Which organism forms reefs by forming colonies?

Which organism forms reefs by forming colonies?

• A. Coral
• B. Earthworms
• C. Giant tube worms
• D. Deer ticks

Answer: (A)

The idea being tested is how reef structures are built by organisms that form colonies. Coral are colonial cnidarians whose individual polyps expand by budding to form a single genetically identical colony. Each polyp secretes calcium carbonate, building a hard skeleton, and as many polyps keep growing and accumulating, a limestone framework emerges—the reef. The mutualistic algae living in the coral tissues provide energy via photosynthesis, helping the colony grow in warm, sunlit waters. This combination of colony formation and calcium carbonate deposition explains why corals are reef builders. The other organisms don’t create reef-like, calcium carbonate structures: earthworms aerate soil but don’t form calcium carbonate reefs; giant tube worms form tubular colonies near vents but not reef frameworks; deer ticks are free-living parasitic arthropods with no reef-building role.