A coral reef is a diverse environment that encompasses a wide-ranging food chain. Trophic levels in a coral reef describe the feeding position of the plants and animals that make up that ecosystem. Plants, which are able to create their own energy, are primary producers. Herbivores, creatures that eat primary producers, make up the second level. Carnivores occupy the final levels.
Primary producers form the basis of the coral reef ecosystem. Mostly made up of sea plants, this group is able to produce its own food and therefore does not rely on another animal or plant for survival. Most primary producers are photosynthetic, meaning they convert energy from the sun for their own subsistence. Phytoplankton, coralline algae and seaweed are photosynthetic primary producers that commonly inhabit the coral reef. In deep reef areas where there is an absence of sunlight, chemosynthetic producers can be found. These organisms are able to convert inorganic compounds, such as ferrous iron and hydrogen sulfide, into usable energy. Archaea are one example; these single-celled microorganisms sustain themselves by a process of chemical conversion in the darkest of coral reefs.
Primary consumers rely on primary producers for sustenance. These are the herbivores in the ecosystem. Herbivorous zooplankton, the most plentiful of the primary consumers, are small sea organisms. Zooplankton span a wide range of organisms: Some float along the surface of the ocean, others are able to swim and still others are the young of larger animals. The flounder is an example of a fish that begins life as zooplankton. Infant flounder cannot swim, so they float and feed on plankton. Once the flounder matures into a fish, however, it settles to the ocean floor and is no longer restricted to a herbivorous diet. Other primary consumers include gastropods, such as sea snails, sponges and sea urchins. Larger species, such as herbivorous crabs and green sea turtles are primary consumers. Herbivorous fish such as parrotfish, surgeonfish and rabbitfish make their homes in the coral reef.
Secondary consumers eat primary consumers. These are carnivorous animals that are also eaten by carnivores. Some sea animals, such as butterflyfish, parrotfish, filefish and coral guard crabs consume coral and are subsequently called corallivores. Lobsters and mantis shrimp subsist on benthic invertebrates -- animals that live on the ocean floor and lack backbones. Examples of benthic invertebrates include mollusks, anemones and various types of worms. Secondary consumers that eat fish are called piscivores. Fish, mollusks and arthropods are examples of secondary consumers that are piscivores.
At the top trophic levels lie the tertiary consumers. Also called apex predators, tertiary consumers are carnivores that are not preyed upon themselves. Quick and agile swimmers that are skilled at catching fish, apex predators include sharks, dolphins, tuna and seals. These animals are often large in size. If they are preyed upon, it is usually the young or sick that are picked off.