The animal commonly known as the starfish is a fascinating marine creature whose name creates a biological misunderstanding. Despite the “fish” suffix, this organism is not a fish and belongs to a completely different branch of the tree of life. Scientists prefer the more accurate common name “sea star” to avoid confusion, recognizing it as a marine invertebrate. Its classification is based on a unique collection of specialized anatomical features. Understanding its true biological identity requires looking beyond its common name to its formal classification.
Correct Biological Classification
The sea star belongs to the Phylum Echinodermata, a major grouping of exclusively marine invertebrates that includes sea urchins, sea cucumbers, and brittle stars. This Phylum name is derived from the Greek words echinos (spiny) and derma (skin).
Starfish are further categorized into the Class Asteroidea, which is exclusive to the approximately 1,900 known living species of sea stars. The name Asteroidea comes from the Greek word aster (star), describing their characteristic shape. This Class is distinguished from other echinoderms by having arms broadly connected to a central body disc, unlike brittle stars. This formal scientific placement clarifies that the sea star is an invertebrate, meaning it lacks a backbone.
Defining Features of Echinoderms
The placement of sea stars within the Phylum Echinodermata is justified by three distinct anatomical trademarks. First, adult sea stars exhibit pentaradial symmetry, meaning their bodies are arranged in five equal parts radiating from a central point, like spokes on a wheel. This is a sharp contrast to the bilateral symmetry seen in humans and fish, where the body can be divided into two mirror-image halves. Although the adult form is radial, the larval stage of the sea star is bilaterally symmetrical.
Another defining feature is the water vascular system, a network of fluid-filled canals that operates like a hydraulic system. Water enters the system through a porous plate on the upper surface called the madreporite and travels through canals to ampullae. The ampullae contract to extend or retract the numerous tube feet, which are used for locomotion, gripping surfaces, and prying open prey shells. This internal pressure system is a primary mechanism for movement, feeding, and gas exchange.
Finally, all members of this Phylum possess an endoskeleton made of plates called calcareous ossicles. These ossicles are composed of calcium carbonate and are embedded beneath the skin, giving the animal its firm texture and protection. The ossicles often have external projections, which are the spines that give the phylum its “spiny skin” name. This unique skeletal structure provides support and is fundamentally different from the bone or cartilage structures found in vertebrates.
Why They Are Not Fish
The common name “starfish” is misleading because the animal lacks the defining characteristics of a true fish. Fish belong to the Phylum Chordata and Subphylum Vertebrata, meaning they possess a backbone, whereas sea stars are invertebrates with no internal spinal column.
The biological differences between sea stars and fish are significant:
- Skeletal structure: Fish possess an endoskeleton of bone or cartilage, while sea stars have interlocking calcareous ossicles.
- Respiration: Fish use gills to extract oxygen, but sea stars rely on small, thin-walled projections on their skin (dermal branchiae or papulae) and their tube feet for gas exchange.
- Nervous system: Fish have a centralized nervous system with a brain and spinal cord, while sea stars operate with a simpler nerve ring surrounding the mouth and radial nerves.
- Movement: Fish swim using fins and a tail, while sea stars slowly crawl across the seabed using the coordinated action of their tube feet.