Echinoderms are a phylum of invertebrates found exclusively in marine environments. These creatures are defined by their unique features, most notably a five-point, or pentaradial, symmetry in their adult form, which arises from a bilaterally symmetrical larval stage. The phylum name, Echinodermata, translates from Greek as “spiny skin,” referencing the calcareous plates and ossicles that form their internal skeleton and often project outward as spines. Familiar examples include sea stars, sea urchins, and sea cucumbers, which are distributed across all the world’s oceans.
The Global Estimate of Living Echinoderm Species
The most recent scientific consensus places the count of living, described echinoderm species at approximately 7,600. This figure is a dynamic estimate, constantly being refined by ongoing marine research and taxonomic revisions. This number makes Echinodermata the largest phylum composed entirely of marine organisms. However, the count represents only the species currently known to science, and the actual global biodiversity is almost certainly higher, with new species regularly being discovered. This diversity is organized into five distinct, extant classes.
Defining the Five Major Classes
The phylum Echinodermata is categorized into five major classes, each representing a unique body plan and ecological role. The largest group is the Ophiuroidea, or brittle stars, which account for around 2,300 described species. Brittle stars are distinguished by their long, slender, and highly flexible arms that are sharply demarcated from a central body disc. These organisms are typically scavengers or detritivores, often found in high densities on the deep-sea floor.
Following closely are the Asteroidea, or sea stars, with nearly 2,000 species cataloged. Unlike brittle stars, sea stars possess thick arms that are broadly continuous with the central disk, and their internal organs extend into these arms. Most sea stars exhibit the classic five-armed structure, though species with up to 50 arms are known, and they primarily function as generalist predators.
The Holothuroidea, or sea cucumbers, comprise the next major class, with an estimated 1,430 to 1,700 species. They exhibit a highly elongated body shape, and while their five-fold symmetry is less apparent externally, it is present internally. They are unique for respiring by drawing water into an internal respiratory tree through their anus.
The Echinoidea class includes sea urchins, sand dollars, and sea biscuits, representing about 900 known species. These echinoderms lack arms entirely, instead possessing a solid, rigid internal shell called a test, which is covered in movable spines. The spines are used for protection and locomotion, and the group is characterized by a complex, jaw-like feeding apparatus known as Aristotle’s lantern.
The smallest class is the Crinoidea, which includes sea lilies and feather stars, with approximately 580 extant species. Crinoids are considered the most ancient group, characterized by a cup-shaped body, or calyx, and numerous feathery, branched arms used for suspension feeding.
The Ongoing Process of Discovery and Classification
The exact number of echinoderm species is not static because discovery and classification is a continuous scientific process. A significant portion of this ongoing work involves the exploration of deep-sea environments, which remain largely inaccessible and represent a major frontier for finding new life. For instance, new species of sea cucumbers and brittle stars are frequently collected from deep-water trawls, underscoring how little is known about the fauna in the ocean’s depths.
Another factor contributing to the fluidity of the species count is the increasing use of advanced genetic analysis in taxonomy. Molecular data often reveals “cryptic species,” which are groups of organisms that look identical morphologically but are distinct species based on their DNA. This genetic evidence frequently leads to the reclassification of a single, widely distributed species into multiple, geographically restricted ones, thus increasing the total number of recognized species. Taxonomic revision is constant, meaning the official count is continually being updated by marine biologists.