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DANCE OF THE MUSSELS

11.18.08 | Comment?

(Video courtesy of Johan van de Koppel.)

It’s a lousy dancer who steps on his partner’s foot, but groups of mussels use a similar foot game to form intricate patterns in a newly discovered underwater dance that benefits all involved.

Scientists in the Netherlands have for the first time shown how individual mussels, acting seemingly independently, naturally self-assemble into large-scale, net-like clusters that balance the competing needs of safety and access to food.

(Image courtesy of Johan van de Koppel)

“Mussel fishermen already knew this,” said Johan van de Koppel of the Netherlands Institute of Ecology. “If you put baby mussels—mussel seeds—in a bucket, they naturally form clumps.” Studying seeded mussel beds in Wales, Van de Koppel’s team was able to show why.

“It’s the best of two worlds,” explained van de Koppel. The mussels’ patterns find the middle ground between a dense uniform patch and sparsely distributed small clumps. Density provides safety from predators and wave action, but limits access to food for the mussels on the inside of the patch. Mussels eat phytoplankton, microscopic, free-floating algae that drift with water currents. In dense patches, many of the phytoplankton are eaten before reaching the interior mussels.

Sparse small clumps, on the other hand, provide bountiful access to food, but offer less protection from predators and waves. Van de Koppel explained the dilemma as follows, “If you take a…dense mussel bed, they are safe, but have nothing to eat. If they are too isolated, they have enough to eat, but aren’t very safe.”

To resolve these competing interests, mussels form a pattern somewhere between a dense uniform patch and sparse clumps. They organize themselves into regularly spaced clusters, each between five and ten centimeters (two to four inches) wide, spaced about ten centimeters apart. In pictures, these patterns look somewhat like a net, with channels of bare sand interwoven among splotches of black mussels. “We were interested in these net patterns initially because they are easy to see, especially from an airplane,” commented van de Koppel.

(Image courtesy of Johan van de Koppel)

After observing these patterns in nature, the scientists set out to reproduce them in the lab. They seeded tanks with baby mussels, starting out in a uniform spread. Within a day, the mussels had organized themselves into patterns that resembled those found in nature. “The general structure follows the pattern we see in the field,” noted van de Koppel. The researchers were then able to model the same patterns using computers, providing a second verification of their results.


Check out what happened in the tank. (Video courtesy of Johan van de Koppel.)

And here’s what the computer model showed.  (Video courtesy of Johan van de Koppel.)

The scientists attributed this self-organizing to individual one-on-one interactions of the mussels. All other variables in the tanks, such as the distribution of sand, were kept uniform. The team found that mussels respond to the number of other mussels that touch their foot. (Like all mollusks, mussels have a single muscular foot that allows them to move.)

Van de Koppel’s results represent a new wave of ecological research, focused on how the actions of individuals create large-scale phenomena. “It’s not mainstream ecological forecasting to start with the individual,” commented Frederic Guichard, an ecologist on sabbatical from McGill University, not involved with the study. “Computationally, it’s very intensive. Also, there are a lot of uncertainties and small uncertainties can have huge effects.”

Guichard explained that a natural mussel bed might have a million individuals and that researchers would like to be able to predict the behavior of the whole bed, given knowledge about the behavior of one mussel. However, this poses a problem. As Guichard clarified, “The effect of a million individuals is not a million times the effect of one individual. The whole is not the sum of its parts.”

The patterns observed by van de Koppel are a prime example of how the behavior of the many can be complex. But powerful computers are starting to allow this complexity to be understood and predicted. Research into natural pattern formation has found related spatial patterns all over the world, from so-called tiger-bushes in Nigeria (bushes that grow in stripe-like patterns), to patterns of coral growth in Australia.

“Ecological interactions can generate spatial patterns…we should take this into consideration when we manage these systems,” van de Koppel concluded. “The formation has a purpose. If we mess with the spatial structure, we mess with the functioning of the ecosystems.”

Check out this Google Earth tour of spatial self-organization in different ecosystems around the planet.  If you have Google Earth and want to take the tour yourself, click here.

(Video courtesy of Johan van de Koppel.)

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