Starfish (sea stars) are classified as deuterostomes, a major division of the animal kingdom. This classification is based not on their familiar adult shape, but on a specific sequence of events during their embryonic development. The term “deuterostome” translates from Greek to “second mouth,” hinting at the defining characteristic of this grouping. Starfish belong to the phylum Echinodermata, which shares this developmental pattern with the phylum Chordata, the group that includes all vertebrates.
What Defines a Deuterostome?
The classification of an animal as a deuterostome relies on three criteria observed during the earliest stages of life.
The most notable characteristic involves the fate of the blastopore, the first opening that forms during gastrulation. For deuterostomes, this opening ultimately develops into the anus, with the mouth forming secondarily.
A second defining feature is the cleavage pattern, referring to how the fertilized egg divides into smaller cells called blastomeres. Deuterostomes exhibit radial cleavage, where cell divisions stack the cells directly. This pattern is indeterminate, meaning that if early cells are separated, each retains the potential to develop into a complete organism.
The final distinction is how the coelom, or main body cavity, forms. Deuterostomes use enterocoelous formation, where the coelom arises from outpocketings of the archenteron (the primitive gut). These pouches pinch off and expand to form the body cavity, contrasting with the schizocoelous formation seen in most protostomes.
Tracing Starfish Embryonic Development
The developmental journey of a starfish embryo provides clear evidence for its placement within the deuterostome lineage. Following fertilization, the zygote begins a series of cell divisions that demonstrate the radial cleavage pattern. The resulting blastomeres stack in a regular arrangement, confirming the indeterminate nature of their early development.
Gastrulation involves the invagination of cells to form the archenteron, the precursor to the gut. The opening created by this infolding is the blastopore, which is destined to become the animal’s anus. The digestive tract subsequently tunnels through the embryo until a second opening forms, which becomes the mouth of the larval sea star.
As the primitive gut extends inward, the coelom begins formation through the enterocoelous method. Pouches of mesoderm bud off from the archenteron wall and expand to form the coelomic cavities. This specific sequence—radial cleavage, blastopore-to-anus formation, and enterocoely—shows that starfish development aligns with the deuterostome definition.
Echinoderms and the Evolutionary Tree
The phylum Echinodermata, which includes starfish, sea urchins, and sea cucumbers, represents one of the main phyla within the superphylum Deuterostomia. This grouping places them in a distinct evolutionary branch from the Protostomes, a collection of invertebrates including insects, mollusks, and worms. Shared developmental traits indicate that all deuterostomes descended from a common ancestor.
The deuterostome trait means starfish are evolutionarily closer to the phylum Chordata (which contains fish, birds, and humans) than they are to a snail or an earthworm. This relationship is not obvious from the adult forms, as starfish exhibit pentaradial symmetry, while chordates are bilaterally symmetrical. However, the starfish larva is bilaterally symmetrical, suggesting the adult’s radial body plan is a secondary, modified trait.
This shared developmental history is a tool for evolutionary biologists tracing the animal family tree. Molecular and genetic studies reinforce the developmental evidence, showing a close phylogenetic connection between echinoderms and chordates. The fact that a sea star shares a more recent common ancestor with vertebrates than with most other invertebrates highlights the significance of early embryonic development in classification.