“The hatch is big enough for a 1,300lb (591kg) manatee to get in,” Rodgers said. “It appears that he was not able to get himself back around to get himself out of the situation.”

Asked if Snooty had drowned, Rodgers said the manatee had been taken for a necropsy, the results of which were not yet known. The three young manatees were not injured, he said.

The necropsy will be carried out by Snooty’s veterinarian of more than 20 years and the Florida fish and wildlife conservation commission.

Besio said: “We’re all devastated about his passing. We’re reviewing what happened and will be conducting a full investigation. Snooty was such a unique animal and he had so much personality that people couldn’t help but be drawn to him. As you can imagine, I – and our staff, volunteers and board members – considered him a star. We all deeply mourn his passing.”

Manatees, sometimes known as sea cows, are large aquatic air-breathing herbivores closely related to elephants. Under constant threat from mankind, whether in American waters, the Amazon or west Africa, manatee are classified as an endangered species by state, federal and world bodies. Numbers of wild manatees in Florida have recently increased as a result of protection programmes.

Manatees generally live into their teens in the wild, with some reaching their 40s or older, museum spokeswoman Jessica Schubick said. Snooty was on record as the oldest manatee in captivity, having spent 68 of his 69 years at the South Florida Museum. He was believed to be the oldest manatee in the world.

Rodgers ruled out malicious or foul play in the manatee’s death, although he said the opening of the hatch remained a mystery as it was designed not to let manatees in the tank push through it. A “Snootycam” trained on the tank was turned off at night, he said, and other security cameras could not see underwater. Snooty was discovered on Sunday morning.

“We’ve been greeting people at the front door to share the heartbreaking news,” Rodgers said of a day through which the aquarium remained closed. He added that staff had helped adult visitors break the news to their children “as they saw fit”.

“Aquarium staff is heartbroken,” he said. “We are working with them as well, with the emotional situation, they’re actively caring for the three rehab manatees today so life goes on for those manatees … we’re just making sure our manatees and our staff are well cared for.”

Snooty was born in captivity in 1948 and came to the South Florida Museum the following year. Since 1979, he has been the mascot of Manatee County. Schubick said staff were “planning ways for the public, locally and round the world, to share their memories”.

“Snooty’s very important to this community,” Rodgers said. “He’s been with us for 68 years – generations have grown up with that manatee. The emotional outpouring today… we grieve right along with these folks. We’ve given a lot of hugs on the front porch of the museum today.

“A lot of people loved that manatee. We loved him too.”