Anthopleura on the Rocks: A Collage of Competing Clones

There are places where Anthopleura elegantissima covers the intertidal rocks. It’s not an overstatement.

Densely packed Anthopleura elegantissima, exposed at low tide
You might mistake this dense colony of A. elegantissima for a solid rock surface

But the covering isn’t unbroken. There are narrow open spaces between patches. Patches of A. elegantissima are clones of genetically identical polyps. Adjacent clones fight, culminating in anemone-free aggression borders. Notice the unoccupied borders in the photo below, from April 2017.

A collage of competing clones

If, in the wild, clones fight, there’s going to be winners and losers. Why so many persistent patches? Why don’t the best fighting clones dominate the rocks? I got an answer at the 2017 Society for the Study of Evolution Annual Meeting in Portland, Oregon, where I made it a point to show up at David Ayre’s talk, Does boldness predict ecological outcomes in a clonal sea anemone? Ayre presented an update on clone competition in A. elegantissima, work he and Rick Grosberg have been collaborating on since the 1990s.

I learned, when Ayre gave the background natural history, that clone borders can be stable for decades. But borders are a low tide phenomenon; they disappear at high tide when clones battle. Polyps at the borders, the ones that do the fighting, are smaller and equipped with more white-tipped fighting tentacles than their interior clone-mates.

In the lab, Ayre and Grosberg showed that there are aggressive and subservient clones, and the losing anemone in one-on-one encounters moves away.

What maintains subservient clones? Ayre explained that the answer lies in tradeoffs between armaments and other anemone attributes. Aggressive clones have a a fighting advantage, but they lose their competitive edge when space opens up for the taking. There is no more prized resource in the rocky intertidal than open space, and subservient clones are better at colonizing it. That’s an advantage for clones that share rocks with aggressive neighbors. Thus, when it comes to persistent clone diversity in Anthopleura elegantissima, a big part of the explanation is tradeoffs between aggression and fighting ability, and colonization ability.

Aggregating anemones in a sand-filled tide pool
Anthopleura elegantissima are sometimes called pink-tipped anemones

Here are some papers by Ayre and Grosberg that explore the A. elegantissima story through their eyes:

Ayre, D. J. and R. K. Grosberg. 1995. Aggression, Habituation, and Clonal Coexistence in the Sea Anemone Anthopleura elegantissima. The American Naturalist 146 (3): 427-453.

Ayre, D. J. and R. K. Grosberg. 1996. Effects of social organization on inter-clonal dominance relationships in the sea anemone Anthopleura elegantissima. Animal Behaviour 51, 1233–1245.

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, 97–110.

While I was preparing this post I enjoyed The original clone wars. Check the Pacific Rocky Intertidal Monitoring: Trends and Synthesis page on Anthopleura for basic natural history.

Anthopleura elegantissima Lives Up to All Its Common Names is a story I wrote about A. elegantissima‘s common names in April 2017.

If you like sea anemones, my Sea Anemones page shows photos of several species from Oregon’s rocky intertidal.



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