Marine Habitats, Part III: Life in a Coral Reef Community

Question: Can you name the largest biologically produced structures to ever exist on Earth? Clue: It is not an extinct dinosaur, nor is it...

coral reef

Question: Can you name the largest biologically produced structures to ever exist on Earth?

Clue: It is not an extinct dinosaur, nor is it the blue whale, the largest creature on Earth today. A Second Clue: These structures existed in the days of the dinosaurs and continue to exist today.

Answer: Coral reefs.

While it is true that individual coral animals, called polyps, are often pea-sized, scientists consider coral reefs to be dynamic biological structures. The Great Barrier Reef, the largest such structure in the world, is more than 1,250 miles/2,013 km long and as much as 150 miles/242 km wide in some places. Quite a bit larger than a mere 120-foot-/37-m-long, 100,000-pound/45,249-kg blue whale, wouldn’t you say? Turneffe Reef, located in Caribbean waters off the Central American nation of Belize, is the second largest coral reef structure at a length of roughly 200 miles/322 km. In contrast, small coral reefs can consist of only a handful of coral heads covering an area that can be easily explored during a single dive.

The world of a coral reef community is one of color, bizarre-looking life forms and intrigue. The moment you place your face in the water and gaze downward in a typical coral reef setting, it is all but impossible not to feel an almost overwhelming sense of wonderment. Everywhere you look, brilliantly colored fishes catch your eye. Some appear to cruise lazily, while others dart about in a hurried fashion. A closer look later will reveal that still others sit motionless, attempting to go unnoticed. Rays of sunlight dance over coral heads adorned with sponges and sea fans, and nearly every nook and cranny teems with life. Everywhere you look can appear so inviting that it is easy to get temporarily frozen with indecision.

Collectively, the myriad organisms that comprise the ecosystem of coral reef communities form one of the most complex and productive living systems on Earth. From a diver’s perspective, few places are as alluring, as we enjoy opportunities to encounter animals ranging from the simplest multicellular organisms, the sponges, to creatures as sophisticated as dolphins and whales.

In this piece, the third in a series of four feature articles examining various marine biomes, we will take a look at life in coral reef communities. The insights gained will help you get the most out of your diving. In the minds of many, coral reefs are associated with tropical islands that we collectively call “paradise,” but in many instances the islands associated with coral reefs are rather small and desolate, with only a few trees and scrubby plants. The true beauty lies beneath the surface, where astonishing coral reefs await.

Coral reefs occur only in areas where the minimum water temperature at the surface is 64.4˚F/18˚C, although many animals that commonly inhabit coral reefs are also frequently encountered in cooler waters. Coral reefs are most commonly associated with tropical seas, though they occur between the latitudes of 30 degrees north and 30 degrees south, artificial boundaries that place them both north and south of the tropics. Most coral reefs are confined to waters no deeper than 100 feet/30 m, although reefs have been documented in waters well in excess of 200 feet/60 m. The distribution of coral reefs around the world is not uniform. Factors such as water temperature, salinity, prevailing weather patterns and prevailing currents influence the pattern of reef distribution.

In terms of biodiversity, coral reefs rival the species-rich tropical rain forests. It is true that coral reef communities lack the proliferation of insects, which gives tropical rain forests a huge percentage of their species counts, but scientists are quick to point out that coral reef communities are characterized by an amazing diversity of marine creatures. The list of coral reef inhabitants includes sponges, anemones, hard corals, soft corals, hydroids, worms, shrimps, lobsters, crabs, snails, scallops, oysters, octopi, squids, barnacles, sea stars, brittle stars, sea cucumbers, sea urchins, tunicates and many more. Many of these animal groups have more species represented in coral reef communities than in any other aquatic ecosystem.

Corals are living animals. While all reefs provide food, shelter and places of attachment for organisms that require them, only coral reefs are living structures. This fact alone makes these reefs significantly different from the rocky reefs of temperate and polar seas.

Described in the phylum Cnidaria, corals are animals that are closely related to sea anemones, jellyfishes, hydroids and coralimorpharians. All of these relatively primitive creatures capture their food and repel predators with tentacles armed with potent stinging nematocysts. It is the ability to sting that distinguishes members of this phylum.

Individual coral animals are called polyps. The living part of a polyp is relatively simple. It is typically cylindrical in shape, and the size of different species varies from less than 1 millimeter in diameter to several centimeters. The living animal may be tall and thin or flat and broad, and it produces a skeleton to match. The mouth of a coral polyp is positioned at the top of the cylinder and is surrounded by one or more rings of tentacles.

