Of all the invertebrates on the Earth, the largest group is composed of arthropods. Worldwide, there are about 80,000 species of these organisms. Many of the arthropods of the coral reef are familiar to nearly everyone and include animals such as shrimp, crabs, and lobsters.
Most arthropods are relatively small animals whose bodies are covered with hard, protective coats called exoskeletons. Exoskeletons give the bodies of animals structural support and protect them from predators. The tough skeletons of arthropods are primarily composed of chitin, an extremely hard, but highly flexible, material made of long chains of molecules that are similar in structure to cellulose.
Like the bodies of bristle worms and other segmented marine worms, arthropods’ bodies are divided into sections. An arthropod has a definite head region that is specialized for handling food and gathering information about the environment. Most have compound eyes, which create multiple pictures and arrange them like tiles in a mosaic. Organisms with multiple eyes cannot focus on objects as well as human eyes, but they are very good at detecting motion.
Even though they are enclosed in a suit of formidable armor, arthropods can move about quickly. Their ease of motion is due to their jointed appendages. An appendage is a leg, antenna, or other part that extends from the main body of the animal.
Sexes are separate in arthropods, and mating is usually a seasonal event with elaborate courtship rituals. In many species the male deposits sperm in the female’s body. The sperm are held here until eggs mature, then, as each egg leaves the ovary, sperm are released and fertilization occurs. Resulting zygotes mature into larvae that swim in the plankton for a short period of time before settling down on the reef floor to mature.