The world’s oldest manatee has died in a “heartbreaking” accident at his Florida home a day after celebrating his 69th birthday.

Snooty, an intelligent and highly sociable male manatee, appears to have died on Sunday after getting trapped in a maintenance hatch he shared with three young rescued manatees.

According to the South Florida Museum, a panel door leading out of the underwater tank which is normally bolted shut had been knocked loose.

“The manatees had access to get into this tight area. The young manatees were able to get in and out of that, and it appears that Snooty was able to get into the area, but he was not able to extract himself from that situation,” Jeff Rodgers, the museum's provost and chief operating officer, said during a press conference on Sunday.

He added: “It took us some time to figure out exactly how we were going to deal with that situation, but when we did finally get to Snooty, he was no longer alive."

The museum said staff were “devastated” by the news of his passing and would be conducting a full investigation into what exactly happened.

“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,” the museum’s chief executive, Brynne Anne Besio, said in a statement.

Snooty was born in captivity at the Miami Aquarium and Tackle Company in 1948 and was the first recorded birth of a manatee in human care. Known by his nickname of “Baby snoots”, he came to the South Florida Museum in Bradenton the following year.

Manatees, sometimes referred to as sea cows, are large aquatic mostly herbivorous mammals closely related to elephants. They measure up to four metres long and weigh as much as 590 kilos.

The animals have long been under persistent threat from mankind spanning across American waters, the Amazon and west Africa. Nevertheless, manatees were reclassified as “threatened” rather than endangered in March. The number of manatees in Florida had fallen to just a few hundred in the 1970s but has now reached more than 6,600.

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Museum spokeswoman Jessica Schubick said manatees tend to live into their teenage years in the wild, with some getting to their 40s or older.

Snooty, who celebrated his birthday with a cake made of fruit and vegetables a day before he died, was on record as the oldest manatee in captivity. He has been the official mascot of Manatee County – a region of Florida – since 1979.

"He's just entertaining and calming at the same time," Marilyn Margold, director of living collections at South Florida Museum, told Guinness World Records in 2016.

The aquarium was closed on Sunday while staff investigated what had happened and those who worked with the legendary manatee had "an opportunity to grieve”.

Snooty’s necropsy will be carried by the Florida fish and wildlife conservation commission and his veterinarian of more than 20 years, The museum said the other three manatees in his tank, Randall, Baca and Gale, were all fine.

Mr Rodgers ruled out malicious or foul play in Snooty’s death but said the opening of the hatch was a mystery because it was made not to let manatees in the tank push through it. He said a “Snootycam” trained on the tank was turned off at night and other security cameras were not able to see underwater.