Legend has it that a variety of crabs contain the ghosts of a drowned samurai army -- and each bears a grimacing warrior face on their backs to prove it. But what can we really gather from this biological peculiarity? In the book and TV series “Cosmos,” Carl Sagan argued that it presented a case of artificial selection, but critics disagree. Join Robert Lamb and Joe McCormick for a discussion of the Heikegani.

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