TOKYO (Reuters) - Fish of the Antarctic, be very afraid. There's an unlikely stealth predator on the loose - Adelie penguins. Forget their ungainly waddling on land or comical bobbing at the ocean's surface. As soon as these penguins dive into the icy Antarctic ocean, they become calculating, efficient killing machines, say Japanese researchers. "You could say the penguins have an amazing stealth mode," said Yuuki Watanabe, a researcher at Japan's National Institute of Polar Research. "They're great at sneaking up on their prey and taking them unaware. ...