The common name “starfish” is misleading, suggesting a relationship that does not exist. Although the organism lives in the water, the correct term, “sea star,” better reflects its distinct biology. A sea star is definitively not a fish; it is classified in a completely different phylum. Understanding this distinction requires looking at its fundamental biological structure, not just its aquatic habitat.
Defining a Fish
To be classified as a fish, an aquatic organism must belong to the Superclass Pisces. A primary requirement is that the organism must be a vertebrate, possessing a spinal column or backbone made of cartilage or bone. This internal skeleton provides structural support and central nervous system protection. Fish also possess gills for extracting dissolved oxygen from the water. Finally, they are equipped with fins for propulsion, steering, and maintaining stability, allowing for swift, agile movement.
The True Identity of a Sea Star
The sea star belongs to the Phylum Echinodermata, which includes sea urchins and sea cucumbers. Echinoderms are exclusively marine invertebrates, and the sea star is specifically placed in the Class Asteroidea. A defining feature is their internal skeleton, or endoskeleton, composed of interlocking plates called ossicles. These calcium carbonate plates give the organism its rigid, spiny structure. While larvae exhibit bilateral symmetry, the adult form develops pentaradial symmetry, meaning body parts are arranged in multiples of five around a central disc.
Key Distinctions in Sea Star Anatomy
The most profound anatomical difference between a sea star and a fish lies in its distinct body symmetry and movement system. Unlike fish, which have bilateral symmetry, the adult sea star features pentaradial symmetry. This arrangement means the animal has no true front or back end, and its sensory and motor functions are distributed equally across its five or more arms.
The sea star’s unique method of movement and respiration is driven by a complex, pressurized structure called the water vascular system. This system operates like a hydraulic network, taking in filtered seawater through a small plate on its upper surface called the madreporite. The water then circulates through a ring canal and into radial canals that extend down each arm.
The system powers hundreds of small, flexible extensions known as tube feet, or podia, located in grooves on the underside of each arm. These feet operate using hydrostatic pressure; muscular sacs called ampullae contract, forcing water into the tube feet to extend them. The tube feet attach to surfaces via small suction cups and are used for slow locomotion, gripping, and prying open prey shells.
The tube feet also facilitate gas exchange, serving the respiratory function that gills perform in fish by allowing oxygen to diffuse directly into the body from the water. Furthermore, the sea star lacks a centralized brain, instead relying on a decentralized nervous system composed of a nerve ring and radial nerves. This simple network coordinates the movement of the tube feet, demonstrating a vastly different biological organization from that of any fish.