Yeah.. lots of gas exchange at the surface. Especially since alot of people keep rays at such high temps, the water doesn't hold as much 02.
Believe it or not, my rays went without a filter or air for 6 hours while I was at work.. The fish room froze, and I did everything I can before having to move the rays into a 100g tank. I had the 180g covered with blankets, heaters from extension cords, and turned off everything but the return pump.. The tank got down to about 65 degrees over a period of 1 night, so I moved them.. No stress, no losses.. no issues. Rays are pretty darn tough once acclimated, I just think they are all half-dead when in LFS.
BTW, My rays came from Cichlids exchange.. they are nice rays at a good price (not charging more than $50 a ray, right?.. CichlidEx is selling em for about $24/ea shhhh) but they did have internal parasites, and I did have to treat them .. my LFS treated em once, and so did I.. the smallest one with the snubbed tail sure doesn't seem to keep up with the other 2 that have been in captivity longer and received extra doses of meds.
Hope that helps..
You must be from the NW?