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.