Starfish (sea stars) are marine invertebrates defined by their unique five-fold radial symmetry, known as pentaradial, where body parts are arranged around a central disk. The central mystery of their origin is how this five-part body plan arose from an ancestor that was organized with bilateral symmetry. Unraveling this transition requires looking back hundreds of millions of years to a time when life in the oceans was first diversifying.
Placing Starfish on the Evolutionary Tree
Starfish belong to a major group of marine life called echinoderms, which also includes sea urchins, sea cucumbers, and brittle stars. This group shares an evolutionary lineage with chordates, the group that contains all vertebrates, including fish, birds, and humans.
The shared ancestry is evidenced by similar patterns of early development in the embryo. During this stage, the developing embryo’s first opening becomes the anus, with the mouth forming later. This characteristic trait defines the superphylum known as Deuterostomia. This places echinoderms on a branch separate from most invertebrates, such as mollusks and insects.
The Puzzle of Bilateral Ancestry
Evidence strongly suggests that the earliest ancestors of all echinoderms, dating back to the Cambrian period around 500 million years ago, possessed bilateral symmetry. Fossils like Atlascystis acantha, an ancient relative found in Morocco, show a flattened, spine-covered body with a clear left and right side. This extinct species lacked the radical body plan twist seen in modern forms, suggesting it was bilaterally symmetrical throughout its entire life.
In a remarkable echo of this deep history, the larval stage of a modern starfish, such as the bipinnaria larva, is still bilaterally symmetrical and free-swimming. During metamorphosis, this larva undergoes a dramatic reorganization, where the body axis is essentially twisted. The left side grows disproportionately to form the main body of the adult, while the former right side is largely absorbed, resulting in the final radial organization.
This switch to pentaradial symmetry was likely driven by the adoption of a sessile or slow-moving, bottom-dwelling existence. A radial body plan offers advantages for sensing the environment and feeding equally from all directions on the seafloor. The evolutionary mechanism involved a duplication event of a primitive ambulacral, or arm-like, structure, transforming a two-part system into the characteristic five-part arrangement seen today.
The Development of the Modern Starfish Body Plan
The first Asteroidea, the class containing modern starfish, began appearing in the fossil record during the Ordovician period, approximately 450 million years ago. These early forms already possessed two defining structural elements that distinguish them and allowed for their subsequent success.
One feature is the endoskeleton, which is composed of numerous tiny, interlocking calcium carbonate plates called ossicles. These ossicles provided both structure and defense, forming the basis of the “spiny skin” that gives the phylum its name.
The second defining feature is the evolution of the water vascular system, a unique adaptation not found in other major animal groups. This intricate hydraulic network functions in locomotion, respiration, and feeding, allowing the starfish to move across surfaces and pry open the shells of prey.
Water enters the system through a perforated plate on the upper surface called the madreporite, flowing into a central ring canal and then out through five radial canals that run down each arm. This pressurized fluid then powers the hundreds of tiny tube feet lining the underside of the arms. This successful body plan allowed the Asteroidea to diversify into the nearly 2,000 species that inhabit all the world’s oceans today.