Grades 6–8 · Organisms
How do tiny coral animals build a reef?
A stony coral colony is made of many small animals called polyps. Each polyp has a soft body and tentacles, but it can build a hard calcium carbonate skeleton beneath itself. The skeleton supports and protects the living tissue. A coral reef is therefore partly a structure made by living animals, rather than a pile of underwater plants.
In the diagramLiving coral polyps add a mineral skeletonMany reef-building corals also receive food from partner algaeEarlier skeleton staysSunlightAlgae make foodPolyps build belowNew mineral layers form beneath living tissue.
A polyp adds new skeletal material as it grows. Older layers can remain below while living tissue stays near the surface. Polyps can also multiply by budding or splitting, expanding a colony across its shared skeleton. Over many generations, the remains of reef-building organisms can form a large framework with spaces used by other animals.
Many shallow-water reef corals have tiny partner algae living inside their tissues. The algae use sunlight in photosynthesis and supply some of the coral’s food. The coral also captures small food particles. This partnership helps support the energy needed for living and building, but the algae are not the animals that make the main coral skeleton.
Adding skeleton is only one side of reef growth. Breakage, erosion and organisms that wear away material can remove it. A reef gains structure when accumulation exceeds those losses. Species and surroundings affect the balance, so the picture’s extra layers show a process without assigning a growth rate or promising that every reef is getting larger.
About this explanation
Starting knowledge: Distinguish animals from plants and understand photosynthesis.
Sources
- NOAA Ocean Service — Corals Tutorial: How do corals grow?
- NOAA Flower Garden Banks National Marine Sanctuary — Coral Basics
- NOAA Ocean Acidification Program — Low net carbonate accretion characterizes Florida’s coral reef