Whereas permanent denizens of fresh water or seawater must deal with just one set of salt and water problems, fish residing in estuaries, and fish migrating between fresh and seawater, must handle both sets.
(permanent denizens of fresh water or seawater; must deal; with just one set of salt and water problems, fish residing in estuaries, and fish migrating between fresh and seawater) (fish; residing; in estuaries)
permanent denizens of fresh water or seawater fish
must deal residing
with just one set of salt and water problems, fish residing in estuaries, and fish migrating between fresh and seawater
in estuaries