Planet Earth

Weirdo Deep-Sea Anemone Kills a Giant Worm, Goes for a Walk

InkfishBy Elizabeth PrestonJul 7, 2015 2:31 PM

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If you already think everything at the bottom of the ocean is slightly terrifying, Iosactis vagabunda won't change your mind. It's transparent, can tunnel underground, and hunts animals 15 times its size. And scientists are now realizing that there might be way, way more of these roaming killers than they'd previously thought. Iosactis vagabunda lives on the Porcupine Abyssal Plain, a seabed southwest of Ireland that ranges from 4,000 to nearly 5,000 meters deep. The species was already thought to be common in this area. But ocean-floor trawls don't drag up many of the animals, thanks to their small, squishy bodies and tendency to burrow. To learn what the transparent creatures are really doing down there, National Oceanography Centre graduate student Jennifer Durden and her coauthors used tens of thousands of photographs. They gathered images from a station at the Porcupine Abyssal Plain, a camera towed by a research ship, and a robotic submarine to help them estimate how populous I. vagabunda anemones are. They sent other cameras to the seafloor to take time-lapse pictures. These cameras snapped shots at 20-minute intervals for 2 weeks, and at 8-hour intervals for another 20 months. Looking over these images, scientists were struck by a few things. For one, I. vagabunda doesn't just withdraw into a burrow when it's feeling shy. It can tunnel underground and pop up in a totally different place. And this isn't a quick process—the time-lapse images showed one anemone taking more than 22 hours to sink into a hole and emerge several inches away. (See a video here.) Durden says this behavior is "quite unusual." Some other anemones are known to burrow, but most species spend their lives glued to one spot—more like a flowering plant than a groundhog. Another series of images showed an anemone eating a spiny sea worm called a polychaete, which it had trapped in its stinging tentacles. The researchers estimated the worm's mass to be about 15 times that of the anemone. It took the anemone the better part of a day to swallow its meal. Durden says this, too, is surprising. There are other examples of anemones eating large prey, including one on the Oregon coast that was seen (rather alarmingly) eating a baby bird. "But I don’t think it has been observed in the deep sea before," Durden says. "Most deep-sea anemones are thought to be suspension feeders." That is, they use their waving tentacles to filter food from the water. But I. vagabunda seems to be a predator. Photographs also showed it using its tentacles to pluck bits of sediment from the ground and pass them to its mouth. Moving between burrows may help the anemones find better parts of the seafloor to feed on, Durden says. And hiding underground protects the animals from being eaten themselves. The anemones stayed in their burrows for an average of 19 days, and as long as 47 days, before reemerging. Yet Durden says it's surprising that this normally cautious animal would also make itself vulnerable by slowly eating and digesting giant prey. The anemone that ate the polychaete stayed upright, its body stretched high into the water, for 56 hours after its meal. Knowing now that I. vagabunda spends so much time underground, the researchers think previous studies may have vastly underestimated how many of these animals live on the Porcupine Abyssal Plain. They guess that about half of all animals in this area might be I. vagabunda anemones—up to 100 times more than previously estimated. If this is true, Durden says, scientists' whole understanding of the seafloor food web would need to change. Along with, presumably, the cast of characters of your anxiety dreams. Image courtesy of Jennifer Durden.

Durden, J., Bett, B., & Ruhl, H. (2015). The hemisessile lifestyle and feeding strategies of Iosactis vagabunda (Actiniaria, Iosactiidae), a dominant megafaunal species of the Porcupine Abyssal Plain Deep Sea Research Part I: Oceanographic Research Papers, 102, 72-77 DOI: 10.1016/j.dsr.2015.04.010

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