Salmon face osmoregulatory challenges during their lifecycle. as hatchlings, they spend their time in freshwater environments for several months before migrating downstream towards the ocean. as adults, they spend most of the time in the ocean, but migrate back to the freshwater stream where they hatched to lay their eggs. describe the physiological modification of the gills necessary for salmon to survive in these two distinct environments. specifically, (1) what changes do you expect to see in the number of both chloride cells (that secrete salt to the environment) and ionocytes (that absorbs salt from the environment) when the salmon move from freshwater to seawater? (2) what ion channels, pumps, transporters, exchangers are newly expressed by the gill epithelial cells when the salmon move from freshwater to seawater? (3) what ion channels, pumps, transporters, exchangers are lost by the gill epithelial cells when the salmon move from freshwater to seawater? (4) what do you think is the biological signal that initiates the coordinated physiological changes described in (1) through (3)?