Chemistry Reference
In-Depth Information
Figure 9.
SEM of the tips of growing
septa of
Porites lutea
over the diurnal
cycle. Extension occurs by growth of fine
spikes like fingers on a hand (a). At night,
calcification results in elongation of the
fingers by accretion of new calcification
centers at the growing tips. Thus the septal
surface appears spiky (c). During the day,
the spaces between adjacent fingers is
filled in by outward growth of aragonite
fiber bundles. By the end of the day the
surface of the septum has a smooth
appearance. (b). Images courtesy of Dr
David J. Barnes (Australian Institute for
Marine Science, pers. comm)
sites for growth of the fasciculi. However, fusiform crystals have not been identified in
any other scleractinian species examined thus far. Throughout the Scleractinia, the
trabecula axes originally described by Ogilvie (1896) are occupied by submicron-sized
granular shaped crystals (Constantz 1986; Cuif and Dauphin 1998; Cohen et al. 2001).
Indeed, the distribution, origin and function of the fusiform crystals described by
Gladfelter are intriguing and should be investigated further.
Constantz (1989) suggested that Gladfelters fusiform crystals are inorganically
precipitated marine cements that nucleate at random on non-zooxanthellate portions of
the corallum and are thus “non-biogenic” crystals precipitated at night in the absence of
zooxanthellate activity. Indeed, the fusiform crystals in
A. cervicornis
bear a remarkable
resemblance to inorganically-precipitated crystals in high magnesium calcite cements
(Given and Wilkinson 1985). However, it is difficult to see how random precipitation
would produce the organized framework of fusiform crystals that Gladfelter describes.
Their architectural arrangement, albeit flimsy, indicates a process over which the coral
exerts some degree of control.
Models of skeletogenesis: physicochemical
The morphology and arrangement of the aragonite fibers in the sclerodermites bear a
remarkable resemblance to spherulitic crystal morphologies common to all inorganic
crystalline systems (Fig. 10). The analogy between spherulites in rocks and the
spherulitic morphology of coral crystals first discussed by Bryan and Hill (1941) is the
basis of a physicochemical model of coral calcification that has dominated thinking about
