Growth-Promotion Effects of Dissolved Amino Acids in Three Species of Hynobius Salamander Hatchlings.

It has been suggested that aquatic vertebrates may be able to meet their energy requirements by using the amino acids dissolved in environmental water. If this ability can be applied to aquatic organisms generally, then conventional ecological theories related to food web interactions should be revisited, as this would likely bring about significant advances in applications. Here, we prepared two 1 mM amino acid (phenylalanine and glycine) solutions in environmental water and conducted laboratory experiments to demonstrate the utilization of dissolved amino acids by hatchlings of three salamander species (Ezo: Hynobius retardatus, Tohoku: Hynobius lichenatus, and Japanese black: Hynobius nigrescens). Compared to controls (no amino acids in environmental water), the growth rate for Ezo salamanders was higher when larvae were reared in phenylalanine solution, while that for Japanese black salamanders was higher in glycine and phenylalanine solutions. Amino acids in environmental water had no effect on the growth of Tohoku salamanders. However, when growth was divided into early (days 1 to 5) and late (days 5 to 7) developmental stages, growth in early-developmental stage individuals was improved by phenylalanine treatment, even in Tohoku salamanders. The results showed that the growth of salamander larvae was improved when salamanders were reared in environmental water with high amino acid concentrations. Although aquatic bacteria may not have been removed completely from the environmental water, no other eukaryotes were present. Our results suggest an overlooked nutrient pathway in which aquatic vertebrates take up dissolved amino acids without mediation by other eukaryotes.

Phenotypic Plasticity of Salamander Hatchlings in the Pre-Feeding Stage in Response to Future Prey.

Vulnerability of animals immediately after hatching may induce plasticity in early ontology that becomes important for subsequent survival and growth. Ezo salamanders (Hynobius retardatus) are amphibians inhabiting ponds in Hokkaido, Japan where ezo brown frogs (Rana pirica) spawn on occasion. The salamander larvae must achieve sufficient size in order to successfully capture frog tadpoles, and we examined whether the presence of tadpoles causes development of greater body and/or gape size in newly hatched salamander larvae, which will in turn result in advantageous future prey-predator interactions. To examine this hypothesis, we conducted three laboratory experiments to demonstrate the phenotypic plasticity of salamander hatchlings in response to the presence or absence of frog tadpoles and to screen the type of signals involved in the expression of the phenotypic plasticity. First, salamander hatchlings were reared alone or with tadpoles, and the growth and morphological traits of the hatchlings were compared. The results showed that hatchling larvae grew faster with a more developed gape in the presence of tadpoles. Next, to identify the type of signals inducing this plasticity, two separate experiments with manipulated chemical and visual signals from tadpoles were conducted. The findings showed that faster growth and a more developed gape were induced by chemical but not visual signals. This plasticity may be an adaptive strategy because it increases the likelihood of preying on tadpoles in future prey-predator interactions.