A group of coral polyps forms a coral head, a structure that some divers commonly call a “bommie.” Collectively, a number of coral heads form a coral reef.

How Coral Reefs Form

Present-day coral reefs are either shelf reefs or oceanic reefs. Shelf reefs occur close to continental landmasses in relatively shallow seas. In general terms, shelf reefs grow upwards from the sea bed toward the surface, then spread out as they near the surface. Should sea level rise, these reefs can grow upwards, but should sea level fall again, the reefs will become stranded and die.

Oceanic reefs develop in deeper water beyond the continental shelf. There are three general types of oceanic reefs: fringing reefs, barrier reefs and atolls.

Fringing reefs form borders near the shorelines of volcanic islands. The sides of the volcano provide living quarters in shallow water, where corals can settle and grow. Many reefs in Hawaiian waters are fringing reefs.

Barrier reefs, such as Australia’s Great Barrier Reef, develop further offshore near continental landmasses. They tend to create relatively deep, wide lagoons between the reef and the beach. Barrier reefs are believed to have originated as fringing reefs.

Atolls are roughly ring-shaped reefs that encircle internal lagoons. The reef top is often exposed, and small islands of accumulated sand and fragments may occur. It is generally believed that atolls were originally fringing reefs that formed around volcanoes. Over time the volcanoes sank, and the present-day atolls are what remain. As land dropped away into the sea, successive generations of corals piled up to maintain their supply of sunlight. Atolls in the Pacific produce many of the world’s most exciting dive sites.

The foundation of all coral reefs is derived from the accumulated skeletal remains of uncountable numbers of deceased corals, many of which lived many thousands of years ago. But not every species of coral contributes to the long-term development of a coral reef. Corals are generally divided into two categories: 1) the hard, or reef-building, corals and 2) the soft, or nonreef-building, corals. When alive, hard coral polyps secrete external skeletons made of calcium carbonate (limestone). The living corals occur along the surface of the reef structure. As individual coral animals perish, deposits of limestone increase and the reef structure expands. All reef-building corals are colonial organisms, though solitary corals occur throughout the world.

In many shallow reef communities, calcareous red algae live among the corals. Along with a variety of other reef inhabitants, the algae deposit thin yet vitally important layers of limestone that are significant contributions to the construction of a reef.

Hard corals, sometimes called true corals, are often said to be the most significant invertebrates found in shallow, tropical seas. More than 2,500 species of hard, reef-building corals occur worldwide. A mere 60 or so can be found in the waters of the Caribbean, the Bahamas and Florida, while more than 10 times as many species can be found in the waters of the Indo-Pacific.

Soft corals also play vital roles in coral reef ecosystems during their lifetimes by providing food and habitat for many animals, but the skeletons of soft corals lack the calcium carbonate found in hard corals. As a result, the impact of soft corals is more temporary than that of hard corals. The colonial animals we commonly refer to as sea fans, gorgonian corals and sea whips are types of soft corals.

The reef-building process is ongoing, and it is not perfectly uniform. The demise of one generation of coral polyps spawns the development of future generations to colonize by creating suitable living quarters and places of attachment for new polyps to settle or bud out. This process has been underway for eons and continues today as the geological clock of the reef continuously ticks away.

But all is not rosy in coral gardens. Violent storms can wreak havoc on coral reefs. There is also a lot of damage done to coral reefs by a variety of coral predators, such as boring sponges, a variety of worms, crabs, snails, sea stars, nudibranchs and fishes. Fishes represent the largest and most diverse group of coral predators by far. Collectively, these fishes feed on the living polyps, algae that live within the corals and the skeletons that provide the foundation for the reef.

Parrotfishes are probably the most noteworthy fishes in this respect. They transform hard corals into sand as they feed. The fishes eat the living corals in order to gain access to the symbiotic algae that live within the polyps. Parrotfishes then defecate the unwanted coral parts, transforming these remains into much of the sand that forms tropical beaches and the sea floor in so many tropical settings. Butterflyfishes and angelfishes also nibble away at colonies of corals. Their activities tend to be more localized and are not considered extensive. Often a nibbled-upon coral will regenerate lost tissue and continue to flourish.

An Indo-Pacific species of sea star known as the crown-of-thorns (Acanthaster planci) is a rather voracious coral predator. This sea star feeds by thrusting its own stomach outside its body and using it to envelope and digest living corals. At a normal density of only a handful of crown-of-thorns on a reef that is several hundreds yards long, there is balance in the natural system. The crown-of-thorns creates living space for other animals by devouring the corals, and in many instances these animals become valuable partners to the still-living corals. However, at times and for reasons still not totally understood, crown-of-thorn populations explode with devastating impact, causing corals to disappear and algae and other “fouling” organisms to dominate the seascape.

