An exhibit at suburban Brookfield Zoo is closed after 16 stingrays were found dead.

Officials at Brookfield Zoo say 16 stingrays died earlier this week after a heating and cooling system apparently malfunctioned. Water in the 16,000-gallon Stingray Bay exhibit was supposed to be kept at around 79 degrees. But it somehow had gotten up to 89 by Monday when the dead stingrays were found.

The creatures are highly sensitive to water temperatures, according to a zoo spokesperson. The saltwater pool housed sharks and several additional rays. Those fish were apparently unharmed.