Larval Forms of Echinodermata

The development of echinoderms is characterized by a unique and intricate process of metamorphosis, which is unlike any other group of animals. Echinoderms exhibit two primary modes of development: direct and indirect. In direct development, there are no distinct larval stages, and the organism proceeds directly from egg to adult. In contrast, indirect development involves the formation of one or more larval stages, which are free-swimming and play a key role in the organism’s life cycle.

In most echinoderms, the larvae are bilaterally symmetrical, a clear contrast to the radial symmetry of the adult form. This bilaterally symmetrical larva is propelled by cilia, typically organized into a ciliated band. Depending on the species, this ciliated band may be augmented by a number of slender arms or projections originating from the body wall. These projections, whether short or long, help distinguish the different classes of echinoderms based on their structural features.

Besides their ciliated propulsion, the echinoderm larvae undergo a significant metamorphosis, transforming from bilateral symmetry to the radial symmetry characteristic of adults. This metamorphosis is a critical phase, marking the transition from the free-swimming larval stage to the sessile or slow-moving adult form. The process of metamorphosis involves substantial reorganization of the body, which reflects the phylogenetic history and evolutionary adaptations of echinoderms.

The development of echinoderm larvae follows a deuterostomous pattern. Deuterostomy refers to a mode of embryonic development where the anus forms before the mouth, a key feature of echinoderms and other deuterostomes. Following fertilization, which occurs externally in the water, the zygote undergoes radial, holoblastic (complete) cleavage. This cleavage is indeterminate, meaning that the fate of the embryonic cells is not fixed early in development, allowing for a greater range of developmental potential.

Echinoderms, such as sea stars, sea urchins, and sand dollars, exhibit variations in larval morphology, and these differences are significant from both functional and evolutionary perspectives. As the larvae mature through successive stages, they are nourished by external food sources, gaining the necessary energy for growth and metamorphosis. Once metamorphosis is complete, the adult echinoderm emerges with its characteristic radial symmetry.

Different classes of echinoderms—such as asteroids (sea stars), echinoids (sea urchins), and holothurians (sea cucumbers)—each produce distinct types of larvae. The structural differences between these larval forms provide valuable insights into their evolutionary relationships. Studying the variations in larval morphology across different echinoderm classes helps researchers understand how these organisms have evolved over time and how they are related to other groups in the animal kingdom.

Larvae of Asteroidea

The development of Asteroidea, a class of echinoderms that includes sea stars, occurs through a distinct process involving three main larval stages. These stages provide crucial insight into the metamorphosis from free-swimming larvae to the sessile or slow-moving adult starfish. The following is a detailed explanation of the larval forms in Asteroidea, which include the Early Bipinnaria, Bipinnaria, and Brachiolaria larvae.

• Early Bipinnaria Larva:
  • The Early Bipinnaria is the first larval stage in Asteroidea development.
  • It resembles the hypothetical dipleurula larva and has an oval body without arms.
  • This larva possesses ciliary bands, which are responsible for its locomotion in the water.
  • A well-developed alimentary canal is present, allowing it to feed and grow.
  • The Early Bipinnaria larva gradually develops into the more advanced Bipinnaria stage.
• Bipinnaria Larva:
  • The Bipinnaria larva, a more developed form, possesses five pairs of ciliated arms.
  • These arms do not contain any internal skeletal support, but they are essential for swimming in the water.
  • The ciliated arms facilitate feeding, as they help the larva capture plankton from the surrounding water.
  • Additionally, there are pre-oral and post-oral ciliary bands present, further assisting in locomotion and feeding.
  • The general appearance of the Bipinnaria larva is similar to that of the Auricularia larva seen in Holothuroidea, another echinoderm class.
• Brachiolaria Larva:
  • The Brachiolaria larva forms after 6 to 7 weeks of life and continued growth of the Bipinnaria larva.
  • This larva is sedentary, attaching itself to a hard substratum with the help of three brachiolarian arms that have adhesive discs at the tips.
  • The ciliated arms, which were prominent in earlier stages, become reduced and thin, ultimately becoming functionless.
  • The mouth, anus, and gut are well-developed in this stage, indicating further maturation.
  • Inside the Brachiolaria, the water vascular system begins to develop, with the formation of three coelomic compartments: axocoel, hydocoel, and somatocoel.
  • The starfish development occurs within the sedentary Brachiolaria, which eventually ruptures, releasing tiny starfish into the water.
• Metamorphosis of Brachiolaria:
  • During metamorphosis, the Brachiolaria larva attaches to a substrate using its adhesive structures.
  • The anterior part of the larva acts as a stalk, temporarily anchoring the organism.
  • Meanwhile, the posterior part, containing the gut and coelomic chambers, transforms into the young starfish.
  • The young starfish eventually detaches itself from the substratum and begins its free-swimming life as an adult.
  • This marks the final stage of development, with the small starfish now able to move independently in its environment.

