The ocean reef is teeming with life created from a base of coral. This coral requires specific temperatures, sunlight and salinity to grow. Generally reefs are positioned rather close to shore and in fairly shallow waters. They contain the bulk of marine life within their borders.
In a reef everything begins with coral and their symbiotic algae. The coral encourage algae to co-habitate with them, providing safety in return for a steady food source. Algae is what gives the coral its brilliant colors.
Once there is coral all sorts of sea creatures have a place to inhabit, find food and hopefully a means to reproduce. However, the temperature, lighting and salinity conditions that cause coral to thrive cause a lack of nutrients to be present in the water. This causes ruthless competition and a great need for balance between the competitors vying for habitat and food resources in the reef. These are only a few of the reasons why reef ecosystems are so fragile.
Despite the lack of nutrients in the water itself, reef communities have found other ways to survive. First there are the producers. Producers are algae, sea weed, and sea grasses. Unlike most land ecosystems these photo-synthesizers do not make up the bulk of reef life.
Herbivores keep the green things under control so they don't hog sunlight from the corals themselves, and they can find enough to eat if their populations are kept in check by predators. In fact, some species of damsel fish even cultivate their own algae gardens and defend this turf from other hungry vegetarians. In the reef, life adapts to find a way to thrive.
After the herbivores are the smaller predators, which are often more deadly than any of the larger. They often use venom and stings to subdue their prey as they may lack the size and sturdiness to take them down by other means. Examples of these venomous or stinging smaller predators are sea snakes, stone fish, lion fish, anemones, jellyfish and blue ringed octopuses.
After the smaller predators the web leads to larger and larger creatures that often use their teeth to kill and then break prey into edible chunks. Eels, sharks and barracudas for example.
Throughout the web turtles, snails, starfish, oysters, urchins and many other sea creatures eat and are eaten. In a reef many species are specifically adapted to consume only certain kinds of food. If these food sources disappear or are depleted by fishermen then the balance of the entire reef will be thrown off. This is why it is important for humans to take only what they need and to find ways to sustain the balance beneath the waves.
Oceans: Life in the Deep by Beverly McMillan and John A. Musick