up anddown the water column. These structuresincrease the weight of the fish, so thesefishes have retained the swim bladder forbuoyancy. As they move up and down,they experience dramatic changes in pres-sure. Vertically migrating fishes can ra-pidly adjust the volume of gas in the swimbladder to prevent it from collapsing orexploding when they change depth. They are also able to tolerate the temperaturechanges they experience as they move upand down across the thermocline