It's enough of a jump that it effectively gets Donnie off of that horrible train of thought.

"Wha? Well, they need to breathe air but they can hold their breath underwater for a long time, even longer if they're asleep because they can slow down their heart rate." He pauses, arching a brow at Leo before he smiiiirks.

"...why, were you breathing through your butt when you were a merturtle?"