Would it be possible for a creature to have gills like a bony fish, but attached to a lamprey-like respiratory tube, so that it can pump water back and forth through the gills without using the mouth?
$\begingroup$ Don't the lampreys pump water back and forth from their gill sacs? Expand the background some please - I probably don't understand how lampreys do it, and reading just now I still don't. $\endgroup$– WillkMay 31, 2020 at 0:49
It's possible but it would be less efficient: the inward moving water would be rich in oxygen and poor in carbon dioxide, while the outward moving water would, on the contrary, be rich in carbon dioxide and poor in oxygen.
Considering that water has a lower content in oxygen than air, the reduction in efficiency would probably be too much for a fish with the metabolic needs connected to an active life style.
$\begingroup$ What if the creature had a spiracle, so that water could enter the tube through the spiracle, and exit through the gills? $\endgroup$ May 30, 2020 at 18:22
$\begingroup$ @IchthysKing, you wrote "back and forth" $\endgroup$– L.Dutch ♦May 30, 2020 at 18:39