duminică, 31 august 2008

Nemo Sniffs His Way Home

Keen Sniffers

Aug. 29, 2008 -- Tiny orange clownfish, made famous by the Disney character Nemo, use the smell of leaves and anemones in the water to find their way home on the coral reef.

That's the finding of a new study using a clever apparatus to measure the fishes' preference for water carrying different odors.

A team led by Geoffrey Jones of James Cook University in Townsville, Australia surveyed waters around Papua New Guinea for clownfish populations.

"The boat captain said, 'If you want to find the orange clownfish, you have to find islands. The fish need to see trees,'" said study lead author Danielle Dixson. The survey confirmed this observation: "There's a huge statistical difference [in the numbers of clownfish] between where there are islands and where there are not islands."

For reasons that are unknown, the two types of anemones that the region's clownfish call home only live near islands with trees and beaches and are not found on "islands" made only of reefs.

But the fish have to search for these anemones, because after eggs hatch near the parents' home anemone, the larvae are carried away by ocean currents. About 11 days later, the juvenile fish settle back into a new anemone, somehow having found their way to their favored abodes.

The researchers set out to figure out how.

They used a chamber with two sources of water flowing side by side. At the top, a wall divides the chamber, separating the water sources. Lower down, the wall disappears, but the water remains unmixed, with the two types of water flowing parallel to each other.

The researchers introduced clownfish into the chambers and measured how much time they spent on either side. This allowed the researchers to test the fishes' preference for water from different sources.

First they compared beach water from near vegetated islands with water from reef islands.

"It was ridiculously high how attracted they were to the beach water," said Dixson. The fish spent more than 99 percent of their time on the side of the chamber with beach water flowing by.

"The next step was to figure out what is in the beach water that is making them able to discriminate beach water from the other," she said.

Researchers have previously shown that clownfish are attracted to a chemical cue from anemones, and the team found that the fish strongly preferred water that had been exposed to anemones versus water that had not.

But the signal from anemones is unlikely to travel very far, so the team wondered whether there were other cues that could draw the fish back to the islands.

"The islands are loaded with trees," Dixson explained, and the water nearby has large numbers of leaves floating on the surface. So, the team exposed ocean water to five different kinds of leaves from the islands, and to a mixture of the leaves, and compared those to ocean water with no leaf exposure. "They were attracted to all of them."

But they were not attracted to the tea tree plant, which grows in swamps nowhere near the islands, so the fish have specific preferences for the "right" kind of trees.

Finally, the team showed that even fish bred in aquariums in synthetic seawater were attracted to beach water and to anemone and leaf cues, suggesting that this attraction is innate.

"The results are just spectacular," said Jelle Atema of Boston University who developed the testing chamber for his own research and shared one with the Jones group. "As humans we don't take very seriously the notion of odor in water. It's very foreign to people: How can you smell in water?"

Jelle agrees with the researchers that the abundance of leaves in the water near these islands serves to bring the fish into the right vicinity, after which they can search out the anemones.

Beyond providing an example of animal abilities, the research also has a broader message.

"It shows that there is a connection between the marine and the terrestrial environment," Dixson said. "It shows that the two can't be treated separately, especially in terms of management."

"If you're trying to protect the reef, but you're not protecting the shoreline that calls these 'Nemos' home, it's not going to work," she said.

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