When we imagine organized animal societies, it’s usually ants or chimpanzees or human armies that come to mind. We certainly don’t think of the spongy, somewhat slimy, dark green blobs that cover rocks in the inter-tidal zone of California's coast.

These are colonies of sea anemones. When the tide comes in, the blobs unfold their pinkish tentacles and wave them mindlessly in the sea, extracting food from the water. A colony begins as an individual anemone that divides into two copies of itself, which in turn divide into two, and so on and on. A clone is born, anchored to a rock. Each anemone is genetically identical to all others in its colony. Hardly a zenith of complex social organization, you might be thinking.

In fact, ecologists working at the Bodega Marine Laboratory at Bodega Bay are finding that anemone society is anything but simple. Professor Rick Grosberg, in the department of Ecology and Evolution at UC Davis, and his collaborator, Professor David Ayre, from the University of Wollongong in Australia, are uncovering the secrets of an intriguing military system within the seemingly homogeneous mass of the anemone clone.

First of all, there’s definitely an ‘us/them’ distinction between groups. While clone-mates live tightly packed together, different clones are separated by a thin, though distinctive, no-man’s land, anemone-free except for the occasional corpse.

In addition, although clone-mates are genetically identical, yet they are different. Grosberg and Ayre have shown that the front-line individuals along the border, the "warriors", are usually smaller than the interior anemones, the "reproductives", and are more heavily armed. Their weapons are special attack tentacles, loaded with poison-filled barbs that they inject into an enemy anemone. Victims that can’t retreat can die form their wounds.

Given this level of inter-clonal enmity, the existence of long-term. stable boundaries is an enigma. Why don’t single clones aggressively take over the available space? What happened to imperialism?

There are two possible explanations: one is that neighboring clones eventually come to tolerate each other and stop fighting. The second is that the armies of different clones are so evenly matched that a stand-off results.

Ayre and Grosberg examined these possibilities by removing individual anemones from seven pairs of neighboring clones in Bodega Bay, and setting up one-on-one contests in the lab. The experimental design was simple: put two anemones together and observe who beats up whom. Acting as referees, the scientists measured size and number of attack tentacles, and scored who struck first, who retaliated and who won or lost, judged by which contestant retreated.

The results of this experiment deepened the puzzle in many ways. Individuals from different clones were always ready to fight each other, suggesting that mutual tolerance does not readily develop. At the same time, neighboring clones were not evenly matched. In fact, there were clear dominance relations between them: the winners of one-on-one fights could be predicted on the basis of their clonal identities. The keys to success were number and size of fighting tentacles and readiness to launch an attack.

Grosberg and Ayre have a hunch that the combative inequalities seen in the lab are somehow watered down on the real life battlefield, when anemones-in-arms fight as a unit rather than as individuals. For example, a clone with whimpy warriors might nevertheless have the ability to create replacement warriors at a high rate. Recruitment rates could vary with clone size, number of neighbors, and other environmental factors. On the rocks in Bodega Bay, it might be unlikely that one clone could consistently dominate another.

To study group aggression in nature, Grosberg and Ayre have taken the warriors of one clone and transplanted them behind the front-line of a neighboring clone, right into the thick of the enemy. They are watching to see if the infiltrated reproductives become warriors and produce another front line, and possibly a clear boundary within the invaded clone.

This raises the question of how warriors develop in the first place. Can civilian anemones be drafted? Back in the lab, the researchers are attempting to turn reproductives into warriors by exposing them to enemy assaults, testing the idea that repeated attacks might stimulate the growth of more fighting tentacles in the victims.

The research at Bodega Bay does more than just satisfy our curiosity about some bizarre marine creatures. It tackles the much larger question of how social behavior is influenced by both genes and environment, an issue that is relevant to everything from blobs with tentacles to human beings.

© Kelly Stewart

Dept. of Anthropology, University of California, Davis, CA 95616
e-mail: kjstewart@ucdavis.edu

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