Coral

Staghorn coral (Peter Leahy / shutterstock.com)

Corals (anthozoa) are cnidarians without free-swimming life stage. They are typically living in colonies of myriads of identical polyps. Most corals obtain nutrients from symbiotic photosynthetic algae known as zooxanthellae. Skin contact with fire corals (technically no true corals) causes severe pain that can last for several days. Full neoprene or Lycra suits provide adequate protection.

skull and crossbones
Some species venomous.

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This supplemental online resource accompanies the book Scuba Diving Hand Signals by Lars Behnke, available on Amazon.