PREDATORS, INVISIBLE PREY, AND THE NATURE OF POLYMORPHISM IN THE CLADOCERA (CLASS CRUSTACEA) 1

The selective forces of reproductive potential and liability to predation that balance a polymorphism of the cladoceran Ceriodaphnia cornuta are examined. Neither Brooks’ “body‐size hypothesis” nor Jacobs’ “behavioral hypothesis” accounts for the observed differential predation rates by the piscine predator Melaniris chagresi on the morphs of C. cornuta. Laboratory experiments show that amounts of pigmentation in the black compound eyes of these otherwise relatively transparent cladocerans can affect the rate of predation by planktivorous fish. Based on these experiments and other measurements a “visibility hypothesis” is proposed to explain the differential predation rates on C. cornuta. Further data suggest that this same visibility phenomenon applies to other polymorphic and cyclomorphic Daphniidae where differential predation rates have been reported previously.