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UPSC MainsZOOLOGY-PAPER-I201212 Marks150 Words
Q1.

Draw labelled sketches of Alima and Phyllosoma larvae. Identify the neuro-endocrine factors that cause metamorphic changes in them.

How to Approach

This question requires both diagrammatic skills and knowledge of neuro-endocrine control of invertebrate metamorphosis. The approach should be to first draw clear, labelled diagrams of Alima and Phyllosoma larvae, highlighting key features. Then, focus on identifying the neuro-endocrine factors – primarily hormones – involved in triggering and regulating their metamorphic transformations. Mentioning the specific hormones and their roles is crucial. Structure the answer with diagrams first, followed by a detailed explanation of the hormonal control.

Model Answer

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Introduction

Metamorphosis, a post-embryonic developmental process involving abrupt changes in an animal’s body structure, is particularly dramatic in marine invertebrates like crustaceans. Crustaceans exhibit diverse larval forms, often radically different from the adult stage, undergoing complex metamorphic transitions. The Alima larva is found in portunid crabs, while the Phyllosoma larva is characteristic of spiny lobsters. These larval stages undergo significant morphological and physiological changes regulated by a complex interplay of neuro-endocrine factors, primarily crustacean hyperglycemic hormone (CHH) family peptides and ecdysteroids. Understanding these hormonal controls is vital for comprehending crustacean development and ecology.

Alima Larva – Sketch and Metamorphosis

Alima Larva

Labelled Features: Carapace shield, pereiopods (walking legs), pleopods (swimming legs), telson, uropods, gut. The Alima larva is a megalopa-like stage, possessing a relatively developed carapace and appendages.

Phyllosoma Larva – Sketch and Metamorphosis

Phyllosoma Larva

Labelled Features: Flattened, leaf-like body, long, slender appendages, reduced gut, eyes. Phyllosoma larvae are transparent, pelagic larvae with a distinctive flattened body and elongated appendages. They are poorly pigmented and have a reduced digestive system, relying on yolk reserves.

Neuro-Endocrine Control of Metamorphosis

Crustacean Hyperglycemic Hormone (CHH) Family

The CHH family of neuropeptides plays a central role in regulating crustacean metamorphosis. Different CHH-like peptides (CHH1, CHH2, CHH3, etc.) have been identified, each with specific functions.

  • CHH1: Primarily involved in regulating carbohydrate metabolism, but also influences molting and metamorphosis.
  • CHH2: Plays a crucial role in initiating and coordinating metamorphic changes, particularly in the Phyllosoma larva. It stimulates ecdysteroid synthesis.
  • CHH3: Involved in regulating water balance and ion transport during metamorphosis.

Ecdysteroids (Molting Hormones)

Ecdysteroids, particularly 20-hydroxyecdysone (20E), are the primary hormones driving molting and metamorphosis in crustaceans.

  • Synthesis: CHH2 stimulates the Y-organ (equivalent to the adrenal gland in vertebrates) to synthesize ecdysteroids.
  • Mechanism: 20E binds to a nuclear receptor complex (EcR-USP), activating gene expression involved in cuticle formation, appendage development, and other metamorphic processes.
  • Alima Metamorphosis: Rising 20E levels trigger the development of the abdomen, the formation of the adult carapace, and the differentiation of appendages.
  • Phyllosoma Metamorphosis: A prolonged period of low 20E levels is followed by a sharp peak, initiating the dramatic transformation into the puerulus stage (a transitional stage) and ultimately the juvenile lobster.

Other Neuro-Endocrine Factors

Besides CHH and ecdysteroids, other factors contribute to metamorphic regulation:

  • Neuropeptide F (NPF): Influences feeding behavior and energy metabolism during metamorphosis.
  • Octopamine: Modulates the effects of ecdysteroids and plays a role in neuronal development.
  • Proctolin: Involved in muscle contraction and cuticle deposition.
Larval Stage Key Hormonal Events Metamorphic Changes
Alima Increasing 20E levels, CHH1 influence Abdominal development, carapace formation, appendage differentiation
Phyllosoma Prolonged low 20E, then sharp peak stimulated by CHH2 Transformation to puerulus, development of gut, appendage remodeling

Conclusion

The metamorphosis of Alima and Phyllosoma larvae is a complex process orchestrated by a delicate interplay of neuro-endocrine factors, with the CHH family and ecdysteroids playing central roles. Understanding these hormonal controls is crucial for studying crustacean development, ecology, and evolution. Further research into the specific functions of different CHH peptides and their interactions with other hormones will provide a more comprehensive understanding of these fascinating developmental transitions. The sensitivity of these hormonal pathways to environmental factors also highlights the vulnerability of crustacean populations to pollution and climate change.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

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Additional Resources

Key Definitions

Metamorphosis
A biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively sudden change in the animal's body structure through cell growth and differentiation.
Ecdysteroids
A class of steroid hormones that regulate molting and metamorphosis in arthropods, including crustaceans. 20-hydroxyecdysone (20E) is the most biologically active ecdysteroid.

Key Statistics

Global crustacean fisheries and aquaculture production reached 8.8 million tonnes in 2020, with a value of US$68.8 billion (FAO, 2022).

Source: FAO. (2022). The State of World Fisheries and Aquaculture 2022. Rome.

Approximately 75% of commercially important marine invertebrate species, including many crustaceans, have a planktonic larval stage (knowledge cutoff 2023).

Source: Pechenik, J. A. (2019). Biology of Invertebrates. McGraw-Hill Education.

Examples

Lobster Aquaculture

Understanding the hormonal control of Phyllosoma metamorphosis is crucial for developing successful lobster aquaculture. Researchers are attempting to manipulate hormonal levels to accelerate metamorphosis and improve survival rates in hatchery-reared lobsters.

Frequently Asked Questions

▶What is the Y-organ?

The Y-organ is an endocrine gland in crustaceans analogous to the adrenal gland in vertebrates. It is the primary site of ecdysteroid synthesis, crucial for molting and metamorphosis.

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