Sixty-nine students in Long Island’s public and private schools were among 498 nationwide named regional semifinalists Tuesday in the elite Siemens Competition in Math, Science & Technology.

Manhasset High School, where two students last year won the $100,000 top team award, has 15 semifinalists — the most of any public school district on the Island.

Manhasset’s Elin Hu said she and research partner Vanessa Zhang, both 16 and juniors, were “in near tears” after learning they were among the semifinalists. The duo’s project, which involved experiments with fruit flies and worms, found that Vitamin E could lessen the symptoms of Parkinson’s disease.

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“It’s really surreal,” Hu said.

Syosset High School has nine semifinalists and Jericho High School has seven. The Great Neck school district’s two high schools have a total of five semifinalists, while three students achieved the recognition at both Paul D. Schreiber High School in Port Washington and Ward Melville High School in the Three Village system.

For the first time, Walter G. O’Connell Copiague High School in Copiague has a Siemens semifinalist. This year was the first time any of the school’s students entered the 17-year-old contest, officials in the district said.

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The announcement came after Long Island students spent months in classroom or university laboratories, working to develop their discoveries into projects viable for the competition. The selection is a point of pride for the school systems, which invest in research programs and recruit faculty mentors to oversee the students.

Some of the investigations, for example, probed just how tiny a colony of E. coli can be to still be identifiable, the use of algae to purify wastewater, and how alteration of the structure of concrete could make buildings more resilient to extreme temperatures.

“It really pumps the kids up seeing that students from Long Island — especially our school, with homegrown projects — can succeed and do exceptionally well in this competition,” said Alison Huenger, director of Manhasset’s science research program.

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In Brentwood, two students, one with a project on waterways and another who studied sea anemones, were named semifinalists. The announcement came as good news for the district that has been reeling from the slayings of four teenagers that police said stemmed from gang violence.

Rebecca Grella, who helped oversee the Brentwood research, said the achievement helps to “shine a light” on an “intellectual pursuit.”

Among the semifinalists were students at three private schools: St. Anthony’s High School in South Huntington; Hebrew Academy of the Five Towns and Rockaway in Lawrence; and the Davis Renov Stahler Yeshiva High School for Boys in Woodmere.

The 498 semifinalists nationally were chosen from 2,146 students who submitted more than 1,600 projects. In the national finals, the Siemens Competition awards two $100,000 grand prizes — one to an individual and one to a team — as part of a total $500,000 in scholarships.

In Copiague, teammates Jade Dickenson and Sofie Wilson, both 17 and seniors, had spent their sophomore year on separate projects before collaborating in their junior year. Dickenson was studying the invasive species Phragmites australis, while Wilson was researching ribbed mussels. They learned that the ribbed mussels could starve the invasive species of nitrogen, which it needs to grow.

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“We could play these two species against each other,” Dickenson said.

“When you have two students who have sufficient knowledge in areas, they own it,” said Renee Locker, chairwoman of the district’s science department.

Regional finalists are to be announced Wednesday. They will present their projects via web-conferencing and other digital means at one of six competitions, divided into regions set by Siemens, that are held during November.

Students from Long Island usually submit their projects to judges at Carnegie Mellon University. Other regional contests are at Massachusetts Institute of Technology, Georgia Institute of Technology, the University of Notre Dame, the University of Texas at Austin and California Institute of Technology.

The national finals are scheduled in December at The George Washington University in Washington, D.C.

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Last year, Manhasset High School seniors Kimberly Te and Christine Yoo won the $100,000 team grand prize. They created a device to clean up oil spills that would generate clean energy to power remote sensors in developing countries.

The nonprofit Siemens Foundation launched the contest in 1999 with the aim of increasing access to higher education for students gifted in science, technology, engineering and math. The competition “seeks to recognize and build a strong pipeline for the nation’s most promising scientists, engineers and mathematicians,” the foundation said.