Gradient/ELF Magnetic Field Affects Metamorphic Behaviors in T<inf>4</inf>- Administrated Axolotls: Regulation of Amphibian Metamorpho-sis Depending on Field Strength and Exposure Timing

Abstract

Introduction Juvenile axolotls (Ambystoma mexicanum) can be readily metamorphosed to mature salamanders by a function of thyroxine (T 4 ) [1]–[5]; therefore, the experimental applications utilizing axolotls must be favorable for a direct examination with respect to aquatic-terrestrial transformations. But, there are almost no previous studies of the observations of axolotl metamorphosis under exposure to magnetic fields. The purpose of this study is to investigate the influences of a gradient or an extremely low frequency (ELF) magnetic field on the T 4 -inducing forced metamorphosis of axolotls. Materials and Methods Thirty-six axolotls (about 120 mm) were bred under the same condition as group feeding. Before performing this experiment, all the axolotls were individually kept in 0.85-L square boxes containing dechlorinated water (0.7 L) without aeration under an illumination of 250 mEm−2s−1 on a 12:12 h L:D photocycle. The water temperature was also strictly controlled at 24°C, employing an original water-renewing system equipped with siphonage (siphon effect) and temperature controls (Fig. 2). After the adaptation of the axolotls to our experimental environment at least a week, they were kept in 0.32–0.80 mM T 4 and were exposed to a gradient magnetic field of 250 mT or an ELF magnetic field of 5.0 mT at 10 Hz. The gradient/ELF exposure was continued up to the morphological completions of all the T 4 -administrated axolotls. The axolotls had become accustomed to being given food rotating of a solid / a tubifex worm. The morphological changes of the axolotls influenced by the presence of the T 4 were monitored every day, and the changes were evaluated minutely based on the reported method [1]. Discussion To begin with, we will discuss the influences of an ELF field of 5.0 mT at 10 Hz on axolotl metamorphosis, from the viewpoints of a metamorphic rapidity and a morphological change. The earliest completion of axolotl metamorphosis in a control experiment was observed at Day 13, and the remaining axolotls completed their metamorphoses by Day 17. However, none of the metamorphoses were completed by Day 14 under exposure to the ELF field. Moreover, there were morphological delays of up to 26% compared with a control. Concerning the timeframe of the morphological changes in the axolotls under our experimental conditions, we detected no particular change in connection with the ELF field. On the other hand, we found that the initiation timings of gradient-field exposure did affect the survival rates of the salamanded axolotls. Our data greatly support the idea that gradient/ELF exposures might modify axolotl metamorphosis minutely, depending on the exposure timing, the field strength, and the frequency, and so on.