Among the many striking images of Japan’s gleaming start to the Rugby World Cup, one in particular showed how sport, and the oval code in particular, is testing traditional notions about the nature of Japanese society.

Following the action at Tokyo Stadium during the tournament’s opening match between the hosts and Russia, the prime minister, Shinzo Abe, sporting a replica Brave Blossoms shirt, and Crown Prince Akishino, briefly rose from their seats to join in a Mexican wave.

While clearly enjoying the communal celebration, the two men also represented a face of Japanese society that the country’s rugby team is now challenging in front of a global audience.

Here was a conservative politician, accompanied by the heir to the 2,500-year-old Chrysanthemum throne, celebrating a team captained by Michael Leitch, of mixed New Zealand and Fijian heritage, in a match in which three of Japan’s four tries were scored by Kotaro Matsushima, a South African-born winger with a Zimbabwean father and Japanese mother. In all, seven of the 15 men who started for Japan that evening were born overseas.

For some in the sport, Japan’s multiracial side is simply the most egregious example of how teams are exploiting the three-year residency requirement for drafting foreign-born players into national squads. Led by coach Jamie Joseph, a New Zealander who represented Japan at the 1999 World Cup, the 31-strong squad includes 16 players originally from seven countries, including their captain in their recent win over Ireland, the South African-born Pieter Labuschagne.

Criticism of rugby’s relaxed attitude towards eligibility followed Japan and other teams into this tournament, with the former Wales captain Paul Thorburn claiming that World Rugby’s liberal criteria had made a “total mockery” of international rugby and turned the World Cup into a “circus”.

But Leitch, who arrived in Japan as a 15-year-old to study at a high school in Sapporo and took Japanese citizenship in 2013, has spoken with pride of the diverse backgrounds of his teammates. “I am very, very proud to be the captain of the Japanese rugby team,” he said on the eve of the tournament. “We have got a very diverse side with a lot of foreign influence.”

Naomi Osaka is among a growing number of mixed-race athletes who represent Japan. Photograph: VCG/Visual China Group via Getty Images

Leitch is not alone in taking his commitment to his adopted country beyond the sporting realms. Hendrik Tui, born in New Zealand of Samoan descent, graduated from a Japanese university, as did Samoan-born and New Zealand–raised Timothy Lafaele. Both are now Japanese citizens. Lomano Lemeki, who was born in New Zealand to Tongan parents and grew up in Australia, took Japanese citizenship after marrying a Japanese woman. Then there is Koo Ji-won, who was born in South Korea but went to high school and university in Japan and became a Japanese citizen in 2015. He now finds South Korea is locked into a bitter row with Japan over wartime history.

Japanese tennis star Naomi Osaka’s victory in the US Open against Serena Williams a year ago had many wondering if traditional theories of Japanese identity were being eroded, whether on a tennis court, rugby pitch or running track. Osaka, the daughter of a Japanese mother and Haitian father, along with Leitch and his teammates are among a growing number of mixed-race athletes who represent Japan, including the US Major League baseball player Yu Darvish, whose father is Iranian, and the Olympic sprinter Asuka Cambridge, whose father is Jamaican.

Basketball player Rui Hachimura is expected lead the Japanese men’s basketball team at next year’s Tokyo Olympics. In June, Hachimura, whose father is from Benin, became the first Japanese player to make the NBA draft.

The 2020 Games will also feature sprinter Abdul Hakim Sani Brown, born to a Japanese mother and Ghanaian father, who was one quarter of the Japanese men’s team that won a bronze medal in the 4x100m relay at the world athletics championships in Doha.

Helen Macnaughtan, a senior lecturer at the School of Oriental and African Studies, London University, agreed that Japan’s racially diverse team was partly the product of World Rugby’s eligibility rules, but saw potential for a more inclusive definition of Japanese identity as exemplified by the Brave Blossoms.

“The team’s sheer visibility as an ethnically diverse team is challenging notions of Japaneseness - and it’s not just Japanese commenting on that,” said Macnaughtan, who is researching the history of Japanese rugby. “You hear non-Japanese observers and rugby fans commenting on that as well.”

Washington Wizards forward Rui Hachimura (right) is the first Japanese player to make the NBA draft. Photograph: Tommy Gilligan/USA Today Sports

Sport, though is only a reflection of broader social change in Japan, home to a record 2.67 million foreign residents – around 2% of the total population – and where one in 50 children are born to international couples.

But Baye McNeil, an African-American writer based in Tokyo, cautions against using the country’s rugby players as a battering ram with which to shatter Japan’s traditional self-image as a racially homogenous country, and where mixed-race people are routinely referred to as haafu (half).

“In the minds of most Japanese in my experience, Japanese still retains a rigid definition which includes first and foremost, you must look Japanese,” McNeil said. “Any athlete who doesn’t meet that criteria, regardless of citizenship, whether biracial or not, will be considered a non-Japanese athlete on a Japanese team.”

The mixed reaction to Osaka’s rapid elevation to sporting icon shows Japan has a long way to go. Months after her US Open victory, one of her sponsors, the instant noodle maker Nissin, was forced to apologise after depicting her in an ad with pale skin, light brown hair and Caucasian facial features.

This month A Masso, a little-known comedy duo, apologised for saying during a live show that Osaka looked “too sunburned” and suggested she “needed some bleach”.

McNeil, though, noted that the first people to complain about the racist remarks had been Japanese people in the A Masso audience. “That is a very good sign that attitudes towards non-traditional looking Japanese people, who happen to be representing the country in record numbers of late, may be evolving,” he said.

If the rugby gods smile on the tournament this weekend and Super Typhoon Hagibis spares Japan’s final pool match against Scotland, the host nation’s multicultural, multinational team will line up again and give a flawless rendition of their national anthem, Kimigayo, in front of 70,000 fans and a TV audience in the tens of millions.

The anthem, based on an ancient waka poem, expresses hope for a long imperial reign, “until the tiny pebbles grow into boulders” – a sentiment that surely applies to Leitch and his eclectic band of Japanese sporting heroes.