Model Answer
0 min readIntroduction
Polymorphism, derived from the Greek words ‘poly’ (many) and ‘morph’ (form), refers to the ability of an organism to exhibit different forms throughout its life cycle. This phenomenon is commonly observed in Cnidarians like *Obelia*, where distinct morphological stages – polyp and medusa – are adapted for different functions, contributing to the organism’s survival and propagation. Understanding polymorphism in *Obelia* provides a classic example of alternation of generations and adaptive strategies in invertebrates. The life cycle of *Obelia* is a prime illustration of this biological concept, showcasing how a single species can exist in multiple, functionally distinct forms.
Polymorphism: A General Overview
Polymorphism is not merely a change in size or color; it involves significant structural and functional differences. These differences are often linked to variations in environmental conditions, reproductive strategies, or feeding habits. In Cnidarians, polymorphism is largely driven by the presence of two basic body forms: the polyp and the medusa.
- Polyp: Typically sessile, cylindrical, and attached to a substrate. Polyps are primarily responsible for asexual reproduction and feeding.
- Medusa: Free-swimming, bell-shaped, and involved in sexual reproduction.
Life Cycle of Obelia: A Polymorphic Journey
The life cycle of Obelia demonstrates a clear alternation between polyp and medusa forms. It begins with a settled hydranth (polyp) and progresses through several stages:
1. Polyp Stage (Hydrant)
The polyp form of Obelia, known as the hydranth, is the dominant stage. It is a colonial organism, meaning multiple polyps are interconnected and share a common gastrovascular cavity. The hydranth is responsible for feeding and asexual reproduction through budding.
- Hydranth: The feeding and reproductive polyp.
- Gastrozooids: Feeding polyps with tentacles for capturing prey.
- Gonozooids: Reproductive polyps that produce medusa buds.
- Dactylozooids: Defensive polyps with nematocysts (stinging cells).
2. Budding and Medusa Production
Gonozooids on the stem produce medusa buds asexually. These buds mature into free-swimming medusae.
3. Medusa Stage
The medusa is a small, bell-shaped, free-swimming form. It is sexually reproductive.
- Gonads: Located on the radial canals within the medusa, they produce gametes (eggs and sperm).
- Fertilization: Occurs externally in the water, leading to the formation of a zygote.
- Planula Larva: The zygote develops into a ciliated, free-swimming larva called a planula.
4. Settlement and Polyp Formation
The planula larva settles on a suitable substrate and undergoes metamorphosis, developing into a new polyp, thus completing the life cycle.
Polymorphism in Obelia: A Detailed Look
The polymorphism in Obelia is evident in the distinct forms and functions of the polyp and medusa stages. The polyp stage is specialized for feeding and asexual reproduction, allowing for rapid colony growth. The medusa stage is specialized for sexual reproduction and dispersal, ensuring genetic diversity and colonization of new areas.
| Feature | Polyp (Hydranth) | Medusa |
|---|---|---|
| Lifestyle | Sessile, attached | Free-swimming |
| Reproduction | Asexual (budding) | Sexual (gamete release) |
| Body Form | Cylindrical, with tentacles | Bell-shaped, with tentacles |
| Dominant Stage | Dominant in Obelia | Reduced, temporary |
Conclusion
In conclusion, polymorphism in *Obelia* is a remarkable adaptation that allows the organism to exploit different ecological niches and maximize its reproductive success. The alternation between the polyp and medusa stages, each specialized for distinct functions, exemplifies the evolutionary advantages of morphological diversity. This life cycle serves as a classic example in zoology, illustrating the principles of alternation of generations and the adaptive significance of polymorphism in Cnidarians.
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.