MATTAPOISETT — Finn Pereira knows that seahorses might be the sea creature most closely associated with Mattapoisett.

But when the Old Colony Regional Vocational Technical High School junior was asked to design a T-shirt to celebrate the town’s annual Harbor Days celebration, she decided to go in a different direction.

When she was about eight years old, she enjoyed seeing horseshoe crabs during family visits to the beach at Ned’s Point.

She appreciates the horseshoe crab’s history. “They’ve been on this earth forever,’’ she said. Indeed, the creatures_ which in fact are not crabs at all but are more closely related to spiders and scorpios _ predate and obviously far outlived the dinosaur.

Her design of a horseshoe crab, which also features a scallop shell depiction and Native American imagery, won the competition over five other shirts designed by her classmates. The winner was selected via an online poll.

“I like seeing how people have reacted to my design,’’ she said. 

The T-shirt contest was sponsored by the Mattapoisett Lions Club, which also oversees the annual Harbor Days festivities. This year’s event will be held July 10-17, with crafters at Shipyard Park July 16 and 17.

Sales from purchase of the shirt help support the festivities. 

For Erin Murray, a graphics design teacher at the school, the contest provided a welcome opportunity for her students to obtain real-world experience in designing an item.

“This gives kids the feel of a real-life job,’’ she said, and provides “an effective learning experience.’’

“It’s not just about making something look pretty,’’ she said. The students she and fellow graphic design teacher Kathy Peterson teach also had to research the Town of Mattapoisett to develop the most fitting depiction that reflects the community.

The competition was open to the entire junior class, Murray said. The top designs were submitted to be judged.

Pereira, who lives in Acushnet, has never attended Harbor Days.

But that will change this summer, she said. She will no doubt walk alongside people wearing her selected design.

“I’m glad that people really like it,’’ she said.