Anticipatory plasticity: frog embryos respond to environmental cues by producing an adaptive phenotype at hatching

Developmental plasticity can occur at any life stage, but a context in which it might be crucial is when individuals that produce specific phenotypes early in development gain a competitive advantage at a later life stage. Here we asked if pre-hatching (embryonic) exposure to a nutrient-rich resource can impact hatchling morphology in tadpoles of Mexican spadefoot toads, Spea multiplicata. Induction of a distinctive carnivore morph can occur when a tadpole eats live fairy shrimp. We investigated whether cues from fairy shrimp, detected as embryos, determine hatchling morphology in a manner allowing individuals to take advantage of this nutritious resource. We found that hatchlings with embryonic exposure to shrimp were larger and had larger jaw muscles––traits that increase their ability to compete for shrimp. Thus, embryos can assess and respond to environmental cues by producing preemptive resource-use phenotypes. Such anticipatory plasticity may be an important but understudied form ..., In July 2020, we added fairy shrimp eggs, Streptocephalus sp., to 5-gallon aquaria of distilled water placed by a window. Streptocephalus sp. occur in the natural ponds where our toads were collected. Their nauplii began hatching within 24 hours. We started feeding the shrimp nauplii Brine Shrimp Food (Northeastern Brine Shrimp LLC) when one day old. We harvested shrimp from aquaria and transferred them to a tub of dechlorinated tap water. We repeated this step to rinse away residual food particles and concentrate the shrimp's density.
In nature, fairy shrimp hatch within 48 hours of pond filling. Depending on the time of day a pond fills, S. multiplicata oviposit within 2-20 hours of pond filling (they breed only on the night after a pond fills). Because S. multiplicata eggs take another 48 hours to hatch, S. multiplicata embryos would typically be exposed to live shrimp for at least 2-20 hours. To mimic these conditions, we bred the frogs approximately 20 hours after the shrimp eggs w...,