The patch reef begins with a coral larva finding a conch shell or some other rocky projection in the seagrass or sand. The larva attaches itself to this piece of the sea floor, and develops into a moderate-sized coral. Subsequently, a storm or a predator kills the coral, leaving the limestone skeleton as vacant real estate. Soon other reef coral larvae settle on the skeleton and grow upward and outward, building and expanding the reef. After a few hundred years, the reef approaches the water's surface. Upward growth ends, but outward expansion continues. When a large coral is cut open and examined, the richness of life is amazing. A coral head two feet in diameter may contain thousands of animals belonging to hundreds of species. Large star and brain corals form the bulk, or framework, of the reef. Smaller corals settle and grow in dead areas on the larger corals. The competition of space is intense on a fully developed reef. Algae, sponges, seawhips, anemones and a multitude of other creatures are in the reef real estate market. The upper surfaces of the massive corals are protected from invasion and encroachment by microscopic stinging cells in their tissues. However, their unprotected undersides are rapidly invaded by a host of excavating plants and animals, such as blue-green algae, sponges, worms and mollusks.

In older reefs, the actions of these coral miners have created large caves and dens within the underside of the reef. These caverns are excellent refuges for fish, crabs, lobsters and turtles. However, the excavation of the coral’s undersurface weakens the structure, and it eventually collapses during a storm or because of its inability to support the weight of the reef’s upper structure. This can spell the demise of the reef or it may regenerate from the rubble and fragments of coral following the collapse.

Patch reefs vary in size, development and the number of species found on them. They are typically only a few hundred yards in diameter. In the Upper Keys, they are usually located seaward of Hawk Channel and inshore of the outer reefs. In some areas of the middle Keys, patch reefs are very close to the coast. They develop and thrive in these locations when they are protected from the direct influence of Florida Bay waters.

Distinct fish populations are found on patch reefs. Bluehead, damselfish, surgeonfish, angelfish, grunt and parrot fish are most frequently found on the upper surfaces. Large predators (grouper, snapper and barracuda) find refuge in caverns and dens around the lower fringes of the reef. In more open areas, puddingwife, wrasses, gobies and goatfish are common. Puffers, squirrelfish, cardinalfish and moray eels lurk in deeper caves, often sharing their dens with lobsters.

Patch reefs experience more variability in their environment because they are located closer to shore without the moderating influence of offshore waters, particularly the Florida Current. In a typical year, water temperatures might range from 62 degrees F in February to 88 degrees F in August in patch reefs off Elliott Key. Since patch reef plants and animals typically experience more environmental change, their populations seem to have acclimated to the extreme temperature changes. For example, during the doldrum periods when the outer reefs often exhibit major bleaching (the corals turn white because all the zooxanthellae are expelled from the tissues due to heat stress), the inshore patch reefs, although under the same conditions, do not discolor or show only minor symptoms.