Model Answer
0 min readIntroduction
Echinoderms, a phylum of marine invertebrates, exhibit a complex life cycle often involving a variety of larval stages. These larval forms are significantly different from the adult body plan and play a crucial role in dispersal and colonization. The larval development varies considerably among the different classes within Echinodermata. Understanding these larval stages is vital for comprehending the evolutionary relationships and ecological adaptations of these fascinating marine organisms. This answer will detail the larval forms found in Asteroidea, Ophiuroidea, and Holothuroidea, accompanied by labelled diagrams to illustrate their key features.
Larvae of Asteroidea (Starfish)
Starfish larvae undergo a characteristic bilateral symmetry larval stage before transitioning to the radial symmetry of the adult. The primary larval forms are the bipinnaria and brachiolaria.
Bipinnaria Larva
- Shape: Bilaterally symmetrical, flattened, and transparent.
- Features: Possesses ciliated bands for locomotion and feeding. It has a gut, and developing arms are visible as buds.
- Development: The initial larval stage, developing from the fertilized egg.
Brachiolaria Larva
- Shape: Derived from the bipinnaria larva.
- Features: Characterized by three adhesive arms (brachiolaria arms) used for attachment to a substrate. These arms are ciliated and help the larva settle and metamorphose into the juvenile starfish.
- Development: Represents a later stage, preparing for settlement and metamorphosis.
Larvae of Ophiuroidea (Brittle Stars)
Brittle star larvae also exhibit a bipinnaria stage, but it is typically shorter-lived compared to starfish. They then develop into a unique larval form called the ophiopluteus.
Ophiopluteus Larva
- Shape: Bilaterally symmetrical, elongated body with skeletal rods extending from the body.
- Features: Possesses five primary skeletal rods that will eventually develop into the arms of the brittle star. These rods are supported by microscopic ossicles. Ciliated bands are present for locomotion and feeding.
- Development: Distinctive larval form unique to ophiuroids, representing a significant departure from the bipinnaria stage.
Larvae of Holothuroidea (Sea Cucumbers)
Sea cucumber larvae undergo a unique developmental pathway, involving a doliolaria larva, which then transforms into a pentactula larva.
Doliolaria Larva
- Shape: Barrel-shaped, transparent larva with a prominent ciliated band around the middle.
- Features: Possesses a fluid-filled cavity and a rudimentary gut. It lacks arms and is primarily planktonic, relying on ciliary currents for feeding and movement.
- Development: Develops from the fertilized egg and is a characteristic larval form of sea cucumbers.
Pentactula Larva
- Shape: Derived from the doliolaria larva, it develops five arms around the anterior end.
- Features: The arms are ciliated and used for attachment and locomotion. The pentactula larva undergoes a complex metamorphosis into the juvenile sea cucumber.
- Development: Represents the final larval stage before settlement and transformation into the adult form.
Conclusion
The larval stages of Asteroidea, Ophiuroidea, and Holothuroidea demonstrate the remarkable diversity in developmental strategies within the Echinodermata phylum. Each class exhibits unique larval forms adapted to their specific ecological niches and life histories. The bipinnaria and brachiolaria larvae of starfish, the ophiopluteus of brittle stars, and the doliolaria and pentactula larvae of sea cucumbers all highlight the complex and fascinating life cycles of these marine invertebrates. Further research into the genetic and environmental factors influencing larval development is crucial for understanding the conservation and management of echinoderm populations.
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.