Stingrays are strange-looking but normally shy creatures whose defenses include poisonous, serrated barbs in their tails.
At least 35 species of stingrays swim in the tropical waters of Australia's Great Barrier Reef, where television personality Steve Irwin died Monday when he was stabbed in the heart by a ray's barb.
Experts called it a freak occurrence.
They said the triangular-shaped rays are usually unobtrusive, gliding through the water, rummaging on the sea bottom for food or burrowing into the sand. But when stepped on or otherwise frightened, they deploy spines up to 10 inches long with breadknife-like serrations as a defense mechanism.
"If it's spooked by someone stepping on it or swimming too closely over it, frightening it, the tail raises involuntarily," said Victoria Brims, a marine life expert at OceanWorld, an aquarium in Sydney, Australia.
The spines emit toxins that can kill many small creatures and that cause excruciating pain in humans. Few people die from the poison, but the spines can badly tear flesh and the wounds are prone to infections, including tetanus.
Simon Pierce of Queensland University's School of Biological Sciences said there were no accurate records of stingray deaths, but estimated there had been about 30 worldwide in recent years.
Witnesses said Irwin was struck directly in the heart.
"It was extraordinarily bad luck," said Shaun Collin, a University of Queensland marine neuroscientist. "It's not easy to get spined by a stingray, and to be killed by one is very rare."