Larvae of Holothuroidea

The class Holothuroidea, which includes sea cucumbers, undergoes a complex larval development process involving two distinct larval stages: the Auricularia larva and the Doliolaria larva. These larval forms play crucial roles in the development of adult holothurians, and each stage has specific structural and functional characteristics that facilitate survival and progression to the next developmental phase.

• Auricularia Larva:
  • The Auricularia larva closely resembles the Bipinnaria larva of Asteroidea, although they belong to different classes.
  • It is a free-swimming larva with a bilaterally symmetrical body.
  • The preoral lobe is well-developed, a feature that aids in the larva’s orientation and feeding.
  • A single, winding ciliated band is present, which may be extended into lobes to assist in movement.
  • The mouth, sacciform stomach, hydrocoel, right and left stomocoels, and anus are all well-defined, indicating the larva’s ability to feed and digest food effectively.
  • After a short period in this stage, the Auricularia larva undergoes a transformation into the next larval stage, the Doliolaria.
• Doliolaria Larva:
  • The Doliolaria larva is the next stage following the Auricularia and is characterized by a bilaterally symmetrical, barrel-like body.
  • This larva has five ciliated bands surrounding its body, contributing to its movement in water.
  • The preoral lobe remains well-developed, continuing to play a role in the larva’s locomotion and feeding.
  • The ciliated band, which initially winds around the body, breaks into 3 to 5 flagellated transverse rings, facilitating more efficient swimming.
  • A neural sensory plate is located on the anterior side of the larva, aiding in sensory perception and feeding.
  • An apical tuft of cilia is present, which helps the larva maintain balance while swimming.
  • The mouth, or vestibule, is located on the ventral side, and the gut is organized into distinct zones, further aiding in the digestive process.
  • While the Doliolaria larva undergoes transformation into an adult, in some species of holothurians, this Doliolaria stage may be absent, and the larva may bypass this phase in its development.

Larvae of Echinoidea

In the class Echinoidea, which includes sea urchins, there is a single larval stage known as the Echinopluteus. This complex and distinctive larva plays a crucial role in the development of adult sea urchins. The Echinopluteus larva is characterized by several key anatomical features that distinguish it from other echinoderm larvae and contribute to its feeding and locomotion abilities.

• Echinopluteus Larva:
  • The Echinopluteus larva is bilaterally symmetrical, marking a contrast with the radial symmetry observed in the adult form of sea urchins.
  • It has an oval body structure that is designed to support its active lifestyle as a free-swimming, pelagic organism.
  • The larva is equipped with long, paired ciliated arms that aid in both locomotion and feeding. These arms are supported by calcareous skeletal rods, providing structural integrity and movement efficiency.
  • The Echinopluteus larva features a preoral arm, which is located anteriorly, helping the larva to move and position itself in water. However, the posterolateral arm, typically found in other echinoderm larvae, is absent.
  • The remaining three arms include the anterolateral, postoral, and posterodorsal arms, each contributing to different aspects of swimming and feeding.
  • The mouth, anus, and gut are all well-developed, allowing the larva to efficiently feed and process food as it drifts through the water.
  • This larva is a feeding form, utilizing its ciliated arms to capture plankton and other microscopic particles from the surrounding environment.
• Mesogen Larva:
  • In addition to the standard Echinopluteus larva, there is an unusual form found in the subantarctic brooder Abatus cordatus, which demonstrates direct development without a distinct larval body plan.
  • The Mesogen larva is unlike the Echinopluteus and lacks the characteristic features of a typical echinoid larva.
  • It does not exhibit bilateral symmetry, and it shows a novel orientation of the hydrocoel. Moreover, it displays unique patterns of morphogenesis in the coeloms, gut, and skeleton, differing significantly from the developmental processes in other echinoids.
  • This larva represents a departure from the typical developmental stages of sea urchins, providing insight into the diversity of larval forms within the class.

