Hatching plasticity in two temperate anurans: responses to a pathogen and predation cues

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Abstract
Water molds are widespread in aquatic environments and are important causes of mortality in amphibian and fish eggs. We tested the ability of two species of North American anurans with different breeding phenologies ( Rana sylvatica LeConte, 1825 and Bufo americanus Holbrook, 1836) to alter their hatching timing in response to three indicators of environmental risk: infection with a water mold, exposure to simulated egg predation cues, or exposure to simulated larval predation cues. When infected with water mold (Saprolegniaceae), B. americanus eggs hatched, on average, 44% earlier than the controls and R. sylvatica eggs hatched 19% earlier than the controls. In addition, B. americanus but not R. sylvatica eggs hatched significantly earlier than the controls when exposed to simulated egg and larval predation cues. Bufo americanus embryos hatched before developing muscular response, suggesting that hatching occurs through enzymatic egg capsule degradation combined with ciliary movement, not through behavior. Bufo americanus breeds later than R. sylvatica and responded to infection and simulated predation cues more strongly. This may reflect a history of stronger selection by pathogens and predators that accumulate in ponds as the breeding season progresses. To our knowledge, these are the first examples of induced hatching of amphibians in response to aquatic pathogens.