Half an hour soaking in a bath will leave you in no doubt about the problems associated with an aquatic life style; we soak up water. And it's the same for freshwater fish. On the other hand, ocean-going species have a different problem; they face desiccation in salty waters. Fortunately, fish have solved their osmotic problems with ion-pumping proteins in their gills to maintain a healthy osmotic balance; sea dwellers excrete salt, while fresh water species absorb salts from the environment. But what about the species that leave their river homes when young to head out to sea? Somehow they have to throw the ion-pumping switch or face certain death. Intrigued by trout's gill metamorphosis, Greg Goss and his colleagues at the University of Alberta began looking at the cellular and molecular changes the fish undergo as they embark on a journey out to sea(p. 905).

First the team monitored the ratios of different gill cell types as the salt levels rose. Categorising cells by whether or not they bound peanut lectin agglutinin, Goss found that the ratio of cells changed dramatically as the salinity rose. And when the team looked at the ion-pumping proteins in each cell type, they changed too as the water became saltier. Not only were the gill cells altering their function, but their distributions changed too. And when Goss took a closer look at one of the gill subtypes, he realised that we have more in common with fish than enjoying a quick dip; the cells were very similar to acid-secreting cells found in mammalian kidneys.

Hawkings, G. S., Galvez, F. and Goss, G. G.(
). Seawater acclimation causes independent alterations in Na+/K+- and H+-ATPase activity in isolated mitochondria-rich cell subtypes of the rainbow trout gill.
J. Exp. Biol.