Coral Biology: What These Animals Really Are
Corals are animals, algae and living rock all at once. Understand polyps, coral types and how corals grow so you can keep them well.
We’ve circled coral biology in every lesson so far: the algae partnership, the mineral skeleton, the need for flow and light. Now we put it together. What is a coral? Understanding the answer makes their care intuitive instead of a list of rules to memorize.
Why this matters for your tank
Corals don’t behave like fish or plants, and treating them like either leads to mistakes. Once you understand that a coral is simultaneously an animal, a farm for algae, and a builder of stone, every part of its care (light, flow, chemistry, feeding) clicks into place as a consequence of its biology.
Core concepts
The polyp is the animal
The basic unit of a coral is the polyp: a tiny animal, essentially a sac with a mouth surrounded by tentacles. It’s closely related to anemones and jellyfish. What we call “a coral” is usually a colony: hundreds or thousands of polyps sharing connected tissue and a common skeleton. When you buy a coral “frag,” you’re buying a small piece of colony that will grow by adding new polyps.
Three ways corals make a living
A coral gets energy from two directions at once:
- Photosynthesis via its zooxanthellae (Lessons 1 and 6), usually the majority of its food.
- Feeding: polyps can capture plankton and absorb dissolved nutrients from the water (the subject of Lesson 8).
And stony corals do a third thing: calcify, pulling calcium and carbonate from the water to build skeleton (the reason alkalinity and calcium matter, from Lesson 2). Light feeds it, water flow delivers particles and minerals, chemistry supplies building blocks. The coral sits at the intersection of everything you’ve learned.
The three families you’ll meet
Reef corals are commonly grouped into three practical categories:
| Type | Skeleton | Difficulty | Examples |
|---|---|---|---|
| Soft corals | None | Easiest | Mushrooms, zoanthids, leathers, GSP |
| LPS (large-polyp stony) | Yes | Moderate | Hammer, torch, frogspawn, candy cane |
| SPS (small-polyp stony) | Yes | Hardest | Acropora, Montipora, birdsnest |
The order of that table is your roadmap. Start with soft corals and hardy LPS. They tolerate the swings a new tank inevitably has while you dial things in. SPS reward a mature, stable system. Chase them once your tank has proven it can hold steady (Lesson 10).
Corals compete and can sting
Corals are animals fighting for space and light on a crowded reef. Many have sweeper tentacles or release chemicals to attack neighbors. This is why you give corals room between them and don’t crowd a fresh aquascape: two corals touching can mean one melting the other overnight.
In your tank
- Begin with beginner corals. Softies and easy LPS first; earn your way to SPS.
- Handle frags gently and acclimate them to your light and flow (Lessons 5 and 6).
- Give corals space to grow. Leave gaps; they’ll fill in, and neighbors that touch may wage chemical warfare.
- Read your corals daily. Extended polyps and good color mean happy; retracted, receding tissue or bleaching means something in the environment needs attention.
You know what corals are and how they feed themselves through light. But even a well-lit coral benefits from a real meal. Next: coral nutrition, how and whether to actively feed your reef.
Key terms
- Polyp
- The individual coral animal: a small sac with a mouth ringed by tentacles. A coral 'colony' is many genetically identical polyps.
- Coral colony
- A group of connected polyps sharing tissue, functioning as one organism and building a shared skeleton.
- Soft coral
- Corals without a hard skeleton (e.g. mushrooms, leathers, zoas). Generally hardy and forgiving, great for beginners.
- LPS / SPS
- Large-Polyp Stony and Small-Polyp Stony corals: hard-skeleton corals. LPS are often beginner-friendly; SPS are more demanding.
- Calcification
- The process by which stony corals pull calcium and carbonate from the water to build their limestone skeleton.