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    Ecological Differentiation in a Hybridizing Cryptic Species Complex

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    Date
    2012-01-04
    Author
    Turko, Patrick
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    Abstract
    The hybridizing Daphnia mendotae and D. dentifera (Crustacea: Cladocera) are sympatric

    throughout much of North America, and are considered a cryptic species complex due to their lack of

    phylogenetically informative morphological characters. They appear to have no biological mating

    barriers: hybrids may dominate or coexist with either or both parental species, and are sexually

    competent, forming both F2 hybrids and back-crosses. Nevertheless, the two species remain distinct.

    There is observational evidence that separation may be enforced by adaptation to different predation

    regimes: D. mendotae, with its greater anti-predator morphological plasticity, may out-compete D.

    dentifera under intense invertebrate predation, while the smaller D. dentifera may be better adapted to

    avoid predation by visually feeding fish. We tested this idea by examining whether D. mendotae and D.

    dentifera differ in ecologically relevant life history and morphological traits. We performed a replicated

    life history experiment involving 6 replicates of 6 clones within each species, and measured time until

    first reproduction, fecundity, and juvenile and population growth rates. In parallel, we examined

    whether these species differed in morphological traits predicted to arise from adaptation to different

    predator types, and tested the ecological relevance of these traits by exposing Daphnia to predation by

    the invasive cladoceran Bythotrephes longimanus. Finally, we examined the plasticity of life history,

    morphology, and susceptibility to predation by rearing Daphnia under exposure to Bythotrephes

    chemical cues for two generations. D. mendotae and D. dentifera differed across almost all measured

    life history and morphological traits in directions that accord with our hypotheses, strongly suggesting

    that their species boundaries are maintained by adaptation to different predation regimes. Plastic

    reaction to Bythotrephes, however, was weak and inconsistent, suggesting that these species either do

    not detect or respond to this recent invader, or that their responses are manifested in other ways.
    URI for this record
    http://hdl.handle.net/1974/6936
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