The common name “starfish” often leads to the misconception that these marine animals are a type of fish. However, despite their aquatic habitat, starfish are not fish. Their biological characteristics differ significantly from those that define true fish, placing them in an entirely separate group of marine invertebrates.
What Defines a Fish?
Fish are aquatic vertebrates, possessing a backbone. They breathe using gills, which extract oxygen from the surrounding water. Most fish also have fins, specialized appendages that aid in propulsion, steering, and maintaining balance within their aquatic environment. Their bodies are often covered in scales, providing protection, and they generally exhibit bilateral symmetry, meaning their left and right sides are mirror images. These combined features are fundamental to the biological definition of a fish.
Unpacking the Starfish Identity
Starfish, correctly referred to as sea stars, belong to the phylum Echinodermata, a group of marine invertebrates known for their “spiny skin.” Unlike fish, adult starfish display a unique body plan called pentaradial symmetry, where their body parts are arranged in five or multiples of five around a central disc. While their larval stages exhibit bilateral symmetry, they transform into this radial form during metamorphosis.
Starfish possess a distinctive water vascular system, a network of fluid-filled canals that power their numerous tube feet, which are small, active projections on their underside. These tube feet enable their movement, attachment to surfaces, and even assist in feeding. Starfish are exclusively marine, found in all the world’s oceans.
Why Starfish Are Not Fish
The fundamental differences between starfish and fish are apparent in their anatomy and physiology. Starfish lack gills, relying instead on their tube feet and specialized dermal branchiae, or “skin gills,” to absorb oxygen directly from seawater through diffusion. Unlike fish, which use fins for locomotion, starfish move slowly across the seafloor using the coordinated action of their tube feet.
A primary distinction is the absence of a backbone in starfish; they are invertebrates. Instead of a bony or cartilaginous vertebral column, their internal support comes from a skeleton of calcium carbonate plates called ossicles, embedded within their body wall. These ossicles form a porous, lattice-like structure, unlike the complex bone structures in fish.
Furthermore, starfish’s radial symmetry contrasts sharply with the bilateral symmetry characteristic of fish, meaning starfish do not have a distinct head or tail. This collection of biological disparities firmly establishes that starfish are not fish.