When a coral head is killed or dies, it is highly unlikely that the entire head will be immediately colonized by new polyps. But that does not mean the space will go unused. A variety of algae and invertebrates soon begin competing for the space. Their life cycles are a vital and necessary part of the ongoing struggle for life on a coral reef.

The ability to attach to the sea floor or to other organisms is a life-and-death priority for many creatures that reside in coral reef communities. Recognizing the fundamental nature of the ongoing competition for space in coral reef communities, and gaining insight into the various survival strategies and life cycles of various reef species, can provide you with a much better understanding of how so many different organisms thrive in coral reef communities.

Zonation in Coral Reefs

The world of corals is quite diverse. Exactly where various species might be found within a typical reef ecosystem is not a matter of chance. Fierce competition describes the relationship between corals, as each species fights for its own niche. All hard corals require plenty of sunlight. As a result, reef-building corals need shallow, clear water that offers maximum penetration by sunlight. Shallow water tends to mean rougher water, so the corals that occur in the shallows must be strong enough to withstand pounding surf and wave action. In a typical Caribbean reef community, sturdy species of staghorn and elkhorn coral are commonly found under and just seaward of the breakers. The latticework created by these corals offers protection to animals such as crabs, shrimps and sea urchins that inhabit the shallows, while the structure also provides habitat for worms and other animals that bore into coral.

To the seaward side of these corals, you will often find a rubble zone. Small heads of star corals, as well as a variety of soft corals, often occur in the rubble zone, as do many invertebrates and small, bottom-dwelling fishes.

Other species are better suited for deeper, calmer waters. As you swim toward deeper water, you are likely to discover stretches of sand that result from the constant battering and degradation of the most seaward corals. The sand provides ideal habitat for stingrays, flatfishes, sea cucumbers and mollusks, such as conchs. Small patch reefs can often be found in the sand. These reefs are oases for a wide variety of invertebrates and small fishes, including many juveniles.

If you could position yourself high above a typical Caribbean reef and magically remove all the water, you would see a series of sand strips separated by long fingers of reef that extend from the shallows all the way to the top of a wall, or drop-off. This layout is known as a “spur-and-groove” formation. Sea fans and sponges often stand out prominently in the upper portion of the reef buttress and along the top of the wall. These organisms tend to flourish where currents flow. Sponges filter nutrients out of the water column, while sea fans form stinging nets that capture nutrients by reaching across flowing waters.

Large star coral (Monastrea cavernosa) is the dominant species on many Caribbean reefs at depths between 40 and 100 feet/12 and 30 m. As you descend you will probably notice that star corals begin to grow more in flattened plates than in the rounded mounds that occur in shallow waters. “Plating out” is an adaptation that enables the corals to increase the amount of surface area that has access to sunlight. Sunlight enables the vital process of photosynthesis to occur in the symbiotic algae known as zooxanthellae.

As you continue your descent, the variety and amount of hard corals diminishes considerably. But various species of reef-building corals, such as brain coral, giant polyp coral and green coral, are commonly encountered along the reef escarpment, and many soft corals, such as deep-water gorgonians, black coral and wire coral, thrive in deeper, calmer waters.

Associated Plants and Animals

Both living corals and their skeletons provide wonderful microhabitat for many other organisms. Some gain shelter, food or living space on a temporary basis, while other organisms form more permanent relationships. Many of these relationships are described as symbiotic, having been derived from the root word “symbiosis,” which means the living together of dissimilar organisms. Uncountable numbers of small organisms, such as flatworms and brittle stars, crawl around on the surface of corals. Other larger animals, such as a variety of snails, crabs, lobsters, shrimps, nudibranchs, octopi, sea cucumbers, sea urchins and feather stars (crinoids), seek cover in the countless nooks and crannies, emerging periodically to feed. These creatures, too, have a variety of relationships with their neighbors. They vary widely from those that are mutually beneficial, like cleaning stations, to parasitism and predator-prey relationships.

Many fishes use corals as shelter, hiding amongst and on species of corals that are branched, such as elkhorn and staghorn, and hiding under coral heads. Many juvenile fishes rarely venture outside their small, protected niches, but adults often spend their days feeding in open water, retiring to the safety of the branches as the darkness of night sets in. Many species of reef-associated fishes, such as sergeant majors and other damselfishes, commonly use portions of coral skeletons as spawning sites, with the eggs being attached to dead surfaces found to be suitable nests.

