and septa are formed. Also, asexual reproduction takes place in an existing coral colony, thereby rendering the distance between coral polyps roughly constant as the surface area of the colony increases.
The microstructure of a coral skeleton reveals an intricate matrix that is reflective of the coral’s habits. The skeleton consists of minute, orthorhombic crystalline needles of aragonite, about 2 µ in diameter. When a planulae first settles to form a coral colony, it accretes a horizontal layer of aragonite called the basal disk (Fig. 1). As the polyp grows upward, the margin of the base turns upward, forming a cup, called the epitheca. The epitheca is believed to contain daily growth bands (8) that are formed as a result of daily changes in theshape of the calcioblastic layer (9). Also extending from this basal disk are vertical sheets of septa that support and separatethe mesenteries in the polyps (Fig. 1). Each septum consists of numerous crystal clusters called sclerodermites. A sclerodermite is made from fibrous aragonitic needles radiating from a single calcification center believed to be an amino acid. Dissepiments are horizontal layers of aragonite that form both inside and outside the corallite. They form skeletal boundaries found at the bottom of the coral polyp and are left behind as the result of upward growth of the polyp.
Massive hermatypic corals are more desirable than the branching varieties as the morphology to use for paleoreconstructions. First, massive corals form round, wave-resistant structures that can include hundreds of years of uninterrupted growth. Second, the accretion rate of calcium carbonate is much higher for hermatypic corals that contain symbiotic zooxanthellea than for deep species. There is a considerable amount of evidence that links zooxanthellar activity with calcification rate (10). The fundamental significance of the association between zooxanthellea and coral host has long been controversial as there are several mechanisms through which scleractinians might benefit from the presence of the algae, including translocation of photosynthate-derived carbon to the coral polyp (11,12).
It is important to note that some authors have observed high-Mg calcite in coral skeletons. MacIntyre and Towe (13) examined Porites lobata from an Oahu reef and concluded that the calcite was contained within the tubular organic network formed by microborers (i.e., fungus or algae). Thus, it appears that the presence of calcite in coral skeletons indicates accretion from noncoral sources and reflects contamination. Methods available to detect calcite are x-ray diffractometry (=1% calcite can be detected) and visual inspection under sunlight (calcite twinkles, whereas coralline aragonite is a dull off-white).
Density Banding in Corals. Ma (14) suggested that groupings of constricted epithecae reflected seasonal variations in the growth rate of coral skeletons. Later, Wells (8) studied middle Devonian fossil corals and cautiously interpreted fine ridges on the surface of the coral epitheca (see Fig. 1) to be daily growth bands. He found about 400 ridges (days) per annum, which agreed with astronomical expectations of the deceleration of the Earth’s period of rotation since the Devonian.
The most significant record contained in most surface coral skeletons are annual density bands. They are primary skeletal characteristics that consist of a high and low density portion per year discernible by x-ray of a thin slab cut along the axis of upward corallite growth (15,16). Annual variations in density represent changes in both the rate of linear skeletal extension and calcification. These growth bands were first conclusively demonstrated as annual by Knutson et al. (17). They found excellent agreement between autoradiographs of Enewetak coral, which depict nuclear bomb products incorporated during nuclear blasts (mostly 90Sr), and x-radiographs, which reveal density structure. At a few locations, multiple banding within an annual growth period has been identified, though this is not common (18,19). Also, Buddemeier and Kinzie (18) have identified lunar bands in P. lobata from Kahe Point, Oahu, Hawaii. Using x-radiography and scanning densitometry of Montastrea annularis from St. Croix, U.S. Virgin Islands, Dodge and Brass (20) found that skeletal extension (linear growth) is negatively correlated with density and positively correlated with calcification (mass addition).
Hudson et al. (21) used alizarin-staining techniques and x-radiography to observe annual density bands in M. annularis from the Florida Straits and the Gulf of Honduras. They noticed that dense aragonitic skeleton accreted during the warm summer months of July through September and that thicker, less dense bands accreted during the cooler months of October through June. At some locations, such as the Gulf of Eilat in the Red Sea, corals (Porites lobata) accrete high density bands during the season of cold sea surface temperatures (SST). In Florida, thicker, dense layers, known as stress bands, also formed during the unusually severe winters of late 1969, 1963, 1957, 1941, 1898, 1894, 1885, and 1856. These cold winters or “cold fronts” are the result of weather phenomena that originate in the Northwest and pass over the reef in a southeastern direction. This correlation between stress bands and recorded cold fronts is proof that the density bands in these corals are indeed annual in nature.
Glynn and Wellington (22) studied Pavona clavus colonies from the Galapagos Islands and revealed that dense bands form during the warm water months of January through March and thicker, less dense bands form during the cold, upwelling season, usually April through December. These workers attributed increased growth rates during certain years as the result of ENSO events. These events are linked to the intensity of the Southern Oscillation Index, which is the difference in air pressure between Easter Island and Darwin, Australia (23). When this index is low, the Peru Current decreases its flow and upwelling along the western coast of South America is restricted to below the sea surface for a period of 4 to 10 months. Simultaneously, a thin skin of warm, low-nutrient tropical surface water moves into the Galapagos region from the West and North. Just as stress bands in M. annularis from the Florida Straits depict recorded cold fronts, unusually high growth rates during El Niño years provide markers of known years for the Galapagos corals. Severe ENSO events, such as that during 1982–83, caused interruption of normal growth of Galapagos corals due to the partial or complete loss of their zooxanthellae or bleaching (22). In areas with smaller temperature changes during this ENSO event, such as Costa Rica, the corals were not bleached and hence contained a complete record of the 1982–83 ENSO (24).
Just as dendrochronologists cross date wood samples to extend tree-ring chronologies (25), so can sclerochronology be used to obtain lengthened records of coral growth. Stress bands, like those observed in the Florida Straits coral, could be used as markers of known years in dead colonies. Though annual growth rates of some massive hermatypic corals show