Anemone Wars: Clone armies deploy scouts, attack tidally

Susan Milius

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.

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References:

Ayre, D.J., and R.K. Grosberg. 2005. Behind anemone lines: Factors affecting division of labour in the social cnidarian Anthopleura elegantissima. Animal Behaviour 70(July):97–110. Abstract available at http://dx.doi.org/10.1016/j.anbehav.2004.08.022.

Further Readings:

For more information about anemones, go to http://hmsc.oregonstate.edu/projects/rocky/aganem.html.

A version of this article written for younger readers is available at Science News for Kids.

Sources:

David J. Ayre
Institute for Conservation Biology
Department of Biological Sciences
University of Wollongong
Wollongong, NSW 2522
Australia

Samuel N. Beshers
Department of Entomology
University of Illinois, Urbana-Champaign
320 Morrill Hall
505 South Goodwin Avenues
Urbana, IL 61801

Rick Grosberg
Center for Population Biology
Division of Biological Sciences
1 Shields Avenue
University of California, Davis
Davis, CA 95616

From Science News, Volume 167, No. 23, June 4, 2005, p. 355.