Describe different larval forms of Echinoderms and comment on their evolutionary significance.

Different Larval Forms of Echinoderms

Echinoderm larvae are broadly categorized based on the class to which the adult belongs, though considerable variation exists within each class. The primary larval forms are:

1. Bipinnaria Larva (Found in Starfish & Brittle Stars)

• Morphology: Bilaterally symmetrical, elongated, and transparent. Possesses ciliated larval arms (bipinnaria) for locomotion and feeding. Has a gut extending the length of the body.
• Feeding: Primarily a microphagous feeder, consuming phytoplankton and other small particles.
• Habitat: Planktonic, drifting in the water column.

2. Brachiolaria Larva (Found in some Starfish)

• Morphology: Similar to bipinnaria but possesses adhesive arms (brachiolaria) used for attachment to suitable substrates for settlement and metamorphosis. These arms have suckers at the tips.
• Feeding: Similar to bipinnaria, microphagous.
• Habitat: Planktonic, but actively seeks out settlement sites.

3. Pluteus Larva (Found in Sea Urchins & Sand Dollars)

• Morphology: Characterized by a distinctive post-oral arm, a long, ciliated structure extending from the posterior end. Bilaterally symmetrical with a gut and water vascular system precursors.
• Feeding: Filter feeder, utilizing the ciliated post-oral arm to capture food particles.
• Habitat: Planktonic, often found in coastal waters.

4. Doliolaria Larva (Found in brittle stars)

• Morphology: Barrel-shaped larva with a ring of cilia around the middle. It is transparent and possesses a rudimentary gut.
• Feeding: Filter feeder, capturing food particles with its ciliated band.
• Habitat: Planktonic, often found in deeper waters.

5. Vitellaria Larva (Found in Sea Cucumbers)

• Morphology: Large, yolky larva with a reduced gut. It is elongated and possesses a ciliated band for locomotion.
• Feeding: Relies heavily on yolk reserves, with limited external feeding.
• Habitat: Planktonic, but often settles quickly.

Evolutionary Significance of Echinoderm Larvae

The larval forms of echinoderms hold significant evolutionary importance, particularly in understanding their relationship to chordates. Several key aspects highlight this significance:

• Bilateral Symmetry: The bilateral symmetry of the larvae contrasts sharply with the pentaradial symmetry of the adults. This suggests that bilateral symmetry is the ancestral condition in echinoderms, and the radial symmetry of adults is a derived characteristic. This is supported by developmental genetics.
• Deuterostome Development: Echinoderms, along with chordates, are deuterostomes, meaning the blastopore (the first opening formed during gastrulation) becomes the anus, while the mouth forms secondarily. This developmental pattern is shared with chordates, indicating a common ancestry.
• Ancestral Chordate Linkages: The larval forms, particularly the bipinnaria and brachiolaria, bear a striking resemblance to the larval forms of some primitive chordates. This has led to hypotheses suggesting a close evolutionary relationship between echinoderms and the ancestors of chordates. The presence of a coelom and a complete digestive system in the larvae further supports this connection.
• Water Vascular System Development: The precursors of the water vascular system, a unique feature of echinoderms, are evident in the larval stages. The development of this system during larval development provides insights into its evolutionary origins.
• Gene Expression Patterns: Studies of gene expression during larval development have revealed similarities between echinoderms and chordates, further supporting their shared ancestry. For example, genes involved in body axis formation are conserved between the two groups.

The evolution of larval forms in echinoderms reflects adaptations to a planktonic lifestyle and the need for dispersal. The diversity of larval forms also suggests a long evolutionary history and a complex interplay between developmental constraints and selective pressures.