Echinodermata: Larval Forms | Zoology Optional Notes for UPSC PDF Download

Echinoderms and Their Larval Forms

Larval Stage

Larval Forms of Different Echinoderm Classes

Larval Form: Bipinnaria

• Bipinnaria is the first larval form of starfish.
• It is bilaterally symmetrical and free-swimming.
• It has an elongated pre-oral region and a broad post-oral region.
• Bipinnaria possesses two ciliated bands: pre-oral and post-oral.
• The anterior end of the archenteron becomes the mouth, while the blastopore develops into the anus.
• Ciliated bands continue as arms.

2. Ophiuroidea (Class: Brittle Stars)

Larval Form: Branchiolaria

• Branchiolaria larvae have three additional arms known as branchiolarian arms that help in substrate attachment.
• These arms lack cilia and calcareous rods and have extensions of the coelomic cavity.
• Three short arms, one median and two lateral, contain adhesive cells at their tips acting as suckers.
• The remaining arms degenerate and become long, slender.

Metamorphosis of Branchiolaria

• After attaching itself to an object using the adhesive structures, the anterior portion acts as a stalk, and the posterior part with the gut and coelomic chambers transforms into a young starfish.
• The young starfish eventually detaches and starts its free-living adult life.

This description outlines some of the larval forms found in the classes Asteroidea and Ophiuroidea within the phylum Echinodermata. These larval forms are integral to the developmental processes of echinoderms, which undergo dramatic changes as they progress from larvae to adults.

Echinoderms, a unique group of marine animals, exhibit various larval forms during their developmental stages, each tailored to the needs of the specific echinoderm class. These larval stages contribute to the fascinating life cycle of echinoderms.

3. Ophiopluteus Larva
Class: Ophiuroidea (Brittle Stars)
Characteristics:

• Bilateral symmetry
• Single ciliated band
• Long arms with ciliated bands at the margin
• Two anterio-lateral, two post-oral, two posterio-dorsal, and two posterio-lateral arms
• Postero-lateral arms are the longest and directed forward
• Smaller pre-oral lobe
• Developed post-anal part
• Contains coelomic chambers and archenteron

Life Cycle:

• Ophiopluteus larvae do not have an attachment stage.
• They are free-swimming and undergo metamorphosis into tiny serpent stars that sink to the seabed to begin their adult lives.

4. Echinopluteus Larva
Characteristics:

• Resembles Ophiopluteus larva with more arms
• Ciliated bands that develop into arms
• Six fully developed arms with calcareous rods
• Short postero-lateral arms directed outwards or backwards
• Some ciliated bands become thickened and are referred to as "Epaulettes"

Life Cycle:

• Like Ophiopluteus larvae, they lack an attachment stage.
• Metamorphosis is extremely rapid, taking place in about an hour.

5. Auricularia Larva
Class: Holothuroidea (Sea Cucumbers)
Characteristics:

• Transparent, free-swimming, pelagic larva
• Lack of arms
• Well-developed ciliated bands
• Alimentary canal with mouth and anus
• Curved intestine with a sacciform stomach

Life Cycle:

• Swims using a ciliated band that forms both pre-oral and anal loops.
• Develops an alimentary canal with mouth and anus.
• Internal organs include a curved intestine with a sacciform stomach.

6. Doliolaria Larva (Holothuroidea)
Characteristics:

• Transitional stage from Auricularia larva
• Barrel-shaped larva with continuous ciliated bands
• Mouth shifts to the anterior, anus to the posterior pole
• Gradual metamorphosis with the appearance of five tentacles and one to two functional podia
• Also known as "Pentacula"

Life Cycle:

• The appearance of additional tentacles and podia leads to the sea cucumber settling on the seabed, transitioning into the adult mode of life.
• In some cases, there is no Auricularia stage, and the embryo directly develops into the Doliolaria larva.

7. Doliolaria Larva (Crinoidea)
Class: Crinoidea (Sea Lilies and Feather Stars)
Characteristics:

• Free-swimming larva with four to five ciliated bands
• Apical tuft of cilia for sensory functions
• Adhesive pit near the apical plate
• Stomodeum or vestibule between the second and third ciliated bands
• Development of a skeleton during this stage

Life Cycle:

• The larva eventually forms a stalk, transitions into a Pentacrinoid larva, and attaches itself.
• Internal organs rotate, and the larva develops a stalk, becoming a Cystidean or Pentacrinoid larva.
• This stage eventually metamorphoses into an adult crinoid.

Homology and Phylogeny of Echinoderm Larvae

• Except for the Crinoids, which are sessile, larvae of Asteroidea, Holothuroidea, Echinoidea, and Ophiuroidea share fundamental similarities.
• They exhibit pre-oral and post-oral loops, V-shaped ciliated bands, presence of a gut with its divisions and openings, and an enterocoelic coelom.
• These shared features suggest a common ancestry among these classes.