Larvae of Ophiuroidea

In the class Ophiuroidea, which includes brittle stars and basket stars, the larval development occurs through various stages, with the Ophiopluteus being the primary and most common larval form. Other forms, such as the Doliolaria and Vitellaria, represent non-feeding larvae, and some ophiuroids also exhibit direct development through a unique mesogen larva. The various larval types provide insight into the complexity of their developmental pathways.

• Ophiopluteus Larva:
  • The Ophiopluteus is the main larval form found in most species of Ophiuroidea and bears a resemblance to the Echinopluteus larva of Echinoidea in its general features.
  • This larva is a complex, feeding organism, primarily adapted to a planktonic existence, where it plays a critical role in the early stages of the ophiuroid’s life cycle.
  • The anterolateral, postoral, and posterodorsal arms are present in the Ophiopluteus, although the preoral arm that is typically seen in other echinoderm larvae is absent. Instead, this larva has long posterolateral arms, which serve critical functions in swimming and capturing food.
  • All of the arms are supported by calcareous skeletal rods, which provide structural integrity and enable efficient movement through the water.
  • After undergoing its planktonic life stage, the Ophiopluteus larva metamorphoses into an adult ophiuroid, completing the transition from a feeding, swimming larva to a sedentary adult.
• Non-Feeding Larval Stages: Doliolaria and Vitellaria:
  • Some ophiuroid species, in addition to the Ophiopluteus, also produce non-feeding larvae such as the Doliolaria and Vitellaria.
  • Doliolaria larvae exhibit diversity in structure and behavior. This larva typically possesses four transverse ciliary rings, which are used in locomotion during its early stages.
  • Vitellaria larvae, in contrast, lack the characteristic ciliated bands seen in other echinoderm larvae. Instead, they possess paired enterocoels and exhibit bilateral symmetry, which sets them apart from the typical radial symmetry of echinoderms.
  • These non-feeding larvae likely have different developmental roles and might serve to further the complex evolutionary history of Ophiuroidea.
• Direct Development: Mesogen Larva:
  • In addition to the previously mentioned larval forms, some ophiuroids exhibit direct development through a mesogen larva, which is significantly different from the traditional planktonic larval stages.
  • The mesogen larva does not display the typical characteristics of Ophiuroidea larvae, and instead, it progresses through direct development, bypassing the free-swimming feeding larval stage altogether.

Larvae of Crinoidea

The class Crinoidea, commonly known as sea lilies and feather stars, demonstrates a fascinating array of larval development stages that reflect both the complexity of their life cycle and their evolutionary relationships with other echinoderms. The larvae of Crinoidea progress through distinct stages, with adaptations to both pelagic and sedentary lifestyles. These stages include the Pentactula, Doliolaria, and Pentacrinoid larvae, each exhibiting unique features that provide insight into their developmental processes and evolutionary connections.

• Pentactula Larva:
  • The Pentactula is considered the fundamental larval stage for Crinoidea, though it is confined within the egg during its early development.
  • This larva is the basic precursor to the more complex stages seen in sea lilies and feather stars.
  • Crinoidea larvae typically pass through one or two developmental stages, with the Pentactula being integral to this process.
  • It represents the initial developmental form from which more specialized forms evolve.
• Doliolaria (Vitellaria) Larva:
  • The Doliolaria, also referred to as the Vitellaria larva in Crinoidea, is a free-swimming, non-feeding larval phase that closely resembles the Doliolaria larva of Holothuroidea (sea cucumbers).
  • It is a lecithotrophic (yolk-feeding) form, meaning it derives its nutrition from yolk stored in the egg rather than external sources.
  • The body of the Doliolaria larva is elongate oval, narrowing towards the posterior.
  • It possesses 4-5 transverse ciliated bands that encircle its body, aiding in locomotion and movement through the water.
  • A critical feature of this larva is its adhesive pit on the ventral side, which allows it to attach to a substrate and settle into a sedentary life, marking a transition towards becoming a sea lily.
  • This attachment is an important stage in the development of a miniature sea lily, signaling the start of its sedentary lifestyle.
  • The resemblance between the Doliolaria larva of Crinoidea and the Doliolaria larva of Holothuroidea suggests a close evolutionary relationship between the two groups.
• Pentacrinoid Larva:
  • The Pentacrinoid larva is a specialized, sedentary larval stage that is unique to Crinoidea, particularly the feather stars.
  • This larva attaches to a substrate using an attachment plate, which helps anchor it in place.
  • The body of the Pentacrinoid larva is supported by a stalk, a feature typical of crinoid morphology.
  • It has 10 ciliated tentacles surrounding the disc, which are used to capture food, aiding in its nourishment during this stage.
  • Both the mouth and anus are located on the same side of the disc, which is a characteristic feature of this larval form.
  • After spending several months in this sedentary form, the Pentacrinoid larva eventually detaches from the substrate and becomes a more mobile organism, completing its transition into the adult stage.