A variety of crabs and shrimps have permanent associations with corals. The caveat is that in almost every case a given crab or shrimp species has a symbiotic relationship only with a certain species of coral, as opposed to a variety of corals. While some crustaceans feed on the mucus secreted by corals, as a rule most shrimps and crabs that associate with corals in this manner assist their host corals by removing and feeding upon debris that settles on the corals. Corals are especially vulnerable to suffocation, and they need clean surfaces in order to survive. In fact, as a rule the direct impact of waves and crashing surf caused by storms does not usually pose as much threat to corals as the sediment that settles on top of them afterwards.

Most of the crustaceans that associate with corals are small, cryptically colored and difficult to locate. However, with a little patience and poking around, especially during night dives, it is often easy to see more species of crabs and shrimps than you can keep track of just by carefully exploring a single coral head.

The vast majority of crabs are fairly active and can often be seen skittering about, but there are some notable and rather interesting exceptions. A species of crab, commonly known as the gall crab, lives in the branches of a particular family of corals. The presence of the crab induces abnormal growth in the coral. The coral soon grows a tiny cage that permanently entraps the crab. From that time forward, the crab is forever trapped inside and must feed by filtering out tiny food particles that float into the cage. An interesting twist to this scenario is that only female gall crabs get trapped. The smaller males live in open depressions on the surface of the corals. The male is thought to fertilize the female prior to her entrapment. A variety of shrimps dwell in similarly enclosed sectors within the coral. In some cases males and females live together, and in other cases they have interconnecting chambers.

A variety of other creatures, such as sea urchins, lobsters, scallops, snails, worms, barnacles and octopi, also live among corals. They usually do little harm other than competing for food, but in extreme conditions some become active coral parasites. Fishes, such as a wide variety of gobies and blennies, commonly use holes in the corals as shelter and vantage points, places from which they can quickly dart out and capture food.

While many of these living relationships are rather passive and nondestructive, a number of animals pose direct threats to corals. A number of species of boring sponges actively dig, or bore, their way through older coral skeletons. Their activity often breaks up coral heads, and the sponges sometimes work their way into the living corals. Although the sponges do not directly harm the living corals, the fragmented structure is susceptible to being knocked around by waves and surge, and the reef begins to break down. A number of mollusks and worms, such as Christmas tree worms, also burrow into corals, and their activity weakens coral heads.

Without question, the world of a coral reef is filled with mystery and intrigue. What is perhaps even more fascinating is that the more we learn about how coral reefs function, the greater our feelings of wonderment, appreciation and curiosity become. As divers, it is difficult to understand how we could ask for more, except perhaps to have the opportunity to spend our lives exploring the coral reefs of the world.

Small Algae, Big Deal

Zooxanthellae live in the tissues of coral polyps and are too small to see with the naked eye, but don’t underestimate their biological importance. These algae are the fundamental building blocks of all coral reefs. The algae use the waste product of corals as fertilizer needed for photosynthesis, while the corals use the by-products of the photosynthetic process for their own nourishment and to enhance their ability to use calcium.

This symbiotic relationship is the most important relationship between any two organisms in tropical reef communities all over the world. (For more information, see Dive Training’s July 1999 article “Zooxanthellae: The Great Algae Mystery” for more information.)

The Threat to Coral Reef is Real

While coral reefs are well-documented to have survived and flourished in the world’s oceans for millions of years, many specialists are gravely concerned today that the very existence of reefs worldwide is severely threatened. The threats range from global warming to commercial development to overexploitation of marine resources.

Although coral reefs have managed to flourish for eons, scientists are well aware that coral reef communities are extremely sensitive to environmental changes. Construction, in some cases near coastlines and in other cases many miles away, causes unwanted sediment to pile up on reefs, while pollution-laden fresh water simultaneously spills its contents into the sea. Logging of tropical forests causes excessive amounts of soil to wash into the sea, and the sediment can suffocate corals. The sediment also blocks the sunlight necessary for vitally important algae that lives within the corals. Not only does the sediment suffocate the corals and block sunlight, it also encourages the growth of algae that is harmful to corals.

Tons of chemicals introduced by farming eventually work their way into coral reef communities, and various studies have demonstrated seriously detrimental impact. Trash dumping also takes its toll, as does the dumping of petroleum-based products.

Dynamite fishing and cyanide fishing, very destructive and all-too-common practices in many Third World nations, reduce reefs to lifeless rubble and introduce pollutants.

For eons mangrove trees and a variety of sea grasses have helped to filter the sediment, but many mangrove swamps and sea grass beds are being destroyed by human development. Careless recreational practices, such as dropping anchors on top of living corals, also have their cost.

Specialists tell us that saving the reefs is probably still possible, but there is no time to waste, and no effort should be spared.