"It now appears that another option for emersion—the behavior of leaving the water—is to enter logs," Taylor said. "We now suspect that logs may be foci for these emersed pool populations."
The research will appear in an upcoming issue of the journal American Naturalist.
"Exceptional" Extremophile
Meanwhile, lab experiments by Patricia Wright, a biologist at the University of Guelph in Ontario, are helping to explain the unusual inner workings of the fish.
Wright has long been fascinated by the animal's extremophile nature—it's ability to survive in extreme circumstances. In addition to its long stints in air, the fish shrugs off large variations in water temperature, pH, salinity, oxygen level, and even the level of pollutants.
It's an "exceptional animal," she said. "It's able to survive in air for a long period of time without going into an altered metabolic state such as a lungfish might."
Her studies suggest that the animals can breathe cutaneously—through their skin—as long as they remain in a moist environment. To increase the time they can do this, the fish fill their gill systems with a mass of cells.
"Otherwise, the delicate [gill] plates would collapse and fuse together," she said.
Wright has also found surprising changes in how the fish deal with waste. Normally, fish excrete waste products such as nitrogen through their gills, but her experiments show rivulus use their skin for this as well.
"They have very thin skin, and you can see this very dense capillary network," she said.
Puzzles Abound
The logpacking find raises many questions, study leader Taylor added. For example, rivulus don't normally like each other.
"[They are] very aggressive to each other in aquaria and also apparently in the field, but they pack together in many numbers without aggression [in logs]," he said. "So it appears that during emersion this behavior ceases."
He speculates that the logs might serve an additional purpose—acting as cheap transport when storms send the wood drifting. But finding out for sure will require additional research.
"Ferreting out [the fish's] natural history and field behavior has been a real challenge, and that work is far from over," he said.
That's compounded by the fish's bizarreness, which has led to "looking in all the wrong places," he added.
"What normal ichthyologist would look in crab burrows or inside logs?"
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