They were on the road for only 4 hours, and was my friend (former owner) who brougth them home. Airstones in both tanks, intestines free of content (last meal 2 days before trip)....still dont know.
I say ammonia during shipping.. Airstones increase pH, increases Toxicity, although your pH was low, is what's strange... Rays metabolism is huge and even with no food for 2 days prior, can produce ammonia in small shipping container..esp. bigger rays.. http://www.stingraysource.com/StingraySource/dreamline/acute.htm Osmorespiratory Compromise you might find interesting..