The nature of skeletal density banding in scleractinian corals: fine banding and seasonal patterns

Abstract

X-radiographs were made of slices cut from the growth axes of two 0.4–0.5 m diameter colonies of Parites lobata, and of a slice cut from a 720 mm long core drilled from the growth axis of a large colony of P. solida. X-radiographs of the core-slice of P. solida were made with the slice tilted lengthways at several different angles. The annual density banding patterns for the two colonies of P. lobata revealed that one grew exceptionally quickly (≈ 19 mm · yr −1) while the other grew more slowly than is usual ( ≈ 8 mm · yr −1). The fast-growing colony maintained a smooth outer surface while the slower-growing colony displayed the more usual bumpy outer surface throughout most of its growth. X-radiographs of the smooth-surfaced fast-growing colony of P. lobata showed a series of alternating dense bands ≈ 1 mm wide and less dense bands up to 3 mm wide. Similar fine bands have been previously reported as lunar banding within the familiar annual density banding pattern in massive corals of one dense band and one less dense band. An annual pattern appeared to be present