Hawaiian Reef Fauna:
The Architects of Reef Structures
Coral Reefs of Today
Coral reefs are the life pulse of planet Earth. No place on Earth contains more organisms per square foot than these bastions of life that we call coral reefs. They contain the greatest diversity of life in the sea and indeed in the entire Earth ecosystem. Coral reefs have been called the "rain forests of the sea". 25% of all the organisms in the world's oceans live in or around coral reefs. Indeed, some researchers place the number of species living in the coral reef ecosystem at over one million. That's nearly 15% of all the living species on Earth!
This abundance of life translates into a phenomenal amount of organic productivity, far exceeding most rain forest ecosystems. Because of their ancient lineage and astonishing fecundity, coral reefs have been called the "cradles of evolution". Countless marine organisms have sprung from the reef ecosystems of Earth's oceans, more so than from any other environment. These massive organic structures are also sensitive indicators of the general health of the planet. Coral reefs sense and respond to subtle changes in atmospheric and hydrospheric conditions. They are gauges of our living world.
Modern-day coral reefs are the product of the skeleton-building activities of tiny coral polyps supplemented by coralline algae and other CaCO3-secreting organisms. These biogenic structures consist primarily of the hard, calcareous exoskeletons of colonial corals which are "glued" together by the carbonate secretions of a multitude of invertebrate organisms, most of which attach themselves to the substrate.
Coral polyps are multicellular, eukaryotic organisms but are really quite simple in body plan. Corals have no circulatory, respiratory, or excretory systems, but consist only of a body cavity (coelum) with a single opening which serves as a mouth. The opening is surrounded by food-gathering tentacles. After attaching itself to the substrate, the coral polyp secretes a CaCO3 cup (or theca) composed of aragonite. This hard exoskeleton provides support and protection to the coral polyp.
Reef-building, hermatypic corals of today enjoy a mutualistic symbiotic relationship with several species of dinoflagellates and algae. Known as zooxanthellae, these tiny protists live within the coral tissue where they conduct their daily cycle of photosynthesis. The coral polyp provides a safe home for the algae and also provides food in the form of CO2. The algae, in turn, absorb waste products, produce sugar and O2 which are used by the polyp, and provide the alkaline pH necessary for calcification of the coral reef itself.
Like all members of the Class Anthozoa, coral polyps do not have a medusa stage - they are polyps throughout their entire life cycle. In addition, the reef-building, hermatypic stony corals are able to reproduce both sexually and asexually. Most of them do so sexually, dispersing their eggs and sperm into the water, usually on the same night.
When coral is healthy, it grows at an average rate of about 1 centimeter (½ inch) per year. In some exceptional cases, growth rates may reach 5 to 10 centimeters (roughly 2 to 4 inches) a year! But conditions must be absolutely perfect for such growth. Indeed, a number of factors must combine for the growth of healthy coral to occur:
SHALLOW WATER : Hermatypic, reef-building coral requires shallow waters less than 150 feet deep to survive and proliferate. The symbiotic algae living within their tissues need sunlight for photosynthesis and at depths greater than about 150 feet, it is simply too dark.
WARM, TROPICAL WATER : Reef-building corals thrive in warm, tropical waters, between 30º N and 30º S latitudes. The ideal temperature range for the growth of healthy coral appears to be from about 78º F to 85º F although some sources suggest temperatures ranging from the mid to upper 80's. At low temperatures, the symbiotic bond between the coral polyp and the resident zooxanthellae is broken. Algal loss followed by bleaching of the coral occurs at temperatures below 68º F.
OXYGENATED WATER : Hermatypic, reef-building coral requires turbulent, well-oxygenated water for healthy growth. These colonial corals will not grow in stagnant, anoxic waters. If O2 levels are too low, macroalgae populations will explode on the reef, covering and encrusting the surface of the coral. The coral polyps are literally smothered by the algae.
LOW SEDIMENT LOAD : Hermatypic, reef-building corals require clear, transparent waters for two reasons. First of all, the zooxanthellic algae residing in the coral tissue needs sunlight for photosynthesis. Turbid waters containing large amounts of finely suspended sediment are too murky for much light to pass through. Secondly, the rain of fine sediment will literally clog the filter-feeding devices of the sessile coral polyps. Water must therefore be of low turbidity.
NORMAL SALINITY : Reef-building corals thrive in waters of normal to slightly elevated salinity. Ideally, salt concentrations should be between 34 and 37 parts per million (3.4% to 3.7%). Coral will not survive in fresh water, brackish water, or in waters that are too saline. Freshwater, in particular, is fatal to reef-building corals.
NORMAL pH : Hermatypic, reef-building corals require alkaline waters with a limited pH range of 8.1 to 8.2. Research has shown that a decrease in the pH of seawater will have a significant effect on the calcification rates of reef-building corals. Most colonial coral species and the encrusting forms of coralline algae show a marked decrease in CaCO3 production as pH is lowered. Indeed, coralline algal growth stops when pH levels dip below 7.7.
IMMERSION IN WATER : Although some coral reefs may be aerially exposed for up to 6 hours or more during low tide, prolonged dehydration and desiccation is fatal to coral polyps.