Perhaps I'm missing something, but I thought this article rather clearly states that the St. John's river population live their entire life in freshwater and have developed an evolutionary strategy to cope with this releasing excess urea and "reduc[ing] the concentration of its body fluids to minimize the osmotic flow into its body". If this put a strain on their osmosis system as you say, certainly they wouldn't be able to form a thriving and reproductively viable population in FW (which they clearly are). And the article states the fw Atlantic ray "boosts its urine output tenfold to rid itself of excess water" but who is to say whether this is significantly more than the urea output of a comparably sized motoro or other river ray (which are also known to excrete huge amounts of waste as part of their adaptive strategy)? Anyways, I'm not advocating these rays be kept, I think there is a lot more that needs to be learned about them before they can be with anything approaching success, just pointing out that this article in no way says these rays cannot be kept in freshwater, in fact, far from it. I think it only confirms that an atlantic ray of this population can in fact live, thrive, and reproduce in a freshwater environment. It would take a highly specialized aquarium and an aquarist not only experienced but with unlimited resources and probably some sort of continual water change system, but I see no reason to say they can not be kept in freshwater, with the important caveat that few, if anyone should attempt it until more is learned about this unique population.
