The first description of clashing armies of sea anemones has revealed unsuspected military tactics.
“Sea anemone fights are amazing,” says David Ayre of the University of Wollongong in Australia. Although anemones move in slow motion, a group-living species from the shores of California, Anthopleura elegantissima, fields a sophisticated army, report Ayre and Richard Grosberg of the University of California, Davis.
Researchers some 50 years ago noticed specialized anemone tentacles that inflate but aren’t used for snagging food. In the 1970s, Stanford University biologist Liz Francis found that these structures, called acrorhagi, lash stinging cells onto enemy anemones. Certain individuals, which she called warriors, have many acrorhagi but a dearth of ripe gonads.
Francis coaxed individual anemones, or polyps, to attach to table tennis balls set afloat in the lab. When she mixed balls carrying various genetic types, the polyps sorted themselves by clone. In the wild, hostilities break out at borders between dense colonies, each made up of genetically identical anemones.
Scientists find it almost impossible to study A. elegantissima patches in the wild, says Grosberg, because at low tide these anemones sit with their tentacles pulled in, and when the water rises and the animals become active, waves block the view. However, he and Ayre moved a large boulder with two adjacent anemone clones into the lab.
Regular flushing of the aquarium built around the boulder revealed that anemone hostilities follow the tides. As water rushed in, warrior polyps located several rows back from the border inflated their acrorhagi, tripled their body length, and began bending around as if looking for trouble. They could reach far enough to strike an alien polyp that had crept close to the patch.
“We’d had no idea they could do this,” says Ayre.
Another surprise came from the small polyps along the outermost edge. A no-clone zone as wide as several polyps lies between hostile patches, and small polyps now and then creep into that zone. Typically, they get stung a few times and then retreat to their home colonies.
One polyp, nicknamed Stumpy, took such a drubbing that when it retreated to its home patch, it was attacked by its own team. Grosberg says that it must have picked up so many alien-clone stinging cells that its clone mates didn’t recognize it. Grosberg says that the incident suggests that these small polyps work as scouts.
The idea that information passes from the border to polyps located farther back in the patch sounds plausible, comments Sam Beshers of the University of Illinois at Urbana-Champaign. “They could be playing ‘telephone’ with some molecules they’ve picked up,” he says.
Studying anemone clones could shed light on the interplay of environment and genetics in the division of labor, Grosberg argues. For example, lab experiments found that repeated contact with alien clones encourages the growth of acrorhagi.
His and Ayre’s studies are described in a report entitled “Behind Anemone Lines” in the July Animal Behaviour.
