Sunday, January 28, 2018

Gary C. Williams, California Academy of Sciences

Parts of the following essay are excerpted in different form, from the introductory chapter originally published in the book: Coral Reef Animals of the Indo-Pacific by T.M. Gosliner, D.W. Behrens, and G.C. Williams (1996); Sea Challengers, Monterey, U.S.A.; 314 pp.

The deepest known point in the oceans of the Earth is the Challenger Deep in the Marina Trench (around 200 km southwest of Guam), which has been recorded at 10,994 meters (36,070 feet) below the surface of the sea. The vast ocean depths can be divided into three convenient zones for research and exploration: shallow water, mesophotic, and deep sea. 

Shallow water ranges from the surface of the ocean to about 40 meters (130 feet) – the extent of non-decompression SCUBA diving. Mesophotic depths – sometimes called the “twilight zone” – is the range of rebreather or mixed gas divers, from 40-150 meters (130-500 feet). The deep sea is every depth beyond the limit of diving, beyond 150 meters (500-36,000 feet) – where exploration takes place by submersibles, remotely operated vehicles (ROV), and autonomous underwater vehicles (AUV).

Corals have long been known to occupy all three zones. In fact, the deepest known coral is a black coral, Bathypathes patula, which has been recorded at 8600 meter (28,000 ft). Coral reefs are now known to occupy all three zones as well -- the familiar and well-known shallow water tropical coral reefs of the Indo-Pacific and western Atlantic, the less well-known mesophotic reefs of intermediate depths, and the recently recognized deep sea or cold water reefs.

The Indo-Pacific is a vast region encompassing the tropical Indian and Pacific Oceans from Africa in the west to Hawaii and French Polynesia in the east. This area represents the largest marine biogeographic region in the world. Many Indo-Pacific coral reefs develop along the margins of the African, Asian, and Australian continents but most are associated with islands. Examples of extensive continental coral reefs include Sodwana and Kosi Bays (South Africa), Eilat and Ras Muhammad (Red Sea), Phuket (Thailand), the Madang Barrier Reef (Papua New Guinea), and the Great Barrier Reef (Australia).
Innumerable islands dot the Indian and Pacific Oceans, creating tropical shallow-water conditions conducive for coral reef growth. Island archipelagos of the Indian Ocean include the Comoros, Seychelles, Mascarene, Lakshadweep (Laccadive) and Maldives, Chagos, Andaman, and Nicobar Islands. Coral reefs have developed on these island chains. However, the greatest number of islands in the world are found in the tropical western and central Pacific. These are included in the modern nations or territories of Indonesia, the Philippines, Papua New Guinea, Belau, the Federated States of Micronesia, Northern Mariana Islands, Marshall Islands, Nauru, Solomon Islands, Vanuatu, New Caledonia, Kiribati, Tuvalu, Fiji, Tonga, Samoa, Cook Islands, and French Polynesia.

The western and central Pacific Ocean is the region of the highest marine biodiversity. It can be divided into three sub-regions based on cultural geography as well as geology: 

Polynesia, Micronesia, and Melanesia. The three points of the Polynesian triangle are Hawaii in the north, Easter Island in the southeast, and New Zealand in the southwest.Polynesia was originally inhabited by people migrating from southeast Asia. Most of Polynesia is made up of widely dispersed larger volcanic islands. The name "Polynesia" means "many islands". Micronesia, on the other hand, is situated between the Mariana Trench in the west and the Line Islands in the east. The name "Micronesia" means "tiny islands." This region is composed of thousands of small islands, mostly atolls, of the central Pacific. Micronesian peoples are thought to be related to islanders of the Philippines, the probable region of their ancient origin. Melanesia is a vast region of volcanic islands including New Guinea, the Solomon Islands, Vanuatu, Fiji, and New Caledonia. The name "Melanesia" means "black islands", referring to the dark-complected peoples inhabiting these islands. 

