Starfish are recognized for their unique shapes and vibrant colors. They are common in oceans worldwide, from shallow tide pools to abyssal depths. Despite their common name, their classification is frequently misunderstood. This article clarifies why they are not fish and details their proper scientific grouping and distinct biological attributes.
Why Starfish Are Not Fish
The common name “starfish” is misleading, as these animals do not possess the defining characteristics of fish. Fish, belonging to the phylum Chordata, are vertebrates with backbones, gills for respiration, fins for movement, and bodies typically covered in scales. Starfish, conversely, are invertebrates, meaning they lack a vertebral column. Their skeletal structure is an endoskeleton made of calcareous plates called ossicles, rather than the bony skeleton found in fish.
Starfish respire differently than fish. They do not have gills, instead utilizing structures like dermal branchiae (skin gills) and their tube feet for gas exchange with the surrounding seawater. Their method of locomotion also differs significantly; while fish use fins and muscular tails for propulsion, starfish move using a unique water vascular system and thousands of tiny tube feet. These fundamental biological distinctions highlight why starfish are not classified as fish.
Their Scientific Grouping
Starfish belong to the phylum Echinodermata and the class Asteroidea. The name “Echinodermata” is derived from Greek words, “echinos” meaning “spiny” and “derma” meaning “skin,” aptly describing the spiny or granular texture of many members of this phylum. This phylum also includes sea urchins, sand dollars, brittle stars, and sea cucumbers, all of which are exclusively marine invertebrates.
Within Echinodermata, starfish are precisely categorized under the class Asteroidea. The term “Asteroidea” similarly comes from Greek, with “aster” meaning “star” and “eidos referring to “shape,” reflecting their typical star-like body plan. Members of this class generally exhibit a flattened body with arms radiating from a central disc, a characteristic that further distinguishes them within the broader echinoderm group.
Unique Features of Starfish
Adult starfish typically display pentaradial symmetry, meaning their body parts are arranged in five equal sections around a central axis. While their larval stages exhibit bilateral symmetry, this five-part radial arrangement develops during metamorphosis and is a hallmark of most echinoderms.
A defining feature of starfish is their water vascular system, a hydraulic network of fluid-filled canals unique to echinoderms. Water enters this system through a sieve-like plate called the madreporite, located on the aboral (upper) surface, and circulates through a ring canal and radial canals extending into each arm. This system powers the numerous tube feet on the underside of their arms, which extend and retract through hydraulic pressure, facilitating locomotion, attachment to surfaces, feeding, and even respiration.
Their endoskeleton is composed of calcium carbonate plates known as ossicles, embedded within their body wall. These ossicles vary in form, from flat plates to granules and spines, providing rigidity and protection. Starfish also exhibit remarkable regenerative capabilities, often able to regrow lost arms, and in some species, even regenerate an entire body from a single arm and a portion of the central disc. This regenerative ability allows them to recover from injuries or evade predators by shedding a limb.