Eating on the Reef: An Intricate Food Web

All animals must find food in order to maintain an energy supply to fuel their metabolic processes. Each individual faces competition for scarce food resources; only those that are consistently successful are able to grow and, in turn, reproduce.

Finding sufficient food is not just about being able to fight for a share when rich pickings are on offer; it is also about innovating and diversifying -- trying new kinds of food and exploiting untapped resources. The intense competition for food on the reef is one important reason why the animals there are so diverse. Reef inhabitants have radiated to fill every available niche, resulting in the formation of complex food webs.

What is a food web?

A food web is a way of depicting the trophic (nutritional) processes in an environment. Simply put, it shows how each living thing links to others according to what it eats and what, in turn, eats it. Food webs almost always have plants as their foundation. Plants use energy from the sun to photosynthesize -- the process by which sunlight and carbon dioxide are converted to oxygen and energy in the form of sugars. When the plants are eaten, this energy is passed up the food chain. But at each level, organisms use energy to fuel themselves, so only a portion of the energy is passed up the food chain, meaning that there is a limit to the number of levels that can exist in a food chain before the energy runs out.

What does the coral reef food web look like?

The coral reef food web is supported at the bottom by single-celled plants known as phytoplankton. These are invisible to the naked eye; you could fit several thousands of them side-by-side in the space of a single millimeter. The phytoplankton live in the well-lit surface waters and are in turn eaten by microscopic animals called zooplankton. Thousands of species fall into this grouping, including both animals that spend all their lives in this stage and others, such as the larvae of larger animals, who are temporary residents of the plankton during their early lives. There is a whole food web within the zooplankton; some species graze on the phytoplankton, others actively hunt other zooplankton.

Tiny crustaceans known as copepods are the most common herbivores and make up almost 75% of the zooplankton. As well as zooplankton, large numbers of fish, clams, urchins, crustaceans, snails, and echinoderms on coral reefs graze on phytoplankton and the algal turf that forms over reefs. A wide variety of reef animals, including the corals themselves, feed directly on zooplankton and then form the diet of carnivores of all kinds further up the food chain. However; things are not that simple. Some animals, such as fish, not only eat zooplankton, but are zooplankton larvae in early life. Others change their diet as they grow, so that one species can occupy more than one position within the same food chain.Thus, a well-known feature of coral reef food webs is their complexity.

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