What Is Coral? A Beginner's Guide to Reef-Building Animals

Q: Is coral an animal, a plant, or a rock?

Coral is an animal. Specifically, it belongs to the phylum Cnidaria, the same broad group that includes jellyfish, sea anemones, and hydroids — and a coral colony is actually made up of many small, anemone-like individuals called polyps living together. The confusion with 'plant' usually comes from the zooxanthellae (symbiotic algae) that live inside coral tissue and give many corals their color and a large part of their energy — but the algae are a separate organism living in partnership with the coral animal, not the coral itself. The confusion with 'rock' comes from the skeleton that many corals build: a hard, calcium carbonate structure that the living animal sits on top of and continually adds to. A piece of dead coral skeleton — like the kind discussed in our brain coral skeleton guide — really is just rock at that point, but a living coral is the animal tissue covering that structure, not the structure itself.

Q: What's the difference between a coral polyp and a coral colony?

A polyp is the basic individual living unit — a small, soft-bodied organism with a central mouth surrounded by tentacles, structurally similar to a tiny sea anemone (and indeed, some corals are essentially solitary polyps that never form colonies). A colony is what you get when many genetically identical polyps, produced by the original polyp budding and dividing over time, remain physically connected and function as a single coral. Most of the corals kept in reef tanks — hammer, chalice, and most others — are colonial: what looks like 'one coral' on a frag plug is actually a group of connected polyps sharing tissue, and often sharing resources, with the colony as a whole. This is part of why corals can be fragmented ('fragged') into multiple pieces that each grow into new colonies — you're not damaging a single organism's body in the way you would with, say, a fish, but separating a colonial organism into smaller colonial pieces.

By Hektor Jorgo Published May 14, 2024 Updated June 14, 2026 5 min read

Close-up of a coral colony showing individual polyps extended over its skeleton

Quick Facts

What Coral Is: An animal — a colony of tiny individual organisms called polyps, related to anemones and jellyfish (phylum Cnidaria)
Animal, Not Plant or Rock: The hard, rock-like material people often think of as 'coral' is a skeleton the living animal builds and grows on top of
Polyps: The basic living unit of a coral colony — small, anemone-like individuals, each with a mouth and a ring of tentacles
Zooxanthellae: Symbiotic algae living inside coral tissue that provide energy via photosynthesis — the same partnership described in our anemone health guides
Hard (Stony) vs. Soft Corals: Hard corals build a rigid calcium carbonate skeleton; soft corals lack a rigid skeleton in the same sense, relying more on a flexible internal structure
LPS vs. SPS: Among stony corals, 'large-polyp stony' and 'small-polyp stony' describe relative polyp size — not a strict difficulty rating, as covered in our LPS beginner guide
How Reefs Form: Over long timescales, the accumulated skeletons of stony coral colonies — living and dead — build up the physical structure of a coral reef
Typical Habitat: Most reef-building corals live in warm, shallow, clear tropical water where sunlight can reach their zooxanthellae

"Is that thing a plant, an animal, or a rock?" is one of the first questions a lot of people have the first time they look closely at a reef tank — and the honest answer is "animal," even when it doesn't look or act like one.

Short Answer

Coral is an animal — specifically, a colony of small, soft-bodied individuals called polyps, related to sea anemones and jellyfish. Many corals build a hard calcium carbonate skeleton (the "rock" part of the popular image), and many also host symbiotic algae called zooxanthellae inside their tissue (the "plant" part of the confusion), which provide a large share of the coral's energy through photosynthesis. Understanding these three pieces — animal, skeleton, and algal partner — explains most of what's confusing about coral at first glance, and sets up nearly everything else covered in our coral and reef tank guides.

Coral Is an Animal, Not a Plant or a Rock

Corals belong to the phylum Cnidaria — the same broad group as jellyfish, sea anemones, and hydroids (the kind of hitchhiker covered in our hydroid identification guide). What looks like a single coral "thing" on a piece of rock is, in most cases, a colony made up of many individual polyps, each one structurally similar to a tiny anemone: a central mouth, a body column, and a ring of tentacles used for capturing food and (in many species) for stinging.

