What Builds Coral Reefs? Tiny Animals Called Coral Polyps

Coral reefs are living geology—built by coral polyps, tiny (1–10 mm) marine invertebrates in the phylum Cnidaria (relatives of jellyfish, anemones).

The Architects: Hermatypic (Reef-Building) Corals

• Colonial – Hundreds to millions of genetically identical polyps.
• Calcium carbonate skeleton – Each polyp secretes a cup (corallite); fused = colony skeleton.
• Symbiosis – Host zooxanthellae (dinoflagellate algae) in tissues. Algae photosynthesize → provide 90% polyp energy (glucose, glycerol, amino acids). Polyp gives CO₂, nutrients, protection.

Life Cycle

1. Spawning – Mass synchronized release of eggs/sperm (often lunar-cued).
2. Planula larvae – Drift days/weeks; settle on hard substrate.
3. Primary polyp – Secretes basal plate; begins budding.
4. Colony growth – Vertical (upward) + lateral; rate: 0.5–10 cm/yr (species-dependent).
5. Death & accumulation – Dead skeletons form reef framework; new polyps grow atop.

Reef Ecosystem Services

• Biodiversity – 25% marine species on <0.1% ocean floor.
• Coastal protection – Dissipate 97% wave energy.
• Fisheries – 1B+ people depend on reef fish.
• Tourism – $36B/yr global value.
• Medicine – Compounds for cancer, HIV, pain, antibiotics.

Existential Threats

Bleaching (heat stress → zooxanthellae expulsion), acidification (reduced calcification), pollution, overfishing, destructive practices. 50% lost since 1950s; 90% at risk by 2050.

Reefs are animals building mountains—and we're dissolving their work in decades.