Model Answer
0 min readIntroduction
Obelia, a marine hydrozoan, is a classic example of a colonial cnidarian exhibiting both polyp and medusa forms. Its relatively simple structure and distinct life cycle make it an ideal organism for understanding fundamental biological concepts like polymorphism and metagenesis. The life cycle of Obelia demonstrates a fascinating alternation between asexual reproduction in the polyp stage and sexual reproduction in the medusa stage, showcasing a unique adaptation to its marine environment. This question requires a detailed, diagrammatic understanding of these features.
Structure of Obelia
Obelia consists of a gastrovascular cavity and a colony of polyps. The colony is anchored to the substrate by a basal disc. The polyps are specialized for different functions.
Figure 1: Obelia Colony showing different polyps (Gastrozooid, Gonozooid, Dactylozooid)
Polyp Structure
The polyp is cylindrical with a mouth surrounded by tentacles. It has a gastrovascular cavity for digestion. The body wall consists of epidermis, gastrodermis, and mesoglea.
Medusa Structure
The medusa is bell-shaped with a mouth on the underside. It has tentacles with nematocysts for capturing prey. The medusa is free-swimming and responsible for sexual reproduction.
Figure 2: Obelia Medusa
Life Cycle of Obelia
The life cycle of Obelia involves both asexual and sexual reproduction, exhibiting metagenesis.
- Polyp Stage: The life cycle begins with a settled planula larva developing into a polyp.
- Asexual Reproduction: The polyp reproduces asexually by budding, forming a branched colony. Different polyps specialize in feeding (gastrozooids), reproduction (gonozooids), and defense (dactylozooids).
- Medusa Formation: Gonozooids produce medusae by budding.
- Sexual Reproduction: Medusae are sexually mature and release gametes (eggs and sperm) into the water.
- Fertilization & Larva: Fertilization results in a zygote, which develops into a planula larva.
- Settlement: The planula larva settles and develops into a new polyp, completing the cycle.
Figure 3: Life Cycle of Obelia
Polymorphism in Obelia
Polymorphism refers to the existence of multiple forms within a single species. In Obelia, this is evident in the different types of polyps within the colony:
- Gastrozooids (Feeding Polyps): These polyps are responsible for capturing food and digestion. They have a large gastrovascular cavity and numerous tentacles.
- Gonozooids (Reproductive Polyps): These polyps are specialized for sexual reproduction and produce medusae.
- Dactylozooids (Defensive Polyps): These polyps are armed with nematocysts and provide defense for the colony.
This division of labor enhances the colony's efficiency and survival.
Metagenesis in Obelia
Metagenesis is the alternation of generations between an asexual polyp stage and a sexual medusa stage. It is not simply a life cycle, but a reproductive strategy. The polyp represents the asexual generation, while the medusa represents the sexual generation. This alternation allows Obelia to exploit both modes of reproduction, maximizing its reproductive potential and adapting to changing environmental conditions. The polyp stage is dominant in Obelia.
Conclusion
Obelia serves as a compelling example of colonial organization, polymorphism, and metagenesis within the phylum Cnidaria. Its life cycle, characterized by the alternating asexual polyp and sexual medusa stages, demonstrates a sophisticated reproductive strategy. Understanding Obelia’s structure and life cycle provides valuable insights into the evolutionary adaptations of marine invertebrates and the principles of biological organization.
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.