Published online 7 November 2008 | Nature | doi:10.1038/news.2008.1215

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Like a fish out of salt water

The sculpin could reveal how some ocean dwellers made the jump to life in fresh water.

Longhorn sculpinDo sculpin offer a window on evolution?K Hyndman

A marine fish that seems to be on the verge of adapting to life in fresh water is shedding light on how some species managed the evolutionary feat in the past.

Unlike salmon and some trout, which can survive in both rivers and oceans, most fish have to live in either fresh water or salt water. But when fishermen in Maine started discovering a marine fish known as the longhorn sculpin (Myoxocephalus octodecimspinosus) in estuaries, Kelly Hyndman and David Evans, zoologists at the University of Florida in Gainesville, decided to take a closer look.

They placed the fish in aquaria containing different concentrations of sea water and, after 24 hours and 72 hours, they measured the concentrations of sodium, potassium and chloride ions in the blood, as well as the levels of three proteins in the gills that help to regulate the presence of the ions. The three proteins were Na+/K+-ATPase, an Na-K-Cl cotransporter and the cystic fibrosis transmembrane conductance regulator.

Staying in control

The team report in Experimental Zoology1 that, even in fresh water, the sculpin produced about the same amounts of the three proteins as they did in sea water. The inability of the sculpin to regulate the number of these proteins makes it more difficult for the fish to survive freshwater conditions in the long term.

"To the best of our knowledge this is the first time the physiological barrier preventing marine fish from entering fresh water has ever been identified," says Hyndman.

But puzzlingly, analysis of the blood samples showed that although sculpin in a mixture of 10% sea water and 90% fresh water were losing these ions from their bodies into the brackish water around them, those in salt water concentrations of 20% or more were not and, according to Hyndman and Evans, could probably have survived indefinitely

The osmolality — the concentration of ions such as potassium, sodium and chloride — of the 20% seawater in which these sculpin were immersed was 190 milliosmoles, or just 56% that of their blood (340 milliosmoles). The difference should mean that salts drain out of the sculpin, eventually killing them. "We were astonished that the fish were somehow able to maintain a steady plasma osmolality in these conditions," adds Hyndman.

The team suggests that there are other proteins, such as sodium–hydrogen exchangers, that bring ions into the body to compensate for the outflow and keep the sculpin from depleting their ion reserves. The fish's kidneys may also be helping to keep ions from escaping.

Evolutionary jump

The sculpin could be on the verge of moving to fresh water, as selective pressures — such as more saltwater predators — threaten to push it over the edge. As the marine fish that have spawned novel freshwater species might have done so by first moving into brackish estuaries, the opportunity to watch sculpin go through a similar transition could be invaluable. Indeed, many of the longhorn sculpin's relatives are species that are able to handle some degree of brackish water and three species in the genera are freshwater species.

"I think we've caught a species in the early throes of inventing, by evolution, a better means to adapt to fresh water," says biologist William Marshall at St Francis Xavier University in Antigonish, Canada, who was not involved with the work.

Salmon and trout, although able to survive in salt and fresh water, are unlikely to be good physiological models of fish that have permanently left the sea for freshwater streams and rivers, he says.

"Estuarine species are the opportunists here. Sculpins, gobies, stickleback, killifish and bass are examples of animals that can tough it out and tolerate brackish water, but that do the trick using unconventional physiological mechanisms," says Marshall. "Although it is immensely inefficient to try to survive in brackish water while actively pumping salt out [as the sculpin seems to], this may still be to the benefit of the species if, for instance, they can use the brackish location as a predator-reduced nursery." 

  • References

    1. Hyndman, K. A. & Evans, D. H. J. Exp. Zool. doi: 10.1002/jez.494 (2008).
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