Coral reefs are the largest structures on Earth that are built and inhabited by invertebrates. Created by a few species of calcium carbonate–secreting organisms, a reef provides homes for thousands of other kinds of animals. The interactions between individuals in this highly diverse group of organisms regulate the ebb and flow of life in reef communities.
Marine invertebrates that live on land are relatively small animals, but some species in the sea grow to spectacular sizes. The giant clam weighs up to a half ton, exceeding the size and weight of any terrestrial cousin. This startling difference in size is due to the advantages that marine environments have over terrestrial ones, especially for invertebrates with shells. The seafloor is a very stable environment, with few changes in physical factors such as light, temperature, and acidity. This constancy facilitates stable conditions within the animals’ tissues. In addition, the water gives structural support to animals, reducing the consequences of weight and permitting animals to reach lengths that would be impossible to attain on land.
Reef invertebrates include animals that are classified as crustaceans, mollusks, and echinoderms. The crustaceans are typified by lobsters and shrimps, animals whose bodies have specialized segments. Shrimps show a tremendous diversity in lifestyle and have adapted for life in every niche of the reef. Their primary defense weapons are their claws, which are often modified for specific tasks.
Mollusks include gastropods, bivalves, and cephalopods. Gastropods such as snails and nudibranchs glide on one large, muscular foot. Snails live in twisted shells that house and protect their soft internal organs. Nudibranchs are without shells, and they depend entirely on their toxins and bright warning colors to keep predators at bay. Bivalves, mollusks with two shells, such as clams and oysters, live quiet lives, sitting on the reef or hiding in the sediment. They are filter feeders that protect themselves from predators by snapping shut their thick shells.
The shell-less members of the mollusk group are the cephalopods, and on the reef they include the cuttlefish, squid, and octopus. Cephalopods are the intellectuals of the reef, capable of lightening-fast integration of sensory input. Octopuses are also masters of disguise that use their chromatophores to imitate deadly predators or to blend into the environment.
Although mollusks may be a little hard to spot on the reef, echinoderms are usually easy to find. Echinoderms have stiff, spiny skin that protects them from hungry predators. They travel across the reef floor, unhurried since most other animals cannot eat them. Echinoderms are capable of “walking” slowly on multiple rows of tiny tube feet. Most prey on smaller invertebrates.
Crustaceans, mollusks, and echinoderms populate every part of the reef. Many are completely dependent on the reef’s hiding places for their survival. A few are so well protected by their bodies’ defense systems that they can exploit more open areas, like the reef floor, which is often covered in hardshelled bivalves and spiny-skinned starfish. Because of their wide range of adaptations, reef invertebrate populations are large and rich.