By Rex Crum, MarketWatch

SAN FRANCISCO (MarketWatch) -- Investors wasted little time Tuesday in showing Apple Inc. how they felt about the company, sending Apple's shares down by more than 8% following a gut-punch combination of disappointing iPhone sales and a weaker-than-expected revenue forecast.

Apple (AAPL) fell more than $48 a share to $502.26 and threatened to drop below the $500 floor for the first time in three months. And nearly all fingers were pointing at the iPhone and concerns about Apple's ability to grow revenue in emerging markets at a time when the U.S. market for smartphones may be reaching a saturation point.

Late Monday, Apple said that for the last three months of 2013, it sold 51 million iPhones, or 7% more than it did in the same period a year ago. Selling 51 million of anything would normally be enough to please anyone, but Apple is no ordinary animal in the tech sector, and many industry analysts had forecast the company to sell between 55 million and 56 million iPhones.

And the concerns about the iPhone were only heightened by Apple forecasting fiscal second-quarter revenue between $42 billion and $44 billion, which fell short of consensus estimates of $46.12 billion. That outlook was disconcerting as China Mobile (CHL), one of the world's biggest mobile-phone network operators, just began offering the iPhone in January.

"The company implies [sales of] iPhones may be close to flat year-over-year in March despite the addition of China Mobile," said Gene Munster, of Piper Jaffray. "We believe a number of factors influenced iPhone [sales] in December and March including a slowdown in US smartphone growth."

Munster noted that there is evidence that the smartphone market in the U.S. "is largely saturated and is close to operating on upgrades as the bulk of product sales," which could imply flat growth for the iPhone and other smartphones going forward.

"There were a lot of reasons to believe iPhone sales would grow [by] double-digit in units in 2014 even as the market matured," added Tavis McCourt, of Raymond James. "But first-quarter results and the March [guidance] pretty much put that thesis to rest."

McCourt trimmed his rating on Apple's stock to outperform from strong buy, and shaved his price target to $550 a share from $700, saying "the clear maturation of the high-end smartphone market limits Apple's growth potential in the near term, and its valuation."

That valuation seemed enough to spur activist investor Carl Icahn to open up his checkbook. Icahn tweeted that he "just bought $500 mln" more shares of Apple stock less than a week after revealing another $500 million purchase of Apple's stock. Icahn has called upon Apple to return more cash to its shareholders either through a larger dividend payment or stock buybacks.

In addition to Apple, Seagate Technology (STX) was also a notable decliner. The hard-disk drive maker's stock fell more than 10%, to $52.05 after it reported fiscal second-quarter results that missed analysts' earnings and revenue forecasts.

Printing technology company Lexmark International (LXK) was a notable gainer, rising more than 7% to $37.46. Before the market opened, Lexmark reported fourth-quarter results that exceeded Wall Street analysts' estimates, led by higher sales of its managed print services and software.

The Nasdaq Composite Index (RIXF) managed to rise almost 4 points to 4,087. The Philadelphia Semiconductor Index also edged into positive territory.

More tech news from MarketWatch:

Apple takes a hit as iPhone disappoints

Apple chip suppliers also take a hit on disappointing report

Perkins's remarks show tech's 1% class has little of it

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