Hitless in 22 straight at-bats, Gerardo Parra decided to change his walk-up song before the Washington Nationals hosted the Philadelphia Phillies in a split doubleheader on June 19.

The 32-year-old outfielder, signed off the scrapheap by Washington a month earlier after being released by the San Francisco Giants, considered various merengue, reggaeton and hip-hop selections, but ultimately chose the maddeningly catchy — and repetitive — children’s tune “Baby Shark.”

“My girl loves that song,” Parra said of his 2-year-old daughter, Aaliyah. “ . . . She sings it a lot.”

Parra went 2 for 4 with a home run and two RBI in the Nationals’ 6-2 win. Naturally, he hasn’t changed his walk-up song since.

By July 5, the Nationals were marking Parra’s brief tenure in D.C. by the date of the switch.

Parra’s teammates in the dugout began shark-chomping along to the music as he stepped to the plate, and fans in the stands got in on the act, too.

On Tuesday, the doo doo doo-ing at the ballpark reached unprecedented levels. When Parra came to bat as a pinch-hitter in the seventh inning with the bases loaded and looking to extend Washington’s 5-0 lead, thousands of fans in the crowd of 22,612 came together as one Baby Shark and chomped as if it were a tradition as old as singing “Take Me Out to the Ballgame.” It helped that the Nationals debuted an animated graphic of Parra clapping along to the song on the center field scoreboard.

“The whole ballpark’s doing it, you gotta see this,” MASN’s F.P. Santangelo said, trying to control his laughter as cameras showed Juan Soto clapping his hands in the dugout. “Everybody’s doing it. Oh my goodness. Look around, this is crazy.”

Parra responded with a single that scored three runs. “Baby Shark” played again, and the chomping resumed. Video of the moment was featured on Wednesday’s episode of “Good Morning America."

“It’s pretty cool,” Manager Dave Martinez said after the Nationals’ 11-1 win. “I guess they’ve got T-shirts now. I wish I could get one, but we don’t get them.”

There are T-shirts, and for anyone lucky enough to have just discovered “Baby Shark,” there are also helpful instructions, posted on the Nationals’ Reddit, for the appropriate hand gestures to make during each verse of the song. This isn’t amateur hour.

“Little pincers — between your index finger and thumb — is called a Baby Shark,” the poster wrote. “Clapping your hands with your wrists together is called a Mommy Shark.”

And so on.

There’s no telling when the next “Baby Shark” attack will happen at Nationals Park — Parra wasn’t in the starting lineup for the first game of a split doubleheader against the Rockies on Wednesday — but the chomping and doo doo doo-ing, much like the Nationals’ post-homer dugout dances, probably aren’t going away anytime soon.

Could shark costumes be next? If so, it would be a throwback to 2013, when speedy Roger Bernadina last roamed the outfield for the Nationals. In 2010, two 20-something Nationals fans decided that Bernadina, in his first year as a semiregular starter, looked “like a shark hunting his prey” while chasing down fly balls and making the occasional spectacular catch.

That August, the fans, Tyler Stoltenberg and Terry Cangelosi, showed up at Nats Park wearing children’s fleece shark hoods. They started a Sharkadina blog and eventually created a small cheering section in right field with fellow fans in shark attire. Bernadina’s nickname stuck. A shark video would play on the scoreboard and the Nats Park crowd would chomp in unison when he came to bat. In 2013, the Potomac Nationals gave out 1,000 Bernadina-inspired Shark-a-rines.

Bernadina, 35, last played in a major league game in 2014. He had a monster year for the Kia Tigers of the Korean Baseball Organization in 2017, but was released after the 2018 season. Earlier this month, he signed with Algodoneros Union Laguna of the Mexican League.

Nationals Park has been considerably less kind to a visiting Shark over the years. Giants pitcher Jeff Samardzija, who was nicknamed “Shark” as a freshman at Notre Dame because some of his teammates decided he looked like the shark in “Finding Nemo,” is 0-5 with a 4.66 ERA in nine games, including five starts, in D.C.

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