The two things that most often cause confusion:

The skeleton — many corals build a hard, calcium carbonate structure (the same general material discussed in our brain coral skeleton guide), and the living animal tissue covers this structure and continually adds to it. A bare skeleton, with no living tissue, really is just "rock" at that point — but a living coral is the animal on top of that structure, not the structure itself.
The algae — many corals host zooxanthellae, photosynthetic algae living inside their tissue, which contribute both energy and color. The algae are a separate organism in partnership with the coral, not the coral itself.

The Polyp: Coral's Basic Living Unit

A polyp is the fundamental living unit of a coral — a small, anemone-like individual with its own mouth and tentacles. Some corals are solitary, consisting of a single large polyp, but most of the corals commonly kept in reef tanks are colonial: many genetically identical polyps, produced as the original polyp grew and divided (budded) over time, remain physically connected and function together as one colony.

This colonial structure is part of why corals can be fragmented ("fragged") — splitting a colony into multiple pieces, each of which can grow into a new, independent colony, is a normal part of how corals are propagated in the hobby (and, in many species, how they spread in the wild too). Our guide to getting started with coral frags covers what this looks like in practice for someone adding their first frags to a tank.

Zooxanthellae: Coral's Solar-Powered Energy Source

Many corals — along with other reef animals, including the anemones covered in our anemone care guides — host zooxanthellae: microscopic, single-celled algae living inside their tissue in a mutually beneficial relationship. The algae get a protected home and access to the coral's metabolic waste products as nutrients; in exchange, they photosynthesize and pass a significant share of the resulting energy on to the coral.

This partnership has two big practical implications:

Light matters. Reef-building corals are generally restricted to shallow, clear, sunlit water because their zooxanthellae need light to photosynthesize. This is the underlying reason lighting comes up so often in coral care guides, including our LPS lighting notes.
Color is tied to the algae (and to health). Zooxanthellae density and type contribute heavily to a coral's color, and a coral that loses too many of its zooxanthellae — a process called bleaching, discussed for anemones in our guide on zooxanthellae expulsion — can pale dramatically and lose a major energy source, even while the coral animal itself may still be alive.

Hard Corals, Soft Corals, and Where LPS/SPS Fit

The hobby splits corals into a few broad structural categories:

Hard (stony) corals build a rigid calcium carbonate skeleton — the kind of structure discussed in our brain coral skeleton guide — with living tissue covering and continually adding to it as the colony grows.
Soft corals lack a rigid skeleton in this sense. Many have small calcium carbonate structures called sclerites embedded in their tissue for support, but nothing resembling the solid, reef-building skeleton of a stony coral — which is part of why soft corals often look and move in a more flexible, leathery way.
Among stony corals, the hobby further divides things into LPS (large-polyp stony) and SPS (small-polyp stony) based on relative polyp size. As our LPS beginner guide covers, this is a structural description, not a difficulty rating — some LPS genera are notably easier than others, and the same is true within SPS.

How Coral Reefs Are Actually Built

Over long timescales, the accumulated skeletons of stony coral colonies — both living and dead — build up the physical structure of a coral reef. Living coral covers the surface of the reef, but underneath is layer upon layer of skeleton laid down by previous generations of coral, cemented together with the skeletal material of other reef organisms. This is also why coral skeletons found outside the water (on a beach, for example) often raise questions about collection — a topic our brain coral skeleton guide touches on, since that skeletal material can still play an ecological role even after the living coral is gone.

