By Lisa Richwine

LOS ANGELES (Reuters) - In its early years, the mobile video game market was dominated by simply designed, family-friendly games like "Fruit Ninja" and "Angry Birds." Action and adventure titles with sophisticated graphics and complex story lines were seen as better suited to gaming consoles and personal computers.

Increasingly, however, developers are bringing console-style games to smartphones, attempting to grab serious gamers when they are on the go.

Today, the world's top-selling mobile game is "Clash of Clans," a combat strategy game in which warring factions battle in a Medieval fantasy world. This week, China's Tencent Holdings Ltd <0700.HK> announced it was purchasing a majority stake in "Clash of Clans" maker Supercell for $8.6 billion, the largest acquisition in gaming history. The deal included all of SoftBank Group Corp's <9984.T> 72.2 percent stake in Supercell.

Designers must simplify graphics and storylines of action-adventure titles for mobile screens. Still, they can offer what avid gamers crave: complex, strategic challenges and a chance to match wits with players around the world, said Peter Warman, CEO and founder of research firm Newzoo.

In "Clash of Clans," "you are setting up your village and battling other players," he said. "It's really hard to be good at."

With the surge in new action titles, mobile gaming revenue is expected to reach $37 billion worldwide in 2016 and surpass sales of PC games for the first time, according to Newzoo. Most mobile games are free to download, but their makers earn revenue through in-game purchases that speed up action or add elements like costumes or weapons.

About 45 percent of mobile game revenue in the United States is now being spent on action, adventure, strategy and role-playing games, Newzoo data from May 2016 showed.

Seven of the world's 10 highest-grossing games in 2015 were the more sophisticated "core" games such as "Clash of Clans," "Monster Strike," and "Game of War - Fire Age," according to AppAnnie, which tracks sales on Apple's <AAPL.O> App Store and on Google Play. "Candy Crush Saga," ranked fifth last year, remains the leading casual game.

With smaller screens and no controllers, developers have to adapt plots and presentation for phones.

In the popular "The Walking Dead: Road to Survival," action is designed to unfold in shorter sessions than traditional video games, said Jori Pearsall, senior vice president of Scopely, the game's publisher. But the company aimed to retain the features action gamers value, he said, with a title that is "gritty, mature and in some cases even shocking."

Early in the game, for example, players must decide whether to trust a seemingly innocent survivor of the apocalypse. If they do, however, he slits the throat of someone on the rescuer's team.

"We are not trying to recreate 'Call of Duty' on mobile," Pearsall said. "We are trying to create a mobile-appropriate gaming experience, content that (players) can engage in more bite-sized chunks."

The Tencent purchase of Supercell would help expand the reach of its games in China, home to more gamers than any other country, Supercell Chief Executive Ilkka Paananen said. About 300 million unique users play games on Tencent platforms, Paananen said in a statement.

For game makers, the growth of more complicated action-oriented games on mobile is raising development and marketing costs, a trend that will likely lead to more industry consolidation, said Joost van Dreunen, CEO of research firm SuperData.

"Previously, mobile gaming was very accessible" for developers, he said. "It is now creating barriers to entry.”

(Reporting by Lisa Richwine; Editing by Sue Horton and David Gregorio)