Significance of Echinoderm Larvae

Echinoderm larvae exhibit remarkable diversity across various classes, with distinct names and characteristics that reflect their evolutionary significance. The study of these larvae reveals insights into the origins, taxonomic relationships, and ecological roles of echinoderms. Understanding the significance of echinoderm larvae can be summarized in several key points:

• Common Origin of Classes:
  • The larvae of most echinoderm classes share fundamental similarities, particularly in their general body plan.
  • Most larvae, with the exception of the sedentary larva of Crinoidea, exhibit bilateral symmetry, a flattened body, longitudinally looped ciliated bands, and well-developed gut and enterocoelic coelom.
  • These shared characteristics suggest a common ancestor for the different classes, likely a coelomate, bilateral, and free-swimming organism.
  • Two hypothetical ancestral larval forms, the Dipleurula and Pentactula, have been proposed by zoologists, indicating that modern echinoderms may have evolved from these ancestral forms.
• Taxonomic Affinities:
  • Despite similarities in larval forms, these characteristics do not reliably indicate taxonomic affinities among echinoderms.
  • Within the Eleutherozoa, there are two distinct larval groups:
    • The Pluteus group, common to ophiuroids and echinoids, is characterized by bilateral symmetry and long arms.
    • The Auricularia group, found in asteroids and holothurians, features a winding ciliated band that may develop into lobes.
  • Based solely on larval similarities, one might suggest that ophiuroids are closely related to echinoids, while asteroids are related to holothurians.
  • However, paleontological and morphological evidence contradicts this notion, indicating that asteroids and ophiuroids share a closer relationship, while echinoids have followed a distinct evolutionary path.
• Phylogenetic Affinities:
  • A survey of larval types reveals numerous instances of convergent evolution among echinoderms, such as the similarities between Ophipluteus and Echinopluteus larvae.
  • This convergence suggests that similar environmental pressures may have led to the development of analogous larval forms among unrelated groups like Asteroidea, Holothuroidea, and Crinoidea.
  • Conversely, closely related groups such as asteroids and ophiuroids may exhibit significant larval differences, indicative of divergent larval evolution.
  • Consequently, the structures of echinoderm larvae do not effectively determine phylogenetic relationships within the phylum.
• Relationship with Chordates:
  • Notably, the Auricularia larva of echinoderms bears striking similarities to the Tornaria larva of certain enteropneusts, such as Balanoglossus.
  • Both larval types demonstrate indeterminate cleavage, and their mesoderm and coelom (enterocoel) have similar developmental origins, suggesting a close evolutionary relationship between echinoderms and lower chordates.
  • Serological studies further indicate a phylogenetic relationship between these two groups, reinforcing the notion that echinoderms and chordates are closely related.
• Aid in Dispersal and Feeding:
  • Given that adult echinoderms tend to be relatively sluggish, their larvae play a crucial role in dispersal.
  • The larvae remain in the planktonic phase long enough to be carried away from their natal environments, enabling them to colonize new areas or replenish original habitats.
  • Additionally, the larval stage allows echinoderms to exploit different food sources than their adult counterparts. This separation in dietary habits reduces competition for resources, particularly in environments where food availability may fluctuate.