Coral Reef Food Web

Coral reef food webs and food chains are exceedingly complex.

It has taken scientists nearly two centuries to begin to unravel or understand the energy sources and processes that enable the development and growth of such large and biologically diverse ecosystems as coral reefs in waters highly depleted of dissolved nutrients. Yet these are exactly the kind of marine environments in which coral reefs thrive.

While considerable progress has been made in answering some of the more fundamental questions in this regard, there still remains much to be learned in terms of more detailed understanding of the roles and interactions of the many forms of life that comprise coral reef communities.

Contributions From Plant Life

The main groups of plants associated with coral reef ecosystems are discussed elsewhere on this web site, but in the context of understanding coral reef food webs it is particularly noteworthy that an obvious anomaly of coral reefs is the marked paucity of visible plant life in those parts of the coral reef biome where the most vigorous growth of hard corals occurs - the reef face and reef crest.

Zooxanthellae. Photo: Courtesy Scott R. Santos, Department of Biological Sciences, Auburn University, Auburn, AL USA.

However, a most unusual and advantageous aspect of hard coral biology is that their body tissues serve as a home for numerous, tiny symbiotic algal cells called zooxanthellae.

These essential biological partners provide the coral polyp with vital nutrients produced through photosynthesis. In return, the plant cells gain the benefit of a ready source of otherwise scarce nutrients which are produced by the polyps' metabolism.

Though barely visible to the naked eye, hard coral surfaces also serve as home for countless tiny microscopic algal cells that form an invisible living layer of plant life upon coral colonies.

Of particular note is the fact that the zooxanthellae and microscopic algae living within and upon hard corals are characterized by very high production rates.

As has often been noted by plant ecologists, a smaller biomass of rapidly-growing plants (like coral reef microalgae) may, in the same amount of time, produce as much new food as a larger biomass of far slow-growing plants (like pine trees).

Coral reef microalgae are classic illustrations of this principle, achieving high production rates with low standing crops (biomass).

Contributions From The Lagoon

So are we to conclude that the contributions of microscopic algal cells alone are sufficient to support the entire coral reef food web? Not by a long shot.

The abundance of animal life concentrated in the outer reef zones (i.e., the reef crest and reef face, which lie seaward of the lagoon, also receive considerable assistance from other sources.

The highly productive plant assemblages (seagrass meadows and mangrove forests) that often occupy the back reef zone support enormous numbers of invertebrate animals, vital links in the coral reef food web.

Fish foraging in back reef seagrass meadow. Courtesy NOAA

Many daytime reef-sheltering fishes such as grunts (Pomadasyidae) and snappers (Lutjanidae) make a living by feeding on such animal prey at night. After returning to their day shelters on lagoonal patch reefs and the upper reef face, the nutrient-rich remains of this food are eliminated.

In this way, part of the prodigous biological production of seagrass meadows and mangroves is used to promote the growth of hard corals throughout the ecosystem.

The exchange of energy and nutrients between the producive back reef habitats and the main reef growth that occurs on the reef crest and fore reef is due to both abiological factors (e.g., water movement) as well as biological factors (active transport by living animals). These processes are an essential aspect of feeding the coral reef community.

Contributions From the Open Sea

Coral reef food webs also gain a good deal of energy input from a quite different neighboring marine biome: the open sea.

Phytoplankton. Courtesy NOAA

By far, the vast bulk of plant life in the sea is composed of microscopic planktonic algae (phytoplankton) that comprise the base of the entire marine food chain.

This wealth of plant life in turns supports quantities of tiny drifting animals (zooplankton) which feed upon the drifting plants, and some of these are ultimately swept across coral reefs.

Thus, along with the nutrients supplied by their symbiotic zooxanthellae (see above), coral polyps also satisfy their nutritional requirements by feeding on tiny zooplankton suspended in the constant flow of sea water that bathes coral reefs.

These are captured by the specialized feeding tentacles and passed to the gut for digestion, with the resultant nutrients shared by the zooxanthellae. In most hard coral species the tentacles are used only at night, when they are relatively safe from hungry coral reef fishes. During daylight hours the tentacles are retracted into the safety of the protective skeleton.

Plankton feeding fishes swarm above a coral reef. Courtesy NOAA

Thus, through capture and ingestion of these creatures by corals, myriad other coral reef invertebrate animals, and plankton-feeding reef fishes, some of the organic production of the open ocean is thereby transferred to the coral reef food web.

The overall energetic contribution of zooplankton to coral reef communities is variable in both time and space, and is difficult to measure. Nonetheless, reef planktivores have been shown to be highly efficient at what they do, removing as many as 60-70% of the drifting animals from the passing water.

Due to this efficiency, the contribution of zooplankton to coral reef food chains is believed to be substantial.

Summary: The Coral Reef Food Web

In truth, hard corals are composite organisms, part animal and part plant; and this is in no small part a reason for their tremendous success in waters where dissolved nutrients are far from plentiful.

Coral reef communities are extremely efficient at retaining and recycling the nutrients received from all sources. The key elements in this recycling process are (1) the plant-animal symbiotic relationship between hard corals and their zooxanthellae, and (2) the rapid and effective exchange of nutrients and energy between different parts of the coral reef biome.

It is the sum of organic contributions from myriad sources both from within the coral reef biome and from neighboring biomes - along with a highly efficient "recycling program" - that allows for the sustenance and growth of coral reefs in the midst of waters so stripped of dissolved nutrients that they are often referred to as biological deserts.