Larval Forms of Echinodermata - Echinoderm Larva

Phylum Echinodermata

The phylum Echinodermata constitutes the largest group of exclusively marine organisms. This may include morphologically diverse species like sea stars, brittle stars, sea biscuits, sea cucumbers, crinoids, sea urchins etc.

These animals are unisexual with no sexual dimorphism. Mostly animals belonging to every class of phylum Echinodermata are viviparous and deuterostomes. The process of fertilization is external and thereby occurs in open water.

The development of the young one indirectly consists of intermediate larval stages. The larva produced after fertilization hatches in the water and undergo multiple successive metamorphosis stages to develop into an adult.

The adult Echinodermata are Pentaradially symmetrical, but the larvae of the majority of them are bilaterally symmetrical and free swimming. However, it loses its bilateral symmetry at the time of metamorphosis and transforms into a radial adult.

All different classes of Echinodermata exhibit morphologically different larval stages. Thus, the study and comparison of the larval stages can reveal the evolutionary ancestry of this phylum.

In this context, we will be studying different larval forms of Echinodermata with their well-labelled diagrammatic representation.

Asteroids – Bipinnaria & Branchiolaria
Ophiuroidea – Ophiopluteus
Echinoidea – Echinopluteus
Holothuroidea – Auricularia
Crinoidea – Doliolaria & Pentacrinoid

Types of Echinodermata Larva

In the following content, we have discussed 8 prominent types of larva in detail.

Dipleurula Larva

Dipleurula larva is universally present and thus regarded as the hypothetical ancestral form of all the echinoderms. In most classes of Echinodermata, it is the early larval stage; therefore, it is considered a fundamental larva.

It is characterized by its microscopic, bilaterally symmetrical, egg-shaped body.
This larva is devoid of a calcareous skeleton, but they have calcareous spines for protection. They possess a gut and two ciliated bands – pre-oral and adoral.
The pre-oral band is located near the mouth, bearing an apical sensory plate and tuft of cilia. Whereas the Adoral band is present inside the mouth.
Three pouches are present in their body coelom. The ventral surface of their body is concave.
The cilia cover the entire surface of the body. Further, the cilia confines in the form of bands surrounding the concavity. These cilia assist the larva in locomotion. When the larva travels forwards via pre-oral ciliated bands, then it tends to swim in a clockwise rotation.
There is an invagination called stomodaeum at the mid-ventral epidermal region of the larva. This stomodeum generates a mouth with a coelomic cavity. The coelomic cavity can be differentiated into the digestive tract with the oesophagus, stomach and intestines.
This larva survives over diatoms and cellular algae.

Bipinnaria Larva

The Bipinnaria larva is the very first larval stage of the class Asteroidea. Generally found in the organisms like Astropecten, Asterina, Starfish etc. After about a week of fertilization, the Bipinnaria larva hatches out of the egg. The Bipinnaria larva looks very much like the Tomaria larva of Balanoglossus.

This larva possesses five pairs of ciliated arms, but still, it is bilaterally symmetrical. The arms are basically for swimming in the water, and therefore, they do not have any calcareous skeletal support.
This larva is free swimming, and it swims through its anterior end in a clockwise rotation. They are free-swimming pelagic larva which feeds on plankton and diatoms. It feeds with the help of food-bearing current produced by ciliary tracts in the Stomodaeal wall.
It comprises a pre-oral lobe which is elongated with brad posterior region. Two ciliated bands are present – the pre-oral ciliated band and Post-oral ciliated band.
These bands continue over a series of prolongated arms. After a while, three of the lateral lobes exist on the lateral sides that remain covered by a post-oral loop of cilia.
Different coelomic apparatus, including the alimentary canal, is produced inside the body. The digestive tract is complete, having a mouth and anus.
After some time, it slowly transforms into its next larval form called the Branchiolaria larva.

The names and numbers of the arms are as follows:

Postero-lateral arm – 2
Post-oral – 2
Postero-dorsal arm – 2
Antero-dorsal arm – 2
Pre-oral arm – 2
Ventro-median arm – 1
Dorso-median arm – 1

Branchiolaria Larva

The Branchiolaria larva is the next stage of the Bipinnaria larva which develops after 6-7 weeks from the time of hatching. It is prevalent in the same organisms, including starfish, Astropecten etc.

The lobes of this larva are converted into a thin, elongated contractile structure having cilia, known as larval arms.
Here, the structure of the Bipinnaria larva transforms to develop three short arms called Brachiolar arms (1 median & 2 lateral). They are present at the pre-oral lobe and comprise coelomic extensions and adhesive discs or suckers at the terminal ends.
There is an adhesive glandular region that acts as a sucker. The presence of this sucker marks the initiation of metamorphosis.
The larva has bilateral symmetry. It is a pelagic larva which is free to swim large distances to establish its colonies away from its over-crowded habitat.
It actively feeds over phytoplankton and other minute organisms.
After sometime, it again undergoes metamorphosis to transform into an adult starfish.
During the metamorphosis, the adult arms are generated from the prolongation of the body. While the mouth, oesophagus, intestines, and anus reduces at the time of transformation.

