The most active and obvious living things in the ocean are the fish, and the greatest diversity of fish species can be found on the coral reef. No other part of the marine environment can match the abundance and diversity of the reef fish populations. Some families of fish are adapted for life among the corals, while others are better suited to live among sea fans, sponges, and sea anemones. Many are specialized to live on the reef floor, while others makes their homes near the water’s surface. The variations in reef fish are almost limitless.
Unlike the reef-building corals, armored crustaceans, and two-shelled clams, fish are vertebrates, animals with backbones. All vertebrates have internal skeletons for structure and support. The skeleton may be made of cartilage, a tough, flexible material; bone; or a combination of the two. Fish are vertebrates that have scales, fins, and gills.
The reef ecosystem can support large populations of fish because most of the inhabitants are specialized feeders. If fish competed with one another for food, many would be driven away by hunger. By specializing, reef fish have been able to develop clearly defined feeding strategies that allow each species to fill a specific role in the community that is slightly different from that of its neighbors’.
One strategy simply involves feeding at different times. Two different species of fish that eat the same food can share the supply if one feeds at night and the other during the day. In the daytime, the reef teems with animals that are grazing, stalking, sifting, or chasing down their next meals. Near dusk, the daytime feeders retreat to find places to hide, and the nighttime feeders emerge. As one shift ends and the next one begins, there is a small lull in reef activity; however, by the time it is dark, the night feeders are hard at work.
Another way populations of reef fish share the bounty is by eating in different parts of the habitat. The seafloor is one of the reef habitats that fish subdivide at feeding time. Some bottom feeders dine on organisms living just below the sand and sediment. These predators float above the reef floor, watching it closely for any signs of activity. If something moves, they rush forward and grab it. Other fish on the bottom search for their prey by “mining,” blowing streams of water over the sediment, or by stirring it to expose prey. A few species of bottom feeders have barbels, long, whiskerlike structures used to feel around in the sediment for prey. A different group of fish actually eats the sand and sediment, filters out the food, then expels the soil back into the environment.
A few species of fish have found niches for themselves by feeding on organisms that nothing else wants to eat. Some of these dine on sponges, despite the fact that sponges produce repulsive chemicals and their bodies contain sharp spicules. Other fish survive by attacking and eating long-spined black urchins, poisonous creatures that are covered with needlesharp spines. Most animals give these urchins wide berths, but the highly specialized urchin eaters have developed methods of flipping the animals onto their backs, enabling them to attack the urchins’ more vulnerable undersides.
Of the more than 22,000 fish species known worldwide, nearly one-third of them have been seen on coral reefs. There are two major groups of fish, the cartilaginous families, which include the skates and rays, and the bony fish families. Bony fish that are characteristic of coral reefs include damselfish and anemone fish (family Pomacentridae), squirrelfish and soldierfish (family Holocentridae), wrasses (family Labridae), parrot fish (family Scaridae), surgeonfish (family Acanthuridae), butterfly fish (family Chaetodontidae), angelfishes (family Pomacanthidae), groupers and sea bass (family Serranidae), blennies (family Blenniidae), gobies (family Gobiidae), cardinal fish (family Apogonidae), and grunts (family Haemulidae).