Plastic Responses to Different Types of Cue : Predator-Induced and Deep-Water-Induced Polyphenisms in a Salamander Hynobius retardatus

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Certain plastic morphological responses of animals induced across a range of environmental conditions may be adapted for effective locomotor performance. Larvae of the salamander, Hynobius retardatus, occasionally swim upward to the surface to breathe air because aquatic respiration alone is insufficient to meet their increasing respiratory requirements for growth. We hypothesized that H. retardatus larvae living in deep water would show an induced plastic response affecting locomotor structures, namely, a deeper tail, similar to that induced by predatory dragonfly larvae (Aeschna juncea), to improve their swimming performance. In this study, larval salamanders responded similarly to different cues (waterborne chemicals in a predatory environment and distance to the water's surface) by developing deeper tails. The similar modifications in tail shape presumably increase a larva's swimming performance, thereby improving its ability both to escape an attacking predator and to swim to the surface for air. The response in tail shape induced by the predatory environment was rapid, but was more gradual in larvae raised in deep water, suggesting that animals may quickly assess a dangerous environment and immediately respond, whereas assessment of an environment not requiring an immediate response for survival may be slower, accounting for the delayed response.