Quick Reference

Coral is an animal (phylum Cnidaria), related to anemones and jellyfish — not a plant or a rock
A coral colony is typically made up of many small individual polyps, each with a mouth and tentacles
Most corals can be fragmented into pieces that grow into new colonies
Many corals host zooxanthellae — symbiotic algae providing energy via photosynthesis and contributing to color
Hard (stony) corals build a rigid calcium carbonate skeleton; soft corals don't, in the same sense
LPS vs. SPS describes relative polyp size among stony corals, not a difficulty tier
Coral reefs are built up over time from the accumulated skeletons of stony coral colonies

Frequently Asked Questions

Is coral an animal, a plant, or a rock?

Coral is an animal. Specifically, it belongs to the phylum Cnidaria, the same broad group that includes jellyfish, sea anemones, and hydroids — and a coral colony is actually made up of many small, anemone-like individuals called polyps living together. The confusion with 'plant' usually comes from the zooxanthellae (symbiotic algae) that live inside coral tissue and give many corals their color and a large part of their energy — but the algae are a separate organism living in partnership with the coral animal, not the coral itself. The confusion with 'rock' comes from the skeleton that many corals build: a hard, calcium carbonate structure that the living animal sits on top of and continually adds to. A piece of dead coral skeleton — like the kind discussed in our brain coral skeleton guide — really is just rock at that point, but a living coral is the animal tissue covering that structure, not the structure itself.

What's the difference between a coral polyp and a coral colony?

A polyp is the basic individual living unit — a small, soft-bodied organism with a central mouth surrounded by tentacles, structurally similar to a tiny sea anemone (and indeed, some corals are essentially solitary polyps that never form colonies). A colony is what you get when many genetically identical polyps, produced by the original polyp budding and dividing over time, remain physically connected and function as a single coral. Most of the corals kept in reef tanks — hammer, chalice, and most others — are colonial: what looks like 'one coral' on a frag plug is actually a group of connected polyps sharing tissue, and often sharing resources, with the colony as a whole. This is part of why corals can be fragmented ('fragged') into multiple pieces that each grow into new colonies — you're not damaging a single organism's body in the way you would with, say, a fish, but separating a colonial organism into smaller colonial pieces.

What is zooxanthellae and why does it matter for coral?

Zooxanthellae are microscopic, single-celled algae that live inside the tissue of many corals (and other reef animals, including the anemones covered in our anemone health guides) in a mutually beneficial relationship: the algae get a protected place to live and access to the coral's waste products as nutrients, and in return, they perform photosynthesis and share a significant portion of the resulting energy with the coral. This partnership is a major reason reef-building corals are generally restricted to shallow, sunlit water — the zooxanthellae need light to photosynthesize, and a coral that loses too many of its zooxanthellae (a process called bleaching, discussed in the context of anemones in our guide on zooxanthellae expulsion) can struggle for energy even if the coral animal itself is otherwise alive. Zooxanthellae are also a major contributor to a coral's color — pigments associated with the algae (and the coral's own pigments) combine to produce the wide range of coloration seen across the hobby, including the dramatic variation discussed in our chalice coral guide.

What's the difference between hard (stony) corals and soft corals?

Hard corals — also called stony corals — build a rigid calcium carbonate (aragonite) skeleton, the same basic material discussed in our brain coral skeleton guide, and the living tissue covers and continually adds to this skeleton as the colony grows. Soft corals lack a rigid skeleton in this sense — many have small calcium carbonate structures called sclerites embedded in their tissue for support, but nothing like the solid, reef-building skeleton of a stony coral, which is part of why soft corals are often described as more flexible or leathery in texture and movement. Among stony corals, the hobby further splits things into LPS (large-polyp stony) and SPS (small-polyp stony) based on relative polyp size — our LPS beginner guide covers this distinction in more depth, including why it's a structural description rather than a reliable difficulty rating on its own.

Sources & Further Reading

About the Author: Hektor Jorgo

Co-Founder & Marine Biologist

Hektor is a co-founder of Sea Life Planet and has kept reef and freshwater aquariums for over 15 years. He holds a background in marine biology and focuses on species care accuracy, water chemistry, and tank husbandry.