The phylum Echinodermata, which includes animals like sea stars and sea urchins, is defined by the radial symmetry of its adult members. This body plan, however, is not present for their entire lives. Echinoderms begin life as bilaterally symmetrical larvae that swim freely in the ocean. This early life stage is a distinct phase that highlights a developmental transition to the bottom-dwelling adult form.
Diverse Larval Forms
The classes within Echinodermata feature a variety of distinct larval forms. Sea stars (Asteroidea) have two larval stages. The first is the bipinnaria larva, which develops into a brachiolaria larva characterized by three additional arms tipped with adhesive cells. It uses these arms to explore and attach to the seabed.
Sea urchins (Echinoidea) and brittle stars (Ophiuroidea) develop a pluteus larva. This form is recognized by its long arms, which are supported by an internal skeleton of calcite rods, giving it a shape compared to an easel. While both groups have a pluteus larva, the ophiopluteus of brittle stars has longer arms than the echinopluteus of sea urchins.
Sea cucumbers (Holothuroidea) produce an auricularia larva, which has a single, winding ciliated band similar to the bipinnaria of sea stars. This feeding larva later transforms into a non-feeding, barrel-shaped doliolaria, which has several rings of cilia. This stage precedes the final settlement and transformation into a juvenile sea cucumber.
The Developmental Journey from Egg to Larva
The life of an echinoderm begins with external fertilization, where eggs and sperm are released into the open water. Once a zygote is formed, it undergoes rapid cell division known as cleavage. These divisions are holoblastic, meaning the entire egg divides, and radial, which is a characteristic of deuterostomes.
This cell division leads to the formation of a blastula, a hollow sphere of cells. The next step is gastrulation, where cells migrate inward to form distinct layers and establish the rudimentary gut. The culmination of these early embryonic events is the hatching of a free-swimming larva.
Life in the Plankton
Most echinoderm larvae exist as plankton, organisms that drift with ocean currents. This planktonic phase is a method for species dispersal, allowing them to colonize wide geographic areas. The majority of these larvae are planktotrophic, meaning they feed on smaller plankton like phytoplankton. They capture food particles using bands of cilia, which create water currents that direct food towards their mouths.
A smaller number of species have lecithotrophic larvae, which do not feed and instead survive on the yolk provided in the egg; these larvae consequently have shorter development periods. As part of the zooplankton, echinoderm larvae also serve as a source of food for larger planktonic predators. Their survival depends on finding enough food while avoiding being eaten.
The Radical Transformation of Metamorphosis
The transition from a free-swimming larva to a bottom-dwelling adult is a process known as metamorphosis. This change begins when the larva is fully developed and receives specific environmental signals to settle. These settlement cues are often chemical, released by bacteria, algae, or even adult echinoderms, indicating a suitable habitat.
Once a location is chosen, the larva undergoes a rapid reorganization of its body. The bilaterally symmetrical larval structures are largely absorbed or discarded. A new body with the pentaradial symmetry of an adult develops from a small group of cells within the larva called the rudiment.
During this transformation, the left side of the larva develops into the adult’s oral surface (the side with the mouth), while the right side becomes the aboral surface. From this process emerges a juvenile echinoderm, a miniature version of the adult. This tiny juvenile will then grow into the familiar sea star, sea urchin, or other echinoderm.