Overview

Echinoderms are a group of exclusively marine invertebrates found throughout the world’s oceans. Familiar examples include sea stars, brittle stars, sea urchins and sea cucumbers. They form a distinct phylum of animals whose members share a set of anatomical and developmental traits that set them apart from other invertebrates.

Distinctive characteristics

  • Calcified internal skeleton: Most echinoderms have a supporting framework of plates or ossicles made of calcite, a mineral composed of calcium carbonate. These plates may be joined to form rigid tests (as in sea urchins) or remain more flexible (as in sea stars).
  • Radial symmetry: Adult echinoderms commonly show a five‑branched or pentameral symmetry, arranged around a central axis. Many species, however, exhibit variations from this pattern.
  • Water‑vascular system and tube feet: A network of internal canals and podia filled with seawater powers the tube feet used for locomotion, attachment, feeding and gas exchange. Tube feet also assist in respiration in several groups.
  • Marine salinity tolerance: Echinoderms are generally stenohaline, meaning they tolerate only small changes in salinity, and as a result are strictly marine organisms.

Ecology, feeding and behaviour

Echinoderms occupy a wide range of benthic habitats from shallow intertidal zones to the deep sea. Some are suspension or filter feeders, others graze on algae, and several groups, notably many sea stars, are important predators of bivalves and other shelled animals such as molluscs. Many species are abundant in nearshore environments and can dominate hard substrates on the shore or on coral reefs, where they influence community structure through grazing and bioturbation.

Fossil record and classification

Echinoderms have a long and well‑preserved fossil history that extends back to the early Cambrian period. Fossils show a diversity of forms and have helped trace the evolutionary development of their unique body plans. Modern counts suggest several thousand living species and many more extinct taxa; approximate figures are cited in many sources as around 7,000 living and 13,000 extinct species. Taxonomists divide the phylum into several major groups; some treatments call these groups subphyla while others rank them as classes, reflecting ongoing revisions in classification.

Human relevance and notable facts

People use some echinoderms as food (for example, certain sea urchin gonads) and others as indicator organisms in environmental monitoring. Their distinctive developmental biology and regenerative abilities make them valuable in scientific research. Because the phylum is the largest composed entirely of marine species, no echinoderm naturally inhabits freshwater or terrestrial environments—a fact linked to their narrow tolerance for changing salinity and often emphasized in general accounts of echinoderm biology.

Echinoderms combine an unusual morphology and ecological importance: their calcitic skeletons contribute to the fossil record, their tube feet shape how they interact with habitat and prey, and their presence is a key part of many marine ecosystems. For introductions or field guides consult general marine biology resources and specialized taxonomic works linked above.