Everyone knows that fish need water to breath, but the amphibious mangrove killifish defies all common sense by living on land for several months out of the year. Find out how the fish survives on land during the dry season. – Global Animal

Leader-Post, Randy Boswell

A team of Canadian scientists has solved a key mystery about one of the world’s most unusual fish — the amphibious mangrove killifish, a tropical species that can survive out of water for as long as two months.

Three researchers from the University of Guelph and a colleague from Hamilton’s McMaster University in Ontario discovered how the small, swamp-dwelling fish is able to use its skin to process oxygen and maintain a crucial ion balance — the way its gills do when submerged — whenever its coastal freshwater habitat dries up.

The killifish can stay alive while stuck in a wet log or stranded in mucky sediments for the duration of the dry season — as long as 66 days — in habitats ranging from Florida to Brazil.

U.S. scientists who documented the fish’s remarkable out-of-water survival capability in 2007 described it as a potential window into evolutionary history and the gradual transformation of many aquatic species into terrestrial ones.

The Canadian study sought to answer how the fish survives out of water.

The research involved laboratory experiments with killifish that demonstrated the species’ ability to automatically alter its skin chemistry in dry conditions — effectively turning its entire body surface into a makeshift set of gills.

“All cells in the body need the right combination of ions and water for an animal to stay alive,” University of Guelph biologist Patricia Wright said in a summary of the study, published in the latest issue of the journal Physiological and Biochemical Zoology.

“Normally, the gills are responsible for these processes in fish. We knew that in mangrove killifish the gills are likely useless on land, so how these fish maintain ion balance out of water was a mystery.”

Wright and her colleagues used a radioactive tracer to observe what happens to the killifish when taken out of water for nine days but kept on a moist surface, where uptake of chemicals through the moisture is still possible.

“The skin of these fish contains a special group of cells called ionocytes, normally only found in gills,” Wright told Postmedia News.

“These cells transport ions (such as sodium and chloride) between the moist surface and the body tissues. In our study, ionocytes in the skin grow larger when fish are out of water, presumably because they need more cell surface to work harder.”

The killifish, which can grow to four or five centimetres in length, are sometimes found clustered by the dozens in a hollow log in order to sustain moisture levels and a collective chemical balance.

“We have discovered that the skin plays an important role in regulating or balancing water and salts in the body fluids and tissues,” Wright said.

“When in water, the gills and skin both play a role, but, when out of water, the skin takes over and the gills are non-functional.”

That ability not only keeps the species alive, the scientists found, it also helps it adapt to different levels of salinity in water, with the ionocytes expanding or retracting as required to deal with different levels of saltiness in the coastal ponds it inhabits.

Wright, a Halifax-born researcher educated at McMaster, the University of British Columbia and University of Ottawa, said the killifish does offer “evolutionary insights” into the role ionocyte cells played in the rise of land animals during Earth’s early history.

Killifish research helps show how “aquatic animals were able to invade terrestrial areas when water conditions became unfavourable,” she said