Problem 258 - Objective Biology for NEET

Question

Assertion: Obelia shows metagenesis. Reason: Obelia is polymorphic organism.

Step-by-Step Solution

Answer

Both the assertion and reason are correct and the reason is the correct explanation for the assertion

Step 1: Understand the Terms

Before proceeding with the problem, it's important to comprehend the primary terms. Metagenesis refers to the alternation of generations in certain organisms' life cycle, and polymorphism refers to the occurrence of different forms among the members of a population or colony.

Step 2: Consider Obelia's Characteristics

Obelia is a genus of marine cnidarians belonging to the class Hydrozoa. In its life cycle, Obelia demonstrates an alternation of generations, with alternating polyp(a sessile, asexual stage) and medusa(a free-living, sexual stage). This is known as Metagenesis.

Step 3: Relate Metagenesis and Polymorphism

The characteristic of Obelia being 'polymorphic' refers to the presence of more than one form of individual in its life cycle, i.e., the polyp and the medusa. Consequently, it can be reasoned that because Obelia shows polymorphism, it also exhibits metagenesis.

Step 4: Evaluate the Assertion and Reason

The assertion 'Obelia shows metagenesis' is correct, and the reason 'Obelia is a polymorphic organism' is also true. Furthermore, the reason accurately explains why the assertion is correct. Hence, the assertion and reason both are correct and the reason is the correct explanation for the assertion

Key Concepts

Obelia

Obelia is an intriguing genus of marine creatures that belong to the class Hydrozoa. They are known for their distinct life cycle and various forms they take during their existence. In general, these creatures can be found in oceans worldwide. One of the most interesting aspects of Obelia is its complex life cycle. It includes both the polyp and medusa stages, which is a characteristic feature of this genus. Additionally, the polyps are usually sessile, meaning they are fixed to a substrate, while the medusae are free-swimming.
The life cycle begins with small, tube-like polyps that are connected by a network of threads called coenosarc. Over time, these polyps can develop into medusae, which then release tiny, free-swimming larvae known as planulae. These planulae eventually settle and form new polyps, continuing the fascinating life cycle of Obelia.

Polymorphism

In biological terms, polymorphism refers to the occurrence of different forms or types among the individuals within a particular species or colony. For organisms like Obelia, polymorphism is evident in their life cycles, where different forms—such as polyp and medusa—exist.
This phenomenon allows species to adapt to varying environmental conditions and enhances their survival and reproduction capabilities. In the case of Obelia, polymorphism is seen when the sessile polyps and the free-living medusae exhibit distinct forms and functions. Understanding polymorphism in Obelia is essential as it links to their ability to cycle between different life stages, contributing to their unique survival strategies.

Hydrozoa

Hydrozoa is a diverse class within the phylum Cnidaria. It encompasses organisms like Obelia, as well as other hydroids, fire corals, and some jellyfish. Members of this class are distinct for their ability to alternate between polyp and medusa forms during their life cycles.
One key feature that sets Hydrozoa apart is their simple body structure, which typically includes a mouth opening surrounded by tentacles. Additionally, Hydrozoans often form colonies, with individuals specializing for various roles, such as feeding or reproduction. This versatility and diversity of forms aid Hydrozoa in thriving in a variety of marine environments from shallow waters to the deep sea.

Alternation of Generations

Alternation of generations is a biological process observed in the life cycle of many organisms, including plants and some animals like Obelia. This process involves the regular alternation between two distinct phases: asexual and sexual reproduction.
For Obelia, this takes the form of alternating between a sessile polyp stage and a free-living medusa stage, which collectively contribute to different elements of the organism's reproduction. The polyp is generally asexual, while the medusa stage is sexual. This switch between forms not only aids in genetic diversity but also allows for different adaptations in response to the environment. Consequently, alternation of generations plays a crucial role in the life cycle and evolutionary success of species like Obelia.