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Invertebrates · 1 species guide

Cnidarians: Corals, Jellyfish & Anemones

Cnidarians are aquatic animals with stinging cells (cnidocytes), including corals, jellyfish, sea anemones and hydroids; they have radial symmetry and a simple body plan, and reef-building corals create habitat for a large share of marine fish species.

Stinging cells
Cnidocytes containing nematocysts are unique to cnidarians — the reason jellyfish and coral both sting.
Radial symmetry
Body parts radiate from a central axis; bilaterian animals have left–right symmetry instead.
Coral polyps
Reef corals are colonies of tiny polyps with symbiotic algae (zooxanthellae) that supply energy in clear water.
Medusa versus polyp
Jellyfish are typically free-swimming medusae; anemones and corals are polyps attached to a surface.

Cnidarians invented the sting. Nematocysts fire venom into prey or threats; the same cells build reefs when coral polyps secrete calcium carbonate. Coral versus anemone, jellyfish versus comb jelly, and bleaching versus disease are the distinctions that decide whether a page answers the query people typed.

Reef-building corals create habitat for a large share of marine fish species in shallow tropical seas. Heat stress drives mass bleaching when polyps expel symbiotic algae. Comb jellies lack cnidocytes and are not cnidarians. Cutting carbon emissions and local pollution are the levers that match the biology. This type page defines the phylum; the coral guide is the main species entry on this site today. WARN currently centres the coral guide as the main cnidarian species entry, with bleaching and reef-fishery context. Readers asking about jellyfish should still start here for the phylum definition, then use newsroom and answer pages for bloom and sting questions until more medusa guides exist.

Key facts

  • Cnidarians include corals, jellyfish and anemones.
  • Stinging cells (nematocysts) define the phylum.
  • Reef-building corals create habitat for countless other species.
  • Coral bleaching is the expulsion of symbiotic algae under heat stress.
  • Comb jellies are not cnidarians — they lack stinging cells.
  • Ocean warming is the primary global driver of mass bleaching events.

How to recognise them

The features that separate this group in the field, in the order they are usually most useful.

Stinging cells
Cnidocytes containing nematocysts are unique to cnidarians — the reason jellyfish and coral both sting.
Radial symmetry
Body parts radiate from a central axis; bilaterian animals have left–right symmetry instead.
Coral polyps
Reef corals are colonies of tiny polyps with symbiotic algae (zooxanthellae) that supply energy in clear water.
Medusa versus polyp
Jellyfish are typically free-swimming medusae; anemones and corals are polyps attached to a surface.
The confusion pairs
Jellyfish versus comb jelly: comb jellies lack cnidocytes and use cilia. Hard coral versus soft coral: skeleton present versus flexible colonies.

1 cnidarians guide

Search or filter by conservation status. Every guide carries sourced facts, IUCN status and FAQs.

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Animal group
Colorful coral colony with many tiny polyps on a sunlit tropical reef
Least Concern

Other animals

C

Coral

Anthozoa

The tiny reef-building animals of tropical seas

Oceans

Open guide

Conservation: what this group faces

Cnidarians that build reefs are foundation species. When bleaching kills colonies, fish and invertebrates lose structure in a single season. Cutting carbon emissions and local pollution are the levers that match the biology.

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Cnidarians: frequently asked questions

What is a cnidarian?
An aquatic animal with stinging cells, including corals, jellyfish, sea anemones and hydroids. They typically show radial symmetry.
Are corals animals?
Yes. Reef corals are colonies of animal polyps that harbour symbiotic algae and build calcium carbonate skeletons.
What causes coral bleaching?
Heat stress and other pressures cause polyps to expel their symbiotic algae, turning colonies white and starving them if stress continues.
Is a jellyfish a fish?
No. Jellyfish are cnidarians — invertebrates. They are not vertebrates and not fish.
Do all cnidarians look like jellyfish?
No. Anemones and corals are attached polyps; jellyfish are usually free-swimming medusae. Both share stinging cells.
Why do reefs matter?
Coral reefs support a large share of marine fish species in shallow tropical seas and buffer coasts — losses cascade through fisheries and tourism economies.

Other types of invertebrates

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