Bank reefs have several notable features. In the shallows, on the inshore side of the reef, seagrasses and sands gradually grade into a mound of coral rubble referred to as the reef flat or rubbleridge. Snorkeling in this region during calm seas reveals masses of broken coral skeletons of various sizes carried there by storms and heavy swells. Coralline algae thrive in the heavy surf that often flows in this area. Few other organisms inhabit this region. Seaward, the next reef component is what scientists refer to as a "spur-and-groove system." Long ridges of coral masses are bisected by sand channels that merge with the reef flat; the relationship of these components is best seen from the air. The ridges are termed "spurs" and the sand channels "grooves." In Florida, the spurs are created by coral growth, principally elkhorn coral. Elkhorn coral grows rapidly (four to five inches a year) and thrives in heavy surf. Broken pieces of this coral become lodged in the reef and grow into additional elkhorn formations. In the shallowest portions of the spur, the banded fire coral and the golden sea mat cover much of the spur surfaces. The spurs may extend 100 yards, often with gaps separating portions of the structure. The seaward ends are 30 to 40 feet deep while the shallower end may barely be awash or exposed at spring low tide.

Inspection of the sides reveals a multitude of small caves and tunnels leading far into the interior of the reef. These caves are homes to fish, crabs, lobsters, urchins and other animals. Lettuce coral forms thin growths on the sides of the spur. When water depth on the top of the spur is five to six feet deep, the elkhorn, star and brain corals cover the reef surface. Small yellow-green-mustard hill corals grow wherever they find a foot-hold. Purple sea fans, seawhips, and plumes are also common in this region of the spur.

The channels separating the spurs are covered with coarse white limestone sand composed of coral fragments, pieces of mollusk shells, and principally, plates from the green calcareous algae (Halimeda). The sand usually has distinct furrows developed from the waves passing through the reef. Since it is being moved constantly, there are few living algae or plants in the material. Many small invertebrates live in the sand. Most remain buried during daylight and come out at night to forage for food. The large sea cucumber is often seen in the sandy areas, feeding on detritus and other organic material in the sand.

Occasionally, corals are dislodged from the spurs and fall into the grooves. Small and fragile ones are usually broken and transported with the other rubble to the reef flat. However, large brain and star corals may survive and continue to grow on the groove bottom.

Beyond the spur-and-groove system, there are a number of reef communities. Each reef is somewhat distinct, depending on topography, water depth and reef age. Two examples of this variability are Carysfort Reef off North Key Largo and Looe Key, seaward of Big Pine Key.

Carysfort Reef has a poorly defined spur-and-groove system, a steep slope to about 45 feet, and a zone of massive coral mounds bisected with low relief spur-and-groove formations. This continues to a 70-foot depth where the bottom is composed of fine silty sand. Seaward there is a reef formation with some corals and sea whips, whose upper surface is about 30 feet.

At Looe Key, the well developed spur-and-groove formation grades into a zone of very large star and brain coral buttresses that continue seaward from 30- to 40-foot depths. In some places they are joined in a linear fashion; in other spots they remain separated. Seaward (to the west) of this habitat are hardgrounds that support an intermediate depth reef community of octocorals, sponges and a few stony corals. At 90 to 105 feet deep, the reef comes in contact with silt-sand deposits. Diminished light will not support sufficient photosynthesis for reef growth at this depth.

Fish populations in the bank reefs are often very large and conspicuous. Snapper, yellowtail, goatfish and porkfish often school in great numbers in and around the branches of the elkhorn coral. Many of these fish are involved in shift work, with some species frequenting the reef by day and others by night, foraging for shrimp and other prey in seagrass and sandy areas. Other species, such as the parrotfish, find a cave, secrete a mucus balloon around themselves, and sleep at night. At dawn, the snapper return and reoccupy the elkhorn corals.

After the fish digest their food, they defecate. The feces contain nitrogen phosphorous compounds that are utilized by the corals and reef organisms. This is a good example of the transport of resources between ecosystem components and the subsequent recycling of nutrients within the reef.

Symbiosis is another form of species interaction on the reef. In Florida reefs, neon gobies and small shrimp are often observed in the gill chambers and mouths of many different fish. These reef dentists clean the teeth and remove parasites from the fish. If you watch a cleaning station on a reef, you can see that there are signals that the fish use to tell the gobie or shrimp that they want to be cleaned. The fish becomes still and slowly rocks from side to side. This is just one of the symbiotic relationships so common on a coral reef.