Problem 55 - Objective Biology for NEET

Question

The umbrella shape and free swimming form of coelenterate reproduced by sexual reproduction is (a) Polyp (b) Medusa (c) Both (a) and (b) (d) None of these

Step-by-Step Solution

Answer

The free-swimming, umbrella-shaped form of coelenterate which reproduces sexually is the Medusa.

Step 1: Understanding the terminologies

Coelenterates are a group of animals that includes jellyfish and corals. These organisms can have two basic body forms: the polyp, which is typically sessile (attached), and the medusa, which is free-swimming and has an umbrella-like shape. Sexual reproduction usually takes place in the medusa form.

Step 2: Matching the description to the correct answer

Here we see that the question asks about a free-swimming, umbrella-shaped form of the coelenterate, which undergoes sexual reproduction. From the above analysis, we understand that this description matches with the 'Medusa' form. Thus, the correct answer is (b) Medusa.

Key Concepts

Polyp and Medusa

Coelenterates are fascinating creatures with diverse body forms, primarily polyps and medusae. A polyp is a cylindrical shape that attaches to a substrate, resembling a tiny sea anemone. It’s well-suited for a sedentary lifestyle. Common examples include corals and sea anemones.
For instance, in a coral polyp, the base is fixed to a rock or dead coral, while the mouth, surrounded by tentacles, is free for feeding. On the other hand, a medusa is free-swimming and exhibits a conspicuous bell or umbrella shape. Jellyfish are the quintessential example of the medusa form.
These body forms are not just distinct shapes but represent different phases in the life cycle of coelenterates. A unique feature of their life cycle is the ability to switch between these two forms, a process called metagenesis. The polyp, being relatively hardy, often asexually reproduces, budding off new polyps or medusae depending on environmental conditions. In contrast, medusae are the ones engaging in sexual reproduction, highlighting a remarkable division of reproductive labor within a single species.

Sexual reproduction in coelenterates

When exploring sexual reproduction in coelenterates, it’s essential to know that this process typically occurs in the medusa stage. During sexual reproduction, medusae release sperms and eggs into the water column where fertilization takes place. This often results in the formation of a planula larva, which eventually settles and transforms into a new polyp, thus completing the life cycle.
Coelenterates can exhibit complex reproductive strategies, some even harboring gonads within the medusa’s body, where eggs and sperm develop. These strategies ensure effective distribution of species in aquatic environments. An interesting aspect is that some species can also reproduce asexually, enabling coelenterates to rapidly colonize an area. This dual mode of reproduction—sexual and asexual—makes them incredibly versatile and successful in various marine habitats.

Coelenterate body forms

Coelenterates boast two major body forms: the polyp and the medusa, as previously discussed. These are not just simple differences in appearance but have evolved as adaptations to their respective environments.

• Polyps have a tubular shape and are often fixed to a solid surface, making them well-adapted to a sedentary existence.
• Medusae have a bell-like structure that allows them to float or swim, typically using passive drifting or pulsations of their bell to move through the water.

Each body form plays a role in their survival and reproductive strategy. For example, since polyps are immobile, they rely on the currents to bring them food and disperse their offspring. Medusae, however, actively swim and can distribute more widely in the ocean, which aids in the genetic dispersal during their sexual reproduction phase.
The adaptation of transitioning between these two body forms is one of the reasons coelenterates have thrived for millions of years. They exploit different ecological niches and optimize their survival and reproductive success in the ever-changing marine environments.