Stephen Hillenburg, the creator of popular cartoon character SpongeBob SquarePants, died at the age of 57 on Tuesday, according to Nickelodeon television network. In 2017, Hillenburg had announced that he was suffering from the neurodegenerative disease ALS, also known as Lou Gehrig’s disease, Reuters reported.

The television network, which has broadcast the series since 1999, said, “We are sad to share the news of the passing of Stephen Hillenburg. He was a beloved friend and long-time creative partner to everyone at Nickelodeon, and our hearts go out to his entire family.”

He was a marine biology teacher in Southern California when he first started creating sea creatures to help his students learn the subject. His most popular creation, SpongeBob SquarePants, is a happy yellow sea sponge who lives in an underwater pineapple.

Hillenburg had also directed The SpongeBob SquarePants Movie in 2004. “Steve imbued SpongeBob SquarePants with a unique sense of humor and innocence that has brought joy to generations of kids and families everywhere,” Reuters quoted the Nickelodeon statement as saying. “His utterly original characters and the world of Bikini Bottom will long stand as a reminder of the value of optimism, friendship and the limitless power of imagination.

The series includes more than 200 broadcasted episodes and two successful animated films. Earlier in 2018, the cartoon was made into a Broadway musical.

The artist is survived by his wife Karen Hillenburg and son Clay.

Following news of his death, Hillenburg’s friends and fans of his work took to Twitter to pay tribute to him.