Gradient/Extremely Low-Frequency Magnetic Field Affects Metamorphic Behaviors in Thyroxine-Administrated Axolotls: Regulation of Amphibian Metamorphosis Depending on Field Strength and Exposure Timing

In this paper, we investigated the influences of a gradient magnetic field or extremely low-frequency (ELF) fields on the forced metamorphosis of the Mexican axolotls, which was induced with a thyroid hormone. After the adaptation of the axolotls to our experimental environment, they were individually kept in an L-thyroxine (T4) solution under exposure to a gradient field of 17 T/m or ELF fields of 1.0-5.0 mT at 10 Hz. In order to remove unnecessary stress for the axolotls, we recently developed an original water-renewing system equipped with siphonage, velocity, and temperature controls. The number of days required for the perfect metamorphosis into salamanded axolotls under the ELF exposure of 5.0 mT was 16.9 days on average, contrary to the time period for a control (15.1 days). We assumed that the morphological delays of up to 26% among the ELF-exposed axolotls might be due to the competing situation between alternating field effects and biological adaptive responses to the magnetic forces in the axolotl subjects' bodies. In addition, we did found that the initiation timings of the gradient-field exposure did affect the survival rates of the following salamanded axolotls. A decrease in the rates was noticeable in the case of an exposure just before the time point of the morphological completion. Our data greatly support the idea that magnetic field exposures might modify axolotl metamorphosis minutely, depending on the exposure timing, the field strength, the frequency, and so on. We strongly propose investigating the magnetic control of axolotl metamorphosis induced with a thyroid hormone.