Most fish are confined entirely to fresh water (FW) or seawater (SW) and cannot live in or adapt to the other environment. What may be surprising is that the internal salts and organic molecules (solutes) in the blood of all bony fishes are maintained in an "intermediate" concentration (which is actually similar to that in mammals). In other words, fish in salt water may suffer stress because they are living in a medium about 3 times as salty as their blood and therefore must "pump out" extra salt that is ingested. This requires molecular transport proteins (such as the sodium/potassium pump and other ion pumps and channels). FW fish, on the other hand, face the problem of becoming "waterlogged". In other words, the higher concentrations of salts in their tissues and blood cause the fish to gain water by diffusion (also called osmosis). These fish must rid themselves of the extra water and conserve salts. They do this by using molecular transport proteins. In general the control of internal salt and water balance (osmoregulation) requires significant metabolic energy to power it. As most people know, a small number of fish like salmon and eels spend a part of their life in FW and part of their life in SW. These fish literally switchover from the FW metabolism to the SW metabolism, a process that may be metabolically stressful. A surprising little fish (3 inches long), the killifish (Fundulus heteroclitus), has been shown to have phenomenal osmoregulatory abilities. This fish can survive indefinitely in FW or in SW up to 3 times more concentrated than ocean water. Furthermore, killifish may migrate daily from SW to FW and back to feed (and to breed and lay eggs in the Spring) making them appear to be unusually adept at osmoregulation. At present there is intense interest in the metabolic machinery and especially the molecular transport proteins that are involved. Indeed, many of the same types of proteins and their responses to salinity change that are found in killifish also are found in salmon and eels. However, with killifish (and perhaps other fish as well) another mechanism, to deal with salinity stress has been suggested, termed behavioral osmoregulation. The heart of this hypothesis is that, all other things being equal, killifish will try to swim up FW streams to the point where their internal salt and water composition resembles that of the external water (about 1/3 strength SW) and stay there conserving metabolic energy that would otherwise be expended pumping salts in or out of the fish. Preliminary data support the hypothesis that killifish may seek salinities about 1/3 that of SW. This new idea has broad implications physiologically and ecologically. The principal investigators will measure the metabolic energy requirements for osmoregulation in killifish. Using DNA based techniques, they will measure the presence of and changes in the molecular transport proteins in killifish. They also will investigate the ecology of wild killifish and attempt to correlate natural distributions and breeding behavior with projected salinity preferences. A very important part of this project is that the principal investigators will lead a team of 8 undergraduate students per year (for each of 4 years) who will work during their academic year on this research at their home institutions and then come to Mount Desert Island Biological Laboratory for 2 months during the summer to do fieldwork, physiology and molecular biology. The students will have the opportunity to do original research while learning modern techniques in many fields at one of the country's finest marine laboratories. It is expected that this experience show these students the passion and fulfillment of scientific research that will motivate them in their future careers.