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The Shark was added in the 4th expansion by Eon Productions. It was ostensibly an effort to add a 7th or 8th player to the game.
Power
You have the power of hunger. You must sit out the game for one challenge, after which you may choose one of the six players who is not currently a main player and take over his position in the game: his hand, power, etc. He must now sit out one challenge and then may "cut-in" on a player as you did.
Restrictions: use only in a 6+ player game.
Wild Flare - Before your first challenge of a turn, you may exchange one game factor (tokens, power, hand, moons, lucre) with another player. Discard after use.
Super Flare - You may cut in on a main player as soon as the defending player has been determined. Note that it makes no sense to have a "super flare" for the Shark, because the Shark player has no� hand. He stops being the Shark as soon as he sits down. What was Eon thinking?
I don't know who made up the above information, but the original Shark and Zilch do not have flares.
Strategy
This is probably one of the least popular powers ever. It is basically a weirder version of the Changeling, which is already plenty weird. Plus it destroys everyone's will to win the game, because they'll just get taken over by the lurking Shark. At least the Changeling doesn't take over your bases as well.