Throughout the centuries, intermixing of coastal peoples from Polynesia, Micronesia, and Melanesia has taken place in areas of geographic overlap. Geologically, the western Pacific is composed of a complex mosaic of tectonic plates, principally the Pacific, Eurasian, and Australian Plates. The smaller Philippine Plate produces an extensive boundary region between the Eurasian and Pacific Plates. Similarly, the Caroline Plate, and the Bismarck, Solomon, and Fiji Microplates create a boundary region separating the Pacific and Australian Plates. The entire region is part of the "Ring of Fire" or Pacific Rim, and is consequently extremely active geologically. Periodic volcanic and seismic activity have produced high mountain ranges and deep-water trenches at the margins of all of these plates and microplates. This constant and often violent geological activity has created numerous archipelagos and island arcs comprising thousands of islands, with extensive regions of shallow-water habitat suitable for coral reef growth.

The first explorers and colonizers of the west-central Pacific were ethnically diverse peoples from southeastern Asia, representing various waves of migration through an extensive period of time beginning in the Pleistocene Epoch, and continuing into the last millennium. These migrations during repeated glacial and interglacial periods, resulted in humans populating virtually all island groups in this vast region including the island continent of Australia, and beyond into the east-central Pacific. A disputed theory holds that migrations also took place from the Americas into the central Pacific.

European exploration began in the sixteenth century with the trans-Pacific voyages of Magellan, Mendaña, Quiros, Drake, Torres, and Tasman. This initial phase of exploration extended from 1519-1644. Later, notable scientific voyages of discovery in the region included the remarkable achievements of William Dampier, Louis de Bougainville, James Cook, Dumont D’Urville, and the Beagle, Challenger, Siboga, and Great Barrier Reef Expeditions. This scientific period of exploration occupied the late 17th through the early 20th centuries.

William Dampier

The shallow subtidal regions of virtually all of these innumerable islands provide optimal conditions for coral reef growth and development. Depending on local conditions including currents, wave action, turbidity, temperature, and salinity, the expression of biotic communities may vary. Four primary types of biotic communities have developed in the shallow-water tropical Indo-Pacific, depending on differing physical parameters. These are coral reefs, seagrass beds, mangrove habitat, and sand flats. Coral reefs are treated in detail in a following section.

Seagrass Beds
Seagrass meadows are considered an important ecosystem of shallow-water tropical regions. They occur in protected areas such as bays and lagoons. The seagrass ecosystem is remarkable for its high ratets of primary productivity. Thick "forests" of rapidly growing seagrasses provide a protective and productive habitat for many animals that live in or on the sandy or muddy bottom (benthic forms), on the plants themselves (epiphytic forms), and in the water surrounding the plants (epibenthic or pelagic forms).
Despite the name, seagrasses are not true grasses but belong to two other families of flowering plants – the Potamogetonaceae and the Hydrocharitaceae. 

Enhalus seagrass bed, Palau

In the tropics, several species often grow together in any particular seagrass meadow. Seventeen species in eight genera (Zostera, Halodule, Cymodocea, Syringodium,
ThalassodendronEnhalus, Thalassia, and Halophila) comprise the seagrass communities in the tropical Indo-Pacific. This represents about 35% of the world species total. The other 31 species are found mainly in monospecific stands in colder temperate regions such as southern Australia and New Zealand, the North Pacific, and parts of the Atlantic Ocean. Green algae comprise an important component of the tropical seagrass ecosystem as well as sand flats with nutrient-rich waters – especially species in the genera Caulerpa, Udotea, Codium, Acetabularia, Avrainvillea, and Halimeda. They often grow as epiphytes on the seagrasses. Brown algae of seagrass beds include species of the genera Padina and Sargassum. Seagrasses are actually consumed by only a few animals including dugongs, sea turtles, and some mollusks and urchins. Many animals, however, graze on the green algal and diatomaceous films that grow on the seagrass blades. For detailed information and identification of seagrasses consult Phillips and Meñez (1988), and Meñez, Phillips, and Calumpong (1983).

A diver sifting sand for sea pens, seagrass beds, central Philippines 

Mangrove Habitat
Mangroves are sometimes situated between coral reefs and a gently-sloping shoreline, often in protected bays and lagoons with limited circulation. Mangroves have adapted to live in saltwater-saturated soils of highly saline condition. Conservation of mangrove areas is important for a number of reasons. The mangrove ecosystem contributes significantly to the productivity of tropical shallow-water regions. Extensive areas of mangrove growth trap silt-laden runoff from the land, thereby protecting coral reefs from sedimentation. The numerous aerial roots produced by some mangroves provide a sheltered habitat for a diverse array of marine life.

