Officials at the Calgary Zoo remain baffled as to why 34 sting rays died, in the space of a few hours, this past weekend. A 35th died Tuesday afternoon. 

"What we know," says research director Cathy Gaviller, "is something suddenly changed, and very likely in the water."

Gaviller says water quality analysis, covering oxygen content and ph levels, has revealed nothing unusual. It's likely, she says, that toxicology tests, lasting several weeks, will be needed to pin down the exact cause of the 35 sting ray deaths. 

In the meantime, surviving rays are doing reasonably well. "The fact they've survived to this stage," says Gaviller, "gives us some guarded optimism that they will do fine."

The sting ray exhibit has been closed since the 34 deaths were discovered, and will remain shut down until further notice.