Study Reveals Secrets of Fish 'Out of Water'

November 23, 2010 - News Release

"Fish out of water" has a new literal meaning for University of Guelph researchers who have discovered how some tiny fish can survive on land.

Skin cells called ionocytes allow killifish - diminutive fish native to the Caribbean mangroves - to actively exchange salt and water. This makes it possible for them to survive for days and even weeks out of water, according to a new study by Guelph biology professor Pat Wright.

In other adult fish, ionocytes occur only on the gills, where they help keep water and salt balanced.

The study reported in the latest issue of Physiological and Biochemical Zoology also involved former graduate student Danielle LeBlanc, now an instructor at Mount Allison University, and U of G biologist Doug Fudge. Articles about the research have appeared recently in U.S. News & World Report and the Vancouver Sun.

Killifish live in Caribbean mangroves. They weigh only about 0.1 grams and are only 30 to 50 millimetres long. But they are known for being able to live under the most extreme conditions, including in very little water.

“All cells in the body need the right combination of ions and water for an animal to stay alive,” Wright said. “Normally the gills are responsible for these processes in fish. So how these fish maintain ion balance out of water was a mystery.”

In experiments with killifish living out of water for nine days, the Guelph researchers found that these fish have ioncytes on their skin. In most fish, skin ionocytes occur only in larval stages and remain only on the gills in adults.

“We found the mangrove killifish have roughly as many ionocytes on their skin as on their gills,” Wright said.

That enables mangrove killish to live out of water for more than 60 days, she said.

Killifish skin also adapts to changing aquatic conditions. The ionocytes enlarge in salt water to help maintain salt balance and return to normal size back in freshwater. “This is part of what makes them so remarkable,” Wright said.

Skin-breathing isn’t their only remarkable feature. Killifish are the only vertebrates with both male and female sex organs that produce and then fertilize their own eggs. Although many fish species are hermaphroditic, individuals typically either make eggs or fertilize those of others, rather than doing both.

Wright has studied killifish and other fish since 1991 with funding from the Natural Sciences and Engineering Research Council. Her research may hold important clues about how the first land animals evolved from aquatic ancestors. “There are lots of applications for what we’re learning from these little fish.”

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