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Physostomes are fishes that have a pneumatic duct connecting thegas bladder to thealimentary canal. This allows the gas bladder to be filled or emptied via the mouth. This not only allows the fish to fill their bladder bygulping air, but also to rapidly ascend in the water without the bladder expanding to bursting point. In contrast, fish without any connection to their gas bladder are calledphysoclisti.
The physostome fish encompass thebichirs,gars, a number ofcarps,trouts,herrings,catfish,eels and thelungfish. While the gas bladder in fish mainly serves as abuoyancy organ, some physostomes (though not all) can use their gas bladder as alung, allowing them to live from atmospheric oxygen in conditions where aquatic oxygen levels have dropped to a point which would kill other fish.
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