Chemistry Reference
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Figure 2.
Detail of the scleractinian septal
structure (s in Fig. 1). At right, aragonite fiber
bundles (f) emerge from centers of calcifciation
(c) and grouped into sclerodermites (s). Groups
of sclerodermites growing upward together
form the trabecula (t). This septum of
Galaxea
sp. is a palisade of trabeculae, shown at left.
From Wells (1956).
fishscale-shaped bundles termed fascicles (Ogilvie 1896) or fasciculi (Constantz 1989).
The diameters and morphologies of individual aragonite fibers are taxonomically distinct.
A number of sclerodermites growing upwards together in the plane of the upfolded tissue
develop into a vertical spine called a trabecula. Groups of trabeculae, united with or
without intervening spaces (or pores) form the septa, the primary structures of the coral
skeleton. Each trabecula depicted in Figure 2 terminates in a dentation at the growing tip of
the septum. The dentation is made up of a delicate array of fine spikes splayed like fingers
on a hand (Fig. 3). At the center of each spike is a vertical line of calcification centers (Fig.
4). Aragonite fibers in fasciculi grow out at low angles from each calcification center until
they meet crystals growing out from the calcification centers of adjacent fingers, at which
point mutual interference prevents further growth. Addition of material at the tip of each
dentation lengthens the fingers and thus the trabecula. This is how the skeleton extends.
Growth of aragonite fibers fills in spaces between the fingers which consolidate basally to
form the hand and eventually the spine shown in Figure 2. The spine will continue to
thicken for as long as the aragonite fibers are in contact with tissue (Barnes and Lough
1983). In porous skeletons such as
Porites
, fingers from adjacent dentations link laterally at
regular intervals to form horizontal supporting rungs called synapticulae (Fig. 3).
Centers of calcification
Examination of thin-sections of coral skeleton in transmitted light indicates that the
fasciculi emerge from dark “blobs” first recognized as calcification centers by Ogilvie
(1896) (Fig. 4). Vertical lines of calcification centers in trabeculae form either discrete
spots (e.g.,
Porites
spp.), continuous lines (as in
Lophelia
spp.) or some combination
thereof that defines the arrangement of the trabecular axes and thus, the septal structures
upon which the classification of the Scleractinian suborders are broadly based (Wells
1956). Apart from their taxonomic usefulness, calcification centers are fundamental to
any model for coral mineralization because they are traditionally considered to be
nucleation sites for growth of the aragonite fibers (Bryan and Hill 1941; Gladfelter 1983;
Constantz 1986, 1989; Constantz and Meike 1989; Le Tissier 1988; Cohen et al. 2001).
Within the calcification centers are submicron-sized granular crystals (Constantz 1986,
1989; Cohen et al. 2001) (Fig. 5) bundled into discrete “nuclear packets” each 2
m
across. Examination of coral skeleton both in transmitted light and with SEM reveals the
intimate relationship between centers of calcification and the fasciculi, the simplest
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4
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