Auricularia Larva

The Auricularia larva is the early-stage larva of class Holothuroidea. It is a primitive type larva and is characterized by a single longitudinal ciliated band.

Its body is elongated with bilateral symmetry.
There is no calcareous skeletal structure. Instead, there are wheel-like, spheroid or star-like structures.
The ciliated band is partially located near the pre-oral lobe and partially on the folds of the body. This band develops many other processes. In the pupal stage, the band break down into multiple ciliary loops present all around the body.
Just like the Dipleurula larva, this larva also possesses some coelomic pouches. For example, Axohydrocoel and Somatocoel.
Before transforming, the ciliary band breaks into 3 to 4 rings that remain arranged all around the body.
This larva is absent in animals like Cucumaria and Leptostyapta. Instead of Auricularia, they bear some other larva.

Ophiopluteus Larva

This is the larval stage of class Ophiuroidea.

It is a free-swimming larva with bilateral symmetry.
The larva comprises a single primitive ciliated band.
Similar to the Bipinnaria larva, it is not divided.

It develops long arms having ciliated and remains suspended by calcareous rods. All the arms are in differential pairs:

Pre-oral
Postero-dorsal
Postero-lateral
Post-oral

Among all of them, the Postero-lateral arm is the longest. It remains directed anteriorly, giving a V-shaped appearance to the larva.
The Hydrocoel produces a ring around the oesophagus. The larval mouth remains the same in adults. Whereas the opening of the anus closes in adults, and thus, they bear only a single opening.
At the time of metamorphosis, the larva continuously swims in water.
The arms reduce, and calcareous rods disappear. At this stage, the larva becomes sedentary and attaches to a substratum.

Doliolaria Larva

Doliolaria larva is present in the Crinoidea group of animals.

It is the next stage of the Auricularia larva.
It hatches as free swimming, bilaterally symmetrical, barrel-shaped larva with 4-5 ciliary bands.
This larva comprises a neural sensory plate in the mid-ventral line and an apical tuft of sensory cilia that help balance while swimming. The stomodaeum or vestibule is present between the second and third ciliated bands.
The mouth lies in the ventral region for feeding purposes.
The adult skeleton also starts to develop at this stage. As the differentiation into prospective organs is complete, the larva rotates the internal organs at 90°, i.e. from the ventral to the posterior position.
Furthermore, it develops a stalk and becomes a Pentacrinoid larva, which later metamorphoses into an adult.

Echinopluteus Larva

Echinopluteus is the larva of class Echinoidea. This is a single larval stage of this class. Its structure is very much similar to the Ophiopluteus larva.

It is a bilaterally symmetrical pelagic larva with an oval body. However, their arms are not alike.
This larva comprises long paired ciliated arms, which remain supported by calcareous rods.
Mainly, Pre-oral, Postero-dorsal and Post-oral arms are present, but the Postero-lateral arm is absent. It comprises a well-defined mouth, anus and gut.
At every terminal end, two ciliated patches called Epaulettes are present. These are nothing but thickened ciliated bands that are used for the purpose of locomotion. The larva can swim directly in the water due to spikes on the aboral surface.

Pentacrinoid Larva

Pentacrinoid belongs to the animals of class Crinoidea. It is known by this name because of its similarity with Pentacrinites.

Organisms like feather stars have to undergo this stalked stage for a few months before developing into mature adults.
This larva is strictly sedentary and thus remains attached to the substratum.
The attachment occurs via an attachment plate or adhesive disc.
It comprises 8 to 10 ciliated tentacles that help in capturing the food. There is a cilia tuft over the larval body’s anterior portion.
It comprises two main cavities – Hydrocoel and Somatocoel.
The anus is absent, and the mouth arises from Enterone.
There is a sac-like structure called a vestibule between the first and the third ciliated band.
The upper portion of the vestibule is quite hard due to the presence of inter-radial spicules known as Deltoids.
The Hydrocoel is present as five-lobed rings in between the vestibule and enterone.

Significance of Echinodermata Larval Stage

The larval stage helps us determine the position of echinoderms in evolutionary ancestry. In addition to that, the bilateral symmetry of the larva keeps Echinodermata associated with the other animals of the animal kingdom. Since the matured echinoderms can’t be compared with the group of other adult organisms due to a high frequency of variation; thus, the larval forms become very significant when it comes to comparison purposes.