What are these spiky balls?

Sea urchins are spiny, globular marine animals belonging to the class Echinoidea. Here are some key points about them:

### Anatomy
- **Spines**: Their bodies are covered with sharp spines that provide protection from predators.
- **Test**: The hard shell, or "test," is made of calcium carbonate and has a unique pattern of holes and grooves.
- **Tube Feet**: Sea urchins have tube feet that help them move and cling to surfaces.

### Habitat
- Sea urchins are found in various marine environments, from shallow coastal waters to deep ocean floors.

### Diet
- They primarily feed on algae, making them important grazers in their ecosystems. Some species are also known to eat coral and detritus.

### Reproduction
- Sea urchins reproduce sexually through external fertilization. They release eggs and sperm into the water, where fertilization occurs.

### Ecological Role
- They play a crucial role in maintaining healthy marine ecosystems by controlling algal growth and serving as prey for various predators, including sea otters and some fish.

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