Coral Reefs

Modern coral reefs (of the Holocene-Anthropocene) exist in the tropics between the latitudes of 30 degrees North and 30 degrees South, with some exceptions. Corals are generally restricted to these areas because their symbiotic relationship with photosynthetic zooxanthellae requires specific temperature, light, and salinity conditions. The major biogeographic regions where coral reefs exist are in the Atlantic Ocean, Australia, Indian Ocean, Middle East, Pacific Ocean, and Southeast Asia. ref

There are four types of reefs:

• Fringing reefs that grow near the shoreline and are the youngest in development
• Barrier reefs which are separated from the shoreline by a body of water called a lagoon
• Patch reefs that are discrete, isolated reefs that are often between fringing and barrier reefs
• Atolls which form on oceanic reefs that surround islands. The island may subside below the surface over time leaving a ring of reef enclosing a central lagoon.

Fringing reef off the East Portland Fish Sanctuary, Jamaica. Photo © Steve Schill/The Nature Conservancy

The different areas of a coral reef are categorized into geomorphologic zones due to differences in light, wave action, temperature, and sedimentation. These zones may differ depending on the type of reef (e.g., fringing, barrier, etc.) but generally consist of a lagoon, back reef, reef crest, reef slope, and fore reef. Community assemblages typically vary in different reef zones and across regions due to different environmental conditions and competitive ability of coral species.

Biological Interactions

There are many biological interactions within coral reef communities that affect the health and fitness of corals including competition, herbivory, and predation (i.e., corallivory). Because physical space is a major limiting resource on reefs, and corals are sessile organisms, they compete with many other benthic organisms, including other corals, algae, sponges, hydrocorals (or ‘fire corals’), and soft corals. Competition between corals and algae is becoming more prevalent with increased disturbances to coral reefs over the last decades.

Healthy and diverse populations of herbivores are critical in mediating coral-algal competition. Herbivorous fishes in particular play an important role in reef resilience by opening space for coral recruitment and reducing stress to existing coral colonies.

Organisms that prey on corals, called corallivores, consume coral tissue, mucus, and skeleton. This includes fishes and invertebrates from nearly every taxonomic group, including fish, snails, worms, and crabs. Damage to coral tissue or skeletons takes time and energy for corals to regenerate and recover, resulting in reduced coral growth rates, ref reproductive ability, ref or increased coral disease through vectoring. ref

The coral-eating snail Coralliophila galea feeding on Caribbean elkhorn coral Acropora palmata, leaving white skeleton behind. Photo © Elizabeth Shaver