Explain how thyroxine triggers metamorphosis in
amphibians.
2. Describe the effect of melatonin in some vertebrates
other than birds or mammals.
1. Thyroxine triggers metamorphosis in amphibians such as frogs and salamanders. Metamorphosis is the process through which these organisms undergo dramatic changes in their body structure and physiology as they transition from the larval stage to the adult stage. Thyroxine is a hormone produced by the thyroid gland, and its levels increase during metamorphosis.
During the larval stage, amphibians have gills for respiration, a fin-like tail, and live exclusively in water. However, as they prepare to become adults, various organs and tissues need to undergo major transformations. Thyroxine plays a key role in orchestrating these changes.
Thyroxine acts on specific cells in the amphibian's body, triggering the breakdown of larval organs such as the gills and tail, and promoting the development of adult structures like lungs and limbs. It also influences the growth of the animal's digestive system, genitalia, and neurological functions.
The increase in thyroxine levels is triggered by environmental factors such as temperature, light, and availability of food. In response to these cues, the hypothalamus in the amphibian's brain releases thyrotropin-releasing hormone (TRH), which signals the anterior pituitary gland to release thyroid-stimulating hormone (TSH). TSH then stimulates the thyroid gland to produce and release thyroxine into the bloodstream. The thyroxine levels gradually increase, leading to the initiation of metamorphosis.
2. Melatonin is a hormone primarily associated with regulating sleep-wake cycles in mammals, including humans. However, melatonin also has effects on certain vertebrates other than birds or mammals.
In fish, melatonin regulates various physiological processes related to reproduction and behavior. It plays a role in the control of sexual maturation, particularly in determining the timing of spawning. Melatonin levels fluctuate with the daily light-dark cycle, and changes in melatonin secretion can trigger the onset of reproductive behaviors and the release of eggs or sperm.
In reptiles, melatonin is involved in the regulation of seasonal behaviors and reproduction. For example, in reptiles like turtles and crocodilians, melatonin levels increase during the winter or dry season, signaling the onset of hibernation or dormancy. It also influences the timing of breeding and nesting behaviors in these animals.
Additionally, in some species of fish and reptiles, melatonin has been found to play a role in the regulation of skin pigmentation. It can affect the synthesis and dispersion of pigment cells, leading to changes in coloration that can help with camouflage or communication.
Overall, while melatonin's main function is associated with regulating sleep-wake cycles in birds and mammals, it also has various effects on reproductive behaviors, seasonal adaptations, and pigmentation control in certain other vertebrates.