In the sea, as well as on land, invertebrates dominate the animal kingdom. Sponges and corals fill the reefscape with bright colors and endless body forms. Worms hide in every crevice, crack, and safe spot they can find. As abundant as these animals are, they only make up a small portion of the reef invertebrate population. Living alongside them are myriad invertebrates that are more complex and sophisticated. Many of these have well-developed body systems, some of which are very similar to those found in vertebrates.
Simple invertebrates, such as sponges, satisfy all of their oxygen needs by absorbing the gas directly through their cells. The larger and more complex invertebrates do not have this luxury. Complex invertebrates need greater quantities of oxygen than can be provided by simple absorption. In addition, their bodies are often covered with waterproof external structures, further reducing the amount of contact they have with oxygen-rich water. These structural characteristics demand a more complex respiratory system and have led to the development of gills.
Gills are respiratory organs made of thin tissues that contain thousands of tiny blood vessels. Because they are tightly folded, gills pack a large surface area into a small space. As water flows over the gills, oxygen that is dissolved in the water diffuses into the bloodstream. At the same time, carbon dioxide dissolved in the blood diffuses into the water and is carried out of the body.