Question

In invertebrates, molting of larvae is stimulated by a. growth hormone. b. cortisol. c. juvenile hormone. d. 20-hydroxyecdysone. e. aldosterone.

Short answer

The correct answer is d. 20-Hydroxyecdysone.

Step-by-step solution

Analyze Hormones

Each provided hormone has a different function. Growth hormone is principally involved in the growth and development of organisms, but not directly with molting. Cortisol is a stress hormone in humans (and other mammals) and doesn't play a role in invertebrate molting. Juvenile hormone is involved in insect development and metamorphosis, but it doesn't directly trigger molting. Aldosterone is a hormone that helps control blood pressure and doesn't have a role in invertebrate molting.

Identify the Correct Hormone

20-Hydroxyecdysone is an ecdysteroid hormone that stimulates ecdysis or molting in insects and other arthropods. It's the primary hormone that controls molting in these invertebrates.

Key concepts

Ecdysis

Ecdysis, commonly known as molting, is a critical process in the life cycle of invertebrates, especially arthropods, including insects, crustaceans, and spiders. It is the shedding of the old exoskeleton to allow for growth and development. This process is not as simple as slipping out of a coat; it is a complex, hormone-regulated event that requires careful synchronization of physiological mechanisms. During ecdysis, an organism must produce a new, often larger exoskeleton beneath the old one. Then, it must extract itself from the old exoskeleton, which involves splitting and discarding the tough outer layer.

The significance of ecdysis cannot be overstated. It allows the organism to grow, as the rigid exoskeleton does not expand like skin. It also may serve as a way to remove parasites and other external damages that may have occurred to the old exoskeleton. However, the process of ecdysis makes an organism vulnerable to predators and environmental stress, as they are temporarily without their protective layer. Recognizing the signs leading up to molting can be important for those managing or studying invertebrates, whether in the wild or in captivity.

20-Hydroxyecdysone

The hormone 20-hydroxyecdysone is the star player in the process of ecdysis. Structurally identified as the primary ecdysteroid hormone, 20-hydroxyecdysone initiates and regulates the molting cycle of invertebrates. Within the invertebrate endocrine system, this hormone sets off a cascade of physiological events leading up to the shedding of the exoskeleton.

In response to internal and environmental signals, levels of 20-hydroxyecdysone rise, leading to a series of cellular and biochemical events that construct a new exoskeleton. Following this, enzymes are activated to detach the old exoskeleton, and behaviors such as molting movements are triggered. Understanding 20-hydroxyecdysone is vital for those studying invertebrate biology, as it has profound implications for development, growth, and metamorphosis.

Juvenile Hormone

While juvenile hormone may not be the direct trigger for ecdysis, its role in invertebrate development is indispensable. Juvenile hormone balances the effects of 20-hydroxyecdysone by regulating the nature of each molt. In other words, it determines whether an insect will molt into a larger larval stage, transform into a pupa, or progress to the adult stage. During larval stages, high levels of juvenile hormone promote molting into successive larval stages; lower levels signal the start of metamorphosis into a pupa, and the absence of juvenile hormone allows for the final molt into adulthood.

The careful balance between juvenile hormone and 20-hydroxyecdysone ensures that the organism goes through the correct stage at the appropriate time. Disruption in the levels of these hormones can lead to developmental abnormalities, making the hormone a key focus in both basic biological research and practical applications such as pest management.

Hormonal Regulation in Invertebrates

Hormonal regulation is fundamental for orchestrating growth and development in invertebrates. Hormones are signaling molecules that travel throughout the body, initiating various processes required for maintaining homeostasis and enabling developmental transitions. In the context of molting, hormonal regulation involves an intricate interplay between multiple hormones, such as 20-hydroxyecdysone and juvenile hormone.

These hormones interact with receptors in target cells, triggering gene expression and enzymatic activities that facilitate the molting process. Environmental factors, such as temperature and light, as well as internal signals reflecting the organism's developmental status, can influence hormone levels. By understanding the hormonal regulation mechanisms, scientists can manipulate these signals to control pest species or to enhance the production of beneficial invertebrates. Therefore, hormonal regulation is not just a keystone in the biological functionality of these creatures, but also a significant area of investigation for applied sciences.