The various flowering plants referred to as mangrove are for the most part unrelated species but share similar growth habits under similar environmental conditions. These taxa include the red mangroves of the family Rhizophoraceae – such as Rhizophora (pantropical), Bruguiera and Ceriops (tropical Asia and Africa), and Kandelia (southeast Asia); the black mangrove Avicennia (family Avicenniaceae); and the white mangrove Lumnitzera (family Combretaceae) from East Africa, Asia, and Australia (Lugo, 1990).

Sand Flats
Extensive sandy areas are found between patch reefs, or in depressions and gullies on the reef proper, or in deeper areas below or beyond a reef. Sand flats are often textured with ripple marks due to the action of strong bottom currents. Sand flats may seem barren

Sand flats with algal turf, Maricaban Island, Philippines

by day, or covered by a green algal turf during periods of elevated nutrient levels in the sea water, but at night the exploration of such areas can startle the diver with a surprising variety of animal life, including the sea pens Veretillum and Virgularia, the gastropods Pleurobranchus and Coriocella, and various cuttlefish, lobsters, crabs, and urchins. These and other common night animals are active and conspicuous on sand flats, but are invisible or highly cryptic during the day.

What are Corals?
The terms "coral" and "coral reef" often produce images of emerald blue waters at bathtub temperatures, calm shallow lagoons, and beaches of sparkling white sand lined with coconut palms. It is true that the majority of coral species are found on tropical coral reefs. However, the term coral refers to a vast array of organisms that are found throughout the world’s seas from freezing polar regions to equatorial reefs, and at all depth from the intertidal zone to the bottoms of the deepest hadal trenches (Williams, 1990).

The word "coral" is derived from the ancient Greek word "korallion", which refers to the precious red coral of the Mediterranean, known to us today as Corallium rubrum (Williams, 1993). The diverse assemblage of organisms known as corals are actually animals belonging to the phylum Coelenterata (Cnidaria) along with such things as hydroids, jellyfish, box jellies, and sea anemones. All corals are coelenterates (also called "cnidarians" by some zoologists), but some are more closely related to other coelenterates than to other corals. For example, hydrocorals are more closely related to hydroids than they are to other corals. Therefore corals are not a monophyletic group, but refer to various coelenterates that produce a calcareous or proteinaceous skeleton.

The life history of a coelenterate is characterized by having alternating life styles – either as a polyp (the attached stationary stage) or a medusa (the swimming or floating jellyfish-like stage). A coral is characterized by having some form of hard skeletal structure, composed of calcium carbonate or a tough fibrous protein known as horn, or a combination of these two. Most corals are colonial, but a few are composed of a solitary polyp throughout their entire life span. The polyp is the living individual of a coral colony, made up of two tissue layers surrounding a thin gelatinous matrix that contains various cells. Each polyp has a ring of tentacles surrounding a central mouth. The tentacles contain specialized stinging structures (nematocysts) within cells called cnidocytes. Nematocysts resemble miniature poison darts, and are important in defense and prey capture.

For the most part, corals are polytrophic feeders – that is, they obtain nutrition in a variety of ways. They are all micropredators and ingest plankton and particulate matter form the surrounding water medium, but they can also directly absorb dissolved organic matter from sea water through the epidermal tissues. In addition, many species can utilize the products of algal symbiosis via photosynthesis. Many reef corals have single-celled algae called zooxanthellae (dinoflagellates) living in their internal tissues. 

Zooxantellae (Symbiodinium sp.) from a stoloniferous octocoral (Clavularia sp.);
each cell is approximately 0.01 mm in diameter.

