
CAPTION
This image shows trails left by sponges as they crawl across the seafloor.
CREDIT
AWI OFOBS team, PS101
Surprise in the deep sea
Researchers discover sponge paths on the ocean floor
Sponges: They are considered to be one of the most primitive forms of animal life, because they have neither locomotion organs nor a nervous system. A team around deep-sea scientist Antje Boetius has now discovered that sponges leave trails on the sea floor in the Arctic deep sea. They conclude that the animals might move actively - even if only a few centimetres per year. They are now publishing these unique findings in the journal Current Biology.
The surprise was great when researchers looked at high-resolution images of the sea floor of the Arctic deep sea in detail: Path-like tracks across the sediments ended where sponges were located. These trails were observed to run in all directions, including uphill. "We conclude from this that the sponges might actively move across the sea floor and leave these traces as a result of their movement," reports Dr Teresa Morganti, sponge expert from the Max Planck Institute for Marine Microbiology in Bremen. This is particularly exciting because science had previously assumed that most sponges are attached to the seafloor or are passively moved by ocean currents and, usually down slopes.
"There are no strong currents in the Arctic deep sea that could explain the structures found on the sea floor," explains expedition leader Prof. Antje Boetius, who works together with deep-sea biologist Dr Autun Purser from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) in the HGF-MPG Joint Research Group for Deep-Sea Ecology and Technology. The recently published recordings were made during an expedition at 87 °North at the Karasik Seamount about 350 kilometres away from the North Pole with the research icebreaker Polarstern in 2016 with a towed camera system OFOBS (Ocean Floor Observation and Bathymetry System). "With OFOBS we can create 3D models from the deep sea. The seamount's summit was densely populated with sponges. 69 percent of our images showed trails of sponge spicules, many of which led to live animals," reports Autun Purser.
Many questions arise from these observations: Why do the sponges move? How do they orient themselves? Possible reasons for locomotion could be foraging, avoiding unfavourable environmental conditions, or to distribute offspring. Searching for food in particular plays a major role in nutrient-poor ecosystems such as the Arctic deep sea. Sponges have an important function there anyway. As filter feeders they can utilize particle and dissolved organic matter and are intensively involved in nutrient and matter recycling by means of their bacterial symbionts. Sponges also provide arctic fish and shrimp useful structures to use as a habitat. However, the scientists still have to investigate the mechanisms of locomotion.
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Original publication:
Teresa M. Morganti, Autun Purser, Hans Tore Rapp, Christopher R. German, Michael V. Jakuba, Laura Hehemann, Jonas Blendl, Beate M. Slaby, Antje Boetius: In situ observation of sponge trails suggests common sponge locomotion in the deep central Arctic. Current Biology (2021); DOI: 10.1016/j.cub.2021.03.014
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