Mr. Mussels first appeared during auditions for the school play. He scared Albert with his loud voice. When Bea came flying into the tank, Mr. Mussels thought she had a amazing entrance, but said she was missing "a little special something". A can of fish flakes then tilted and fell into the tank, onto the stage, looking like confetti. Mr. Mussels ended up giving Bea the part ("Fish Out of Water"). Mr. Mussels got put in a bowl by Milo because he said that his voice scares everybody ("Milo Gets a Ninja").
At Mr. Mussels's drama class, he yelled at his students that everyone has to have a dream, although Milo was the only one in his class who didn't have a dream. When Milo asked Mr. Mussels what happens if you don't have a dream, Mr. Mussels told him that dreams feed the heart and keep you company. He also told him that dreams are so important that if you don't have one, you'll end up cold, hungry, and alone. Later, when Milo went to the fridge to get a drink, an imaginary Mr. Mussels appeared, and led Milo to seeing a bird cage, which made Milo want to tame a bird so he could fly. When Milo failed to, Mr. Mussels, who had just woken up, found Milo getting dragged back into the tanks by Bea and Oscar. Mr. Mussels asked Milo if he would sacrifice everything to fly, and Milo nodded. He then told Milo that that was a real dream and that his friends should support him ("Flying Fish").
When Oscar was about to take his drivers test, he found out that Mr. Mussels and Coach Salmons were his driving instructors. Oscar didn't really like the fact that two energetic and crazy teachers were his driving instructors, but got in the car anyway. Mr. Mussels and Coach Salmons told Oscar the rules of driving, and that if he got 3 "dings", he would fail his driving test. Oscar started driving, and was doing well in the beginning until they couldn't figure out how to get back to the DMV. Oscar was about to look at the map to figure out how to get back, but Mr. Mussels snatched the map, saying that he will get one "ding" if he reads while driving, so he ripped up the map, causing them to be stranded. Mr. Mussels told Oscar that he knew the way out. Coach Salmons suggested that they should ask someone for directions, but Mr. Mussels told him that he knows what he is doing, causing the two to argue for a while. When Oscar drove out of the tanks, Coach Salmons and Mr. Mussels apologized for fighting, and when Oscar asked Mr. Mussels where they were supposed to go, Mr. Mussels said he actually didn't know the way out. Oscar got 2 "dings" when he didn't look at the road, causing him to try and be more careful. After unsuccessfully trying to get directions from Milo, Mr. Mussels and Coach Salmons asked Oscar if they could have a meal at the Burger Bunny restaraunt. When they finished their food, Coach Salmons asked if he could use the bathroom. Oscar pulled over, but got 3 "dings" for not using his turn signal, which meant Oscar failed his driving test. Right at that moment, Wilford the cat attacked the car. Oscar drove the car away from the cat, and parallel parked in between two boxes of cat food, which caused the cat to not see them and walk away. They ended up finding the DMV, and Mr. Mussels and Coach Salmons were extremely proud of Oscar for saving them, so they let him pass the driving test. Oscar took a picture with Mr. Mussels and Coach Salmons and received his driver's license ("Riding in Cars with Fish").
When Oscar did a realistic impression of the Queen of Fish England, he ran into Mr. Mussels, who believed that Oscar really was the queen. Mr. Mussels then took Oscar, Bea, and Milo on a ride in his limousine to Fish England. Later, when Oscar was about to marry the duke, Mr. Mussels came to the wedding to stop the queen from marrying the duke and make the queen marry Mr. Mussels instead. The wedding officiant told Mr. Mussels and the "duke" (who was Milo in disguise) to compete in a "Slow Jams Rap Battle". Mr. Mussels ended up beating Milo since Milo did his poor impression of Mr. Mussels himself. However, Oscar didn't marry Mr. Mussels after he and Milo went back home with Bea after she interrupted the wedding with a wax sculpture of the queen ("Oscar Makes an Impression"). Mr. Mussels was the one who wrote the "Potatoes for Winter" play. When Bea asked Mr. Mussels if he had anything to say to his students before the play started, he said that he wrote the play with nothing more than words, and that they should all try their best for the play. When Jocktopus was getting stage fright, Mr. Mussels encouraged him a little by saying that true power comes from the heart and not from strength. Later, when Bea trapped Jocktopus in the trapdoor, she went to Mr. Mussels and told him that Jocktopus was trying to ruin the play on purpose. However, Mr. Mussels explained to Bea that one day, when Mr. Mussels was typing the script for the play, Jocktopus came to his room and told him how he needed to be a part of the show. The two secretly practiced and rehearsed for the play until Jocktopus was truly ready. This caused Bea to go back and fullfill Jocktopus' dream of being part of the show ("Fish School Musical").
Mr. Mussels announced to his students that the school talent show was coming up, and that the winner would get a free coupon book. At the end, Mr. Mussels gave the coupon book to Bea's dance group, who were the winners of the talent show ("Fish Talent Show").
- He thinks friendship is important.
- He has his own motorcycle as shown in a short clip with a Hermit Crab.
- He wants everybody to have a dream ("Flying Fish").
- Strangely, mussels can't be kept as pets, making it unreasonable that Mr. Mussels lives in a pet store.
- In human form, Mr. Mussels resembles Tommy from Rugrats.
- "Fish Out of Water" (First appearance)
- "Milo Gets a Ninja"
- "Flying Fish"
- "Two Clams in Love"
- "Legend of the Earth Troll"
- "Parasite Fright" (Non-speaking cameo)
- "Pamela Hamster Returns" (Non-speaking cameo)
- "Riding in Cars with Fish"
- "Oscar Makes an Impression"
- "Fish School Musical"
- "Halloween Haul" (Non-speaking cameo)
- "Fish Talent Show"
- "Bea's Commercial"
- "Merry Fishmas, Milo" (Non-speaking cameo)
- "Just One of the Fish"
- "Busy Bea: Rise of the Machines"
- "Little Fish Sunshine"
- "Principal Bea" (Non-speaking cameo)
- "Everything but the Chicken Sink"
- "A Charity Fair to Remember"
- "South Pafishic"
- "Pool Party Panic"
- "Hare and Back Again"
- "Hats Amore!"
- "Camp Camp"
- "Algae Day" (Non-speaking cameo)
- "Surfing the Interwet"
- "The Big Woo"