Earlier induction of metamorphosis in Japanese eel leptocephali stimulated by thyroid hormone

Title : Earlier induction of metamorphosis in Japanese eel leptocephali stimulated by thyroid hormone

Abstract:

Metamorphosis of teleosts, including Anguilliformes, is well known to be induced by thyroid hormone (TH), although the underlying mechanism is not fully understood. In this study, we investigated the experimental conditions needed to induce normal metamorphosis in artificially spawned Japanese eels (Anguilla japonica), including initial larval size, TH concentration, and timing of TH immersion. We found that around 37 mm TL was the minimum size of larvae that underwent successful metamorphosis induced by L-thyroxine (T₄), suggesting that small leptocephali are not sufficiently responsive to TH. Furthermore, successful completion of metamorphosis depended on sensitivity to TH, which changed with the metamorphic stage. Prolonged exposure to higher TH concentrations led to morphological defects. Our results reveal that the induction of metamorphosis by TH is dependent on larval size and that the concentration of TH must be adjusted in line with the metamorphic stage to achieve successful progression of metamorphosis. While metamorphosis of the leptocephali of 50 mm TL or more is a well-recognized phenomenon in normal breeding, we found that even leptocephali of around 40 mm TL have the potential to undergo metamorphosis in this study. The problems with current eel seedling production are that it is not possible to raise large numbers of eels in a tank, and that the growth speed is slow. As a result of this research, it is possible to shorten the production period of glass eels to the same period as that of wild glass eels. Our findings will contribute to improving production technology in the aquaculture of Japanese eels by facilitating the earlier induction of metamorphosis in artificial leptocephali.

Biography:

Dr. Kawakami conducted research on the Japanese eel migration mechanism at Kyushu University, Japan. He received his PhD degree in 1998, with SEM and EPMA analyses of Otoliths. He then joined the eel reproductive research group at Hokkaido University, using molecular biological methods to study the metamorphosis mechanisms of Anguilliformes. Next, he was involved in aquaculture research at Kinki University, research on Germline chimeras at the Nanae Station, Hokkaido University, and eel seedling production and development at the Research Institute for Aquaculture, Japan. From 2012, he led the development of a research environment for eel seedling production at Shin Nihon Kagaku Ltd., Japan.