CALGARY - Another cownose ray has died in the mysterious case that has wiped out the Calgary Zoo’s exhibit.

“We lost another ray Tuesday afternoon, unfortunately,” said zoo spokeswoman Laurie Herron.

“The other eight are holding in.”

The death toll is now at 35.

Yesterday, zoo officials were waiting for the test results from water samples taken from the tank.

The results are supposed to show whether there was a problem with the salinity or other aspects of the water chemistry.

Animal experts are not ruling out anything, but zoo spokeswoman Laurie Herron said veterinarians believe it is unlikely that bad water or tainted food led to the deaths. They also don’t believe it could be a disease because it happened so fast — over a 24-hour period.

Zookeepers found 26 of the cownose rays dead on Sunday. Then, eight more died on Monday morning. The rays were suffering from breathing problems and inflamed gills.

“It’s absolutely baffling and disturbing to have them all go so bad so quickly,” Ms. Herron said yesterday.

Only eight remain and they have been removed to a holding tank with oxygenated water to be monitored closely, she said.

Ms. Herron said two of the larger rays managed to eat some food yesterday morning. Officials have added more oxygen to the tank to assist in their recovery.

“They seem to be doing better,” she said. “But we’re not out of the woods yet.”

Necropsies on the dead animals were conducted on Monday and tissue samples sent to labs for toxicology studies.

However, it could be a few weeks before zoo officials know the exact cause of death.

Zoo veterinarian Doug Whiteside, who performed the necropsies, said the diseased rays had changed from their normal light-grey colour to having splotches of dark grey and yellow.

An early investigation into the deaths indicates possible toxins in the water rather than a problem with the food or the water itself, Ms. Herron said.

“There are a number of household chemicals that could do this,” Ms. Herron said. “You don’t want to speculate, but people do strange things sometimes.”

A lab in British Columbia that specializes in testing fish and invertebrates has offered to help test the tissue samples.

There was no mechanical failure in the life-support system, said Cathy Gaviller, the zoo’s director of conservation, research and education.

Numerous water tests are conducted daily since the exhibit opened, and readings have been consistently within normal range, she said.