Zooxanthellae (Symbiodinium sp.) from a scleractinian coral (Acropora sp.); 
each cell is approximately 0.01 mm in diameter

This symbiotic relationship allows for the production of enough calcium carbonate for coral reefs to originate and grow. Corals that take part in this relationship are called zooxanthellate corals and include the fire corals and blue corals, soft corals such as SarcophytonSinularia, and Lobophytum, some sea fans such as Rumphella, a few sea pens such as Virgularia and Cavernularia, and all the reef-building hard corals. Zooxanthellate corals often have a golden-brown or greenish coloration and are not brightly-colored. The coral provides a protected habitat for the algal cells and at the same time utilizes products of algal photosynthesis to produce more calcium carbonate than it could without the algae. This excess production of calcium carbonate is what builds coral reefs. The building of reefs is therefore a bipartisan effort between coral host and algal tenant, and this close working relationship explains why coral reefs are restricted to the warm, clear, sunlit waters of the shallow-water tropics. Corals that do not contain zooxanthellate are called aposymbiotic.

Corals can reproduce either sexually or asexually. Sexual reproduction involves internally fertilized eggs which are brooded on the inside or outside of the parent polyps, or externally ldcommon in some reef corals and involves cloning by budding or fragmentation, which either originates within the body of the coral itself, of is due to external causes (Hughes, 1985).

Corals have a long fossil record dating back 450-500 million years to the Ordovician Period of the Paleozoic Era. Three groups of early corals – the heterocorals, the tabulate corals, and the rugose corals – are now all extinct, having died out by the end of the Paleozoic. Tour other groups of corals, which developed during the Mesozoic and Cenozoic Eras, survive to the present day – these are the hydrocorals, the black corals, the hard corals, and the octocorals. All four of these groups inhabit Indo-Pacific coral reefs, but is is primarily certain hard coral species that actually build reefs as a result of the deposition of calcium carbonate onto the surface of the reef by the living tissues of the corals themselves. Coral reefs are actually biogenic geologic structures of limestone – having been created by countless generations of living corals. Corals can be classified as hermatypic (reef-building) or ahermatypic (non-reef-building) (Schumacher & Zibrowius, 1985). Hermatypic corals are for the most part hard corals (scleractinians), but also include the octocoral Heliopora (blue coral) and the hydrocoral Millepora (fire coral). All hermatypic corals are zooxanthellate but not all zooxanthellate corals are hermatypic. Some ahermatypic zooxanthellate corals include the mushroom coral (Fungia) and Neptune’s Cap (Halomitra).

Some barnacles form commensal associations with particular coral reef invertebrates. These include pyrgomatid barnacles on the fire coral Millepora, as well as Fungia and other scleractinians; Conopea spp. on gorgonians; Oxynaspis spp. on black corals; Acasta spp. on sponges; and cf. Acasta on the gorgonian Rumphella.

The Five Kinds of Corals

Hydrocorals belong to the Class Hydrozoa. The four other groups of corals are anthozoans. Members of the Class Anthozoa are exclusively polypoid, having lost the medusoid stage, while hydrozoans retain both polypoid and medusoid stage in their life cycles.

Hydrocorals (bordered in  yellow  above) include both the milleporine and stylasterine corals. Milleporine corals are also known as the fire corals or stinging corals, representing a dozen or so valid species of the single genus Millepora. Found in both the Caribbean and Indo-Pacific, fire corals probably account for more toxic stings received by divers than any other coelenterate (Auerback & Geehr, 1989: 955). Stylasterine corals, also known as lace corals, include delicate and colorful species belonging to the genera Stylaster and Distichopora, both commonly found on Indo-Pacific reefs. All hydrocorals are characterized by a massive and relatively brittle calcium carbonate skeleton, with numerous pinpoint-sized pores, from which emanate two kinds of hydroid-like polyps, which are finger-shaped with knob-like tentacles. The two kinds of polyps either have a defensive function (dactylozooids) or a feeding function (gastrozooids).

The fire coral Millepora ap.

Antipatharians (bordered in  purple  above) are the black or thorny corals, characterized by having internal axes of dark horn - calcium carbonate is absent. The axis is covered with minute thorny or spiny projections, much like the branches of a rose bush. Three genera of black corals, StichopathesCirripathes, and Antipathes, are commonly encountered on Indo-Pacific coral reefs. Very thin tissues overlay the axis of a living black coral, usually resulting in a bright yellow or greenish-yellow appearance. Each black coral polyp has six finger-like tentacles surrounding the mouth.

Antipatharians (Black Corals) -- Left, Stichopathes sp.; Center, scanning electron micrograph of axis of Antipathes sp.; Right, Bathypathes sp., photo courtesy NOAA, Greater Farallones National Marine Santuary.

Scleractinians (bordered in  red  above) or hard corals (also called stony corals) comprise most of the framework of a living coral reef. Hard corals have massive calcium carbonate skeletons with relatively large polyps (>5 mm in diameter), each containing internal radiating ribs called septa. Many important hermatypic species of Indo-Pacific reefs are colonial hard corals of genera such as AcroporaMontiporaPocilloporaGoniopora, and Turbinaria. Common ahermatypic taxa include FungiaTubastraea, and Dendrophyllia. Several ahermatypic reef-inhabiting hard corals such as the mushroom coral, Fungia, are solitary (only one polyp is present) and do not form colonies.

The hard coral Pocillopora eydouxi

Gold corals (bordered in  gold  above) are skeleton-forming zoanthids represented by the genus Kulamanamama (= Savalia and Gerardia). Gold corals produce a conspiculously gold-colored scleroproteinaceous axis that when tranversely sectioned, reveals concentric layers of skeletal material.

Gold coral axis

Octocorals (bordered in  green  above) include the soft corals, sea fans, sea whips, and sea pens. All octocorals are easily identified by the eight feather-like tentacles that surround the mouth of each polyp. The soft corals (octocorals that do not have an internal axis) are important members of Indo-Pacific reef communities - their abundance, diversity, and biomass rivals or exceeds hat of the hard corals in some regions. The blue coral (Heliopora coerulea) has a massive aragonitic skeleton and is an important reef builder in some areas. The organ-pipe coral (Tubipora musica) is the only other hermatypic octocoral. Other octocorals are ahermatypic and have skeletal elements composed of calcitic spicules known as sclerites.

Octocorals -- Left, a gorgonian or sea fan Rumphella aggregata; Center, scanning electron micrograph of a gorgonian axis Muricea appressa; Right, scanning electron micrographs of sclerites, most of them are less than 0.5 mm in length.

What are Coral Reefs?

Biogenic limestone reefs are geologic structures built over time by living organims. The two most important types are algal reefs and coral reefs. Algal reefs are formed primarily by lime-secreting green and red algae such as certain species of Halimeda and Lithothamnion, while coral reefs are formed primarily by various species of hermatypic corals. Coral reefs represent the accumulated remains of the skeletons of lime-secreting organisms, primarily hermatypic corals. The thin living veneer of tissue lives on and builds upon the skeletal remains of past generations of corals below.

Coral reefs are home to an indeterminable number of species of organisms. The variety and abundance of marine life on coral reefs is overwhelming. Virtually all of the more than thirty major animal groups (phyla) are represented on coral reefs and many species no doubt have yet to be discovered and described in the scientific literature.

The importance of conserving coral reefs cannot be over-stressed. They are the world's most diverse marine communities representing banks of biological diversity. They are indicators of environmental stress such as pollution, sedimentation, and sea temperature fluctuations. They are also sources of pharmaceutically important compounds such as prostaglandins and anti-cancer agents.

Coral reefs are distributed in a circumtropical band mostly between 20° North Latitude and 20° South Latitude. The tropical western Atlantic and the Indo-Pacific are the two main coral regions in the world. The tropical western Atlantic is the region of tropical and subtropical America between Bermuda in the north and Brazil to the south, including the Gulf of Mexico and the Caribbean Sea. The Indo-Pacific covers the vast region form East Africa and the Red Sea to the Hawaiian and Tuamotu Archipelagoes. In a comparison of biodiversity, the Indo-Pacific is roughly ten times more diverse than the western Atlantic. For example, there are approximately sixty species of hermatypic corals inhabiting the coral reefs of the western Atlantic compared with an estimated 500-600 species in the Indo-Pacific. Coral reefs are rare or absent from the tropical Atlantic of South America and Africa due mainly to the great influx and circulation of fresh water and silt from the Amazon and Congo River systems.

Coral reefs need warm, clear, relatively quiet water for optimal growth. The distribution of coral reefs at any given point in time is determined by various limiting factors. The most significant of these are water temperature, depth and light intensity, salinity, water turbulence, and sedimentation. The optimum temperature for the growth of hermatypic corals and the development of coral reefs is 20-28°C. At temperatures below 18-20°C coral growth is limited or ceases. Temperatures above 28°C frequently induces incidents of coral bleaching - the evacuation and depletion of zooxanthellae from coral tissues.

Bleaching in the Brain Coral Platygyra lamellina

Some researchers feel that global warming will lead to widespread bleaching of corals, and could have a serious negative impact on marine biodiversity. Reef growth ceases with a widespread breakdown in the algae/coral symbiotic relationship. Water depth is important as it determines the intensity of light reaching the coral tissues for photosynthesis to occur. Most reefs grow best in depths of less than 25 meters. Light intensity, particularly ultraviolet radiation, may limit coral growth at the surface or in very shallow water, and in depths greater than 25 meters the diminution of light inhibits photosynthesis. Although many hermatypic corals can tolerate fluctuations in salinity as low as 18 parts/thousand and as high as 70 parts/thousand, around 35 parts/thousand is optimal. The amount of constant or periodic water turbulence can limit the extent of coral growth and reef development. Wave action and surge can lead to physical breakage, as well as limit growth and select for certain morphological types (i.e. robust and mound-like colonies vs. delicately branched forms). 

Lastly, the amount of sediment suspended in the ambient water can be a strong determinant of reef growth. Turbid water cuts down on light intensity and can also result in direct physical stress to coral colonies.

Natural threats to coral reefs and coral reef organisms include cyclones and hurricanes, periodic population explosions of echinoderms such as the crown-of-thorns starfish

Crown-of thorns starfish Acanthaster planci.

(Acanthaster planci), periodic ocean warming events (El Niño), and the actions of earthquakes and volcanoes. Man-made threats include chemical and nutrient pollution, sedimentation from land clearing and coastal development, over fishing and collecting for the international aquarium, jewelry and sea shell trades, recreational use (ship anchor damage and tourism impact), and destructive fishing techniques including the use of dynamite and cyanide.

The evolutionary history of coral faunas and coral reefs is an ancient one extending back in geologic time to the early Paleozoic when the first corals appeared. During the breakup of the supercontinent Pangaea into Laurasia and Gondwanaland during the early Mesozoic, the Tethys Sea became circumglobal. A uniform worldwide coral fauna presumably flourished until the various continents began to split up. The splitting up of the two great land masses into various continents gave rise to a diversity of localized faunas. Approximately 1-6 million years ago, the Isthmus of Panama formed a complete closure between the Atlantic and Pacific. Independent evolution of corals then took place in isolation, leading to the separate faunas of the western Atlantic and the Indo-Pacific that we encounter today.

The Formation of Coral Reefs

Fringing Reef, Southern Luzon, Philippines

Most coral reefs can be classified as fringing reefs, barrier reefs, coral atolls, table reefs, or patch reefs. Fringing reefs are the most common type of reef in the Indo-Pacific as well as the tropical western Atlantic. These reefs project seaward directly from the shore and form a fringe of stony coral around an island or along part of the shore of a large land mass. Barrier reefs are located further from shore than fringing reefs and are separated from adjacent land by a lagoon. The largest barrier reefs in the world are the Great Barrier Reef off Queensland, Australia, and the Belize Barrier Reef off Belize I the Caribbean Sea. Atolls are low, ring-shaped, limestone islands with a central lagoon, encountered predominantly in Micronesia, Polynesia, and parts of the Indian Ocean. The word "atoll" is derived from "atolu", a native name in the Maldive Islands. Several quadrilateral atoll-like coral reefs just to the east of the Belize Barrier Reef in the Caribbean Sea, are not true atolls but have a very different origin and development. Table reefs are small open ocean reefs with no central islands or lagoons, confined to the tops of guyots or seamounts. Lastly, patch reefs rise from the floor of lagoons and represent discrete units surrounded by sand or other non-reef substratum.

The modern theory of coral atoll formation was originated by Charles Darwin as a result of his observations at Tuamotu Archipelago (south central Pacific) and Cocos Keeling Island (eastern Indian Ocean) made between November 1835 and April 1836, during the voyage of the H.M.S. Beagle. His book, The Structure and Distribution of Coral Reefs, was first published in 1842 as Part 1 of the Geology of the Voyage of the 'Beagle.'
Darwin believed that the fringing reefs, barrier reefs, and atolls of volcanic islands represented a successional series through geologic time. He hypothesized that the transition from fringing to barrier reef to atoll could result from the upward growth of coral on the edge of a gradually sinking volcano. Gradual subsidence and continuous reef growth were fundamental to his theory. He believed that barrier reefs represented and intermediate stage between fringing reefs and atolls, and that the ring-like appearance of an atoll with a central lagoon represented an intermediate stage resulted from the total submergence of the summit of a volcano. The sequence of events in atoll formation can be summarized as follows.
  1. An emergent oceanic volcano that is no longer active, is colonized by reef-building coral.
  2. Initial coral growth forms a fringing reef around the island. As the magma chamber of the volcano is depleted the island begins to sink, but coral growth continues, building upon past generations of corals.
  3. As the volcano continues to subside, a barrier reef is formed with a lagoon between the island and the reef.
  4. Finally, the volcano completely disappears below sea level, leaving an atoll composed of low coral islets in a ring with a lagoon in the center.

Darwin was not the only naturalist interested in the origin of coral islands. The geologist James Dwight Dana, and the conchologist Joseph Couthouy, as members of the United States Exploring Expedition between 1838 and 1842 in Fiji, made observations similar to those of Darwin. They contributed significantly to the developing coral island theory initiated by Darwin, by recognizing that sea temperature can restrict coral growth and hence can help explain the distribution of coral reefs. They also found indirect evidence to support the idea that subsidence of some oceanic volcanoes actually does take pales. Dana believed that in island such as Tahiti, a deeply embayed and irregular coastline coupled with an extremely eroded and dissected topography, was evidence for partial island submergence since the sea was not capable of eroding the shore in such a way. Neither of these principles were recognized by Darwin. Dana also recognized what appeared to be uplifted coral reef coastlines on islands of the western Pacific (as opposed to the submergence that occurred in the central Pacific). Dana's 1849 map of the Pacific represents an extraordinary achievement based on his numerous observations.

It was not until the acceptance of the Theory of Glaciation in the latter part of the nineteenth century that we had a full and vigorous explanation for coral island formation. The Swiss naturalist Louis Agassiz came to the United States to a professorship at Harvard in 1837. He began to espouse the concepts of the newly developing Theory of Glaciation and the "Great Ice Age" - a theory that can be traced back to the observations and speculations of various Swiss geologist in the Alps since the early 1800s. It was not until the 1870s that the Theory of Glaciation became fully accepted by the scientific community. Between 1910 and 1948, the American geologist Reginald Daly developed the Glacial Control Theory to explain the many elevated or submerged notches and erosion terrace found on the coastlines of many coral islands, as well as recently exposed Pleistocene reefs. The theory can be summarized as follows:
  1. An island emerges from the surface of the sea.
  2. Sea levels drop significantly during an Ice Age.
  3. Horizontal terraces and ledges are cut by erosion during the period of low sea level.
  4. As the Ice Age ends the sea level rises and coral reef growth takes place on the newly created submerged platforms.

The modern explanation of coral reef development should correctly be called the Darwin/Dana/Daly Theory (The 3D Theory!) of Coral Reef Formation since only the works of these three men taken together can fully explain the characteristics observed on atolls today.

Experimental verification of Darwin's and Dana's part of the theory dealing with subsidence came over a century later in the early 1950's during studies made in the Marshall Islands prior to atomic and hydrogen bomb testing. As part of an environmental assessment of the region, the United States Navy drilled a series of deep holes at Bikini and Eniwetok Atolls between 1947 and 1952. The earlier experiments at Bikini reached 767 meters below the surface of the atoll and yielded only coralline rock. In 1951-52 deeper holes were drilled at Eniwetok and at 1266-1389 meters the volcanic rock basalt was encountered. The drilling passed entirely through the 1300 meter limestone cap composed of shallow-water coral reef rock. Fossil corals from the base of the cap were dated from the Eocene Epoch (approximately 37-54 million years old). It was during the early Atomic Age (1947), while Bikini Atoll was in the news, that the marketing name was coined for the newly miniaturized two-piece swimsuit.

The 1947 bikini prototype and the 1946 Bikini Atoll nuclear explosion

The drilling studies not only provided direct evidence for the subsidence of volcanoes during atoll formation, but also showed that coral reef growth, which allowed for the formation of Eniwetak Atoll, has been occurring for perhaps as long as the past 50 million years. The rate of subsidence has not been constant, but has averaged only a fraction of a millimeter per year.

Coral Reef Landscapes

Volcanic subsidence has occurred mainly in the central Pacific regions of Micronesia and Polynesia - areas confined to the Pacific Tectonic Plate. This is the region where most of the world's coral atolls are found on isolated hot-spot volcanoes. The western Pacific, on the other hand, is part of the Pacific Rim or "Ring of Fire", and is consistently very active geologically. The region is reticulated with crustal plate margins, where earthquakes, uplift, and volcanic activity are commonplace occurrences. An example of this is from the early 1990s when a strong earthquake in the Madang Barrier Reef region of northern Papua New Guinea resulted in the sudden uplift of shallow water reef by 10-12 cm, thus exposing blue corals to the desiccating effects of the air.

It is probable that both geologic uplift as well as sea level fluctuations are responsible for the elevated notches and terraces in the coastlines of certain areas such as New Guinea, the Solomon Islands, the Philippines, and Indonesia. The combined effect of changing sea levels and geologic uplift is also responsible for the remarkable limestone landscapes known as karst. Karst theory states that sea level changes and uplift combined with terrestrial erosion and air exposure of biogneic reef regions have given rise to hummocky landscapes often impregnated with sinkholes and caves. Examples of such striking karst topography include the Blie Holes of Andros in the Bahamas, the limestone towers of Kwangsi Province in China, the Naru Hills of Papua New Guinea, and the Chocolate Hills of Bohol in the southern Philippines.

The karst landscape of Bohol Island, Philippines

The study of biogeography is concerned with the distribution of plants and animals as well as two concepts known as vicariance and dispersal. Vicariance biogeography is based on the assumption that fragmentation of the environment and the creation of barriers promote biotic evolution by the division of populations into isolated subpopulations (a vicariant event). In contrast, dispersal biogeography states that members of a population may occasionally disperse across barriers to establish new populations and subsequently evolve in relative isolation (a dispersal event).

The two great coral faunal regions of the world are the Indo-Pacific and the American. The latter is subdivided into three distinct and geographically isolated coral faunas - Panamic, Caribbean, and Brazilian. It is believed that two significant vicariant events took place during the Tertiary Period, which resulted in the fragmentation of the American coral fauna. These events were: (1) the closing of the Isthmus of Panama, which differentiated the eastern Pacific from the tropical western Atlantic; and (2) the development of the enormous outflow of the vast Amazon riverine system into the Atlantic Ocean, which acted to isolate the Caribbean and Brazilian faunas. A more ancient event, the gradual development of the vast Eastern Pacific Basin, acted to isolate the Indo-Pacific and American shallow-water faunas.

It is estimated that about 3/4 of all species of living organisms are animals. Regarding animal species, only about 7% are vertebrates. On land about 75% of all animal species are arthropods (insects and arachnids), but in the marine realm mollusks are the largest animal group, and the arthropods are represented mostly by crustaceans. A pie chart shows relative estimated species diversity for the major groups of marine animals.

Although the vast Indo-Pacific is the most diverse biogeographical region in the marine realm, it is actually comprised of a complex mosaic of faunal subregions, each with differing species compositions and endemics, The Red Sea and the western Indian Ocean as well as the western Pacific are three such areas with very high diversity. The region with by far the highest diversity is defined by a geographic triangle (the high diversity triangle) formed by the Philippines in the north, Indonesia to the southwest, and New Guinea to the southeast.

A composite theory to explain tropical high diversity includes these elements: greater energy input near the equator together with great spatial heterogeneity and favorable physical factors, punctuated with periodic disturbances.