Well, if you take a cod and drop in in freshwater, it will die. The euryhalaine species are able to change the direction their osmotic processes. The shark able to live in all waters are more salty inside than sea water. I don't know how the shark does it, but fishes has an opercular membrane in the gill flaps that regulate salt balance in the animal. In an very old, but efficient instrument, called an Ussing chamber you can measure the voltage over the membrane and depending on salilinity, the euryhaline opercula membrane reverses the transport of salt; in fresh water, they absorb salt and in sea water they excrete salt. The gill blood passes over the inside surface of the membrane, where there are so called chloride cells that have salt sensors and if blood salinitynis either high or low, the membrane responds by pumping NaCl in or out of the passing blood.
We were visiting a university in the middle of nowhere up in Nova Scotia, Canada. To demonstrate it, our host took a killifish adapted to fresh water and just dumped it i sea water. The fish seemed confused for 2 seconds before it swam happiliy.
The Ussing chamber is a simple set up, and by manipulating salinity of solutes, adding drugs blocking certain signal pathways in the cell, the researcher can find out how the environmental cue is translated into a particular set of molecular signals in the cell regulating everything from cell homeostasis, growth, cell division or programmed cell death. I will soon shut up, but ...I have to mention programmed cell death. There are two ways to die: necrosis, which is an uncontrolled cell death, we can see that on plants with severe nute burn. And then we have the programmed cell death, apoptosis, where the cell recieves a signal, activating a suicide programme, where the cells destruct and consume themselves. This process prevents cancer, it made our fingers and toes and it is how an organism remodells its tissues.
Hardcore morning lecture!
