Fish Out of Water: an Unexpected Discovery

The amphibious mangrove rivulus, Kryptolebias marmoratus (photo by P. Wright)

All living things must carefully regulate the balance of ions (salts) and water in their body.  Such “ionoregulation” can become especially tricky for amphibious fish that migrate from water to land. Whether they do so by choice or not, they experience a dramatic change in their environment that threatens their survival.

Megan Ridgway, an undergraduate student in Dr. Patricia Wright’s lab in the Department of Integrative Biology, recently discovered what triggers the ability of amphibious fish to survive the sudden shift to living in air. They use cortisol, the same hormone that increases with stress and anxiety in humans.

The amphibious fish in question is the mangrove rivulus (Kryptolebias marmoratus). It is found in the mangrove forests of Belize and can survive for up to two months out of water. 

“It’s an amphibious superstar,” says Louise Tunnah, a PhD candidate in the same lab who worked with Ridgway on the study.

Moving from water to land is a challenge not unlike that faced by other fish that move from saltwater to freshwater to spawn. To cope with the changes in salinity, these fish enact changes to the cells of their gills responsible for exchanging salts with the environment (ionocytes). These changes are coordinated by a spike in cortisol.

But when rivulus leaves the water, its gills collapse.  Instead, it undergoes a rapid and major functional shift to use its skin to exchange salts with the environment.   

Even though rivulus uses its skin rather than its gills, Ridgway wondered whether the same hormone might still coordinate the changes in both the gills and the skin.

“Cortisol is an important ionoregulatory hormone in fish but it has never been measured in this species,” explains Ridgway.

Previous work in Dr. Wright’s lab had found that ionocytes in rivulus skin increase in size when they leave water, and have higher abundance of proteins that exchange salts across the skin. These changes increase the efficiency of salt transport across the skin, but for Ridgway and colleagues, the question remained: what exactly triggers this response?

To find out, Ridgway first had to confirm that cortisol levels changed in rivulus exposed to air. She compared fish exposed to air on a moist substrate for up to seven days to those immersed in water.

“Adapting the usual cortisol assay to work with this new species proved to be quite challenging,” says Ridgway. 

“It’s always a challenge when working with an unusual species like rivulus,” agrees Tunnah. But little did the pair know just how unusual rivulus would ultimately prove to be.

After much perseverance, they were finally able to relate cortisol levels to both increased ionocyte size, and the abundance of salt-shuttling proteins in ionocytes. And just as importantly, inhibiting cortisol prevented those changes from happening, confirming that cortisol does indeed play an integral role in coordinating ionocyte remodelling.

With these two clear results, Ridgway’s hypothesis was supported and the experiment was deemed a success. But rivulus had one more surprise to throw the team’s way.

Most ionocytes tend to be a regular, smooth shape, but Ridgway and Tunnah found that almost half of the skin ionocytes in air-exposed rivulus were “spikey”, with many finger-like projections.

Initially, Ridgway was concerned about these never-before-seen-cells.

“Our immediate thought was that we did something wrong,” Ridgway commented. “Maybe it was something random or something was wrong with those samples but we just kept seeing these spikey cells and some of them were really incredible.”

Even more incredible was that the spikey ionocytes only occurred in the skin, meaning they might be unique to amphibious fish for regulating salts through the skin. The spikey projections also seemed to reach out toward blood vessels to more easily create a bridge between the blood and the skin surface.

“Think about the transition from water to land, it is hard to imagine two more different environments,” says Ridgway. “Rivulus have to go from using one organ - their gills - for critical ionoregulation to an entirely different strategy - their skin. We characterized how the process begins.”

The incredible ability of rivulus to survive in radically different environments may help us understand not only how other species may cope with new environmental challenges in our ever-changing world but also how ancient fishes were able to traverse the water-air boundary.

This study was funded by the Natural Sciences and Engineering Research Council.  

Read the full study in the journal Proceedings of the Royal Society B – Biological Sciences.

Read about other CBS Research Highlights.