Echinoderms are a diverse group of marine animals, including starfish, sea urchins, and sea cucumbers. They inhabit various ocean environments, from shallow tide pools to the deep sea. These invertebrates possess distinctive body plans that contribute to their success in marine ecosystems.
Understanding Different Types of Body Symmetry
Body symmetry describes how an organism’s body parts are arranged around a central axis or plane. Asymmetry refers to a lack of symmetry, where there is no pattern or balance in body shape, such as in sponges. Radial symmetry allows an animal to be divided into similar halves by multiple planes passing through a central axis, like a pie. Jellyfish are a common example, with body parts arranged around a central point, enabling them to detect stimuli from all directions.
Bilateral symmetry, found in humans and insects, means an animal can be divided into two mirror-image halves along only one plane. This symmetry results in a distinct head and tail end, as well as a dorsal (back) and ventral (belly) side. Bilateral symmetry is associated with directional movement and the development of specialized sensory organs at the anterior end.
The Unique Symmetry of Adult Echinoderms
Adult echinoderms exhibit a distinctive form of radial symmetry known as pentaradial symmetry. Their body parts are arranged in fives, or multiples of five, around a central axis. Unlike simple radial symmetry, where any plane through the center creates mirror images, pentaradial symmetry specifically involves five radiating parts. For example, starfish have five arms extending from a central disc.
Sea urchins and sand dollars also display pentaradial symmetry, even without visible arms, as their plates and internal organs are arranged in a five-part pattern. Sea cucumbers, while appearing somewhat elongated and cylindrical, retain an underlying pentaradial organization. This five-part body plan differentiates adult echinoderms from other radially symmetrical animals.
Echinoderm Larvae: A Different Symmetry
Echinoderms exhibit a difference in symmetry between their larval and adult stages. While adults display pentaradial symmetry, echinoderm larvae are bilaterally symmetrical, resembling the larval forms of many other animal groups. For instance, the bipinnaria larva of a starfish is a free-swimming, bilaterally symmetrical organism with distinct anterior and posterior ends. Brittle star larvae, known as ophiopluteus larvae, are also bilaterally symmetrical and transparent, with long arms supported by calcareous rods.
This bilaterally symmetrical larva undergoes metamorphosis to transform into the radially symmetrical adult form. During this transformation, the larval body is reorganized, with the left side of the larva developing into the adult body, while the right side is absorbed. This shift in body plan throughout their life cycle is a defining characteristic of echinoderms.
The Evolutionary Significance of Dual Symmetry
The presence of different symmetries in echinoderm larvae and adults provides insights into their evolutionary history. Echinoderms are believed to have evolved from bilaterally symmetrical ancestors, a common feature among deuterostomes. The bilateral symmetry of their larvae supports this hypothesis, suggesting a retained ancestral trait. The free-swimming, bilaterally symmetrical larval stage is well-suited for dispersal and seeking out new habitats.
The evolution of pentaradial symmetry in adults is an adaptation to a sessile or slow-moving, bottom-dwelling lifestyle. This radial arrangement allows adult echinoderms to interact with their environment equally from all directions, which is advantageous for filter feeding or scavenging on the seafloor. While adult echinoderms exhibit pentaradial symmetry, some studies suggest they may retain some underlying bilateral tendencies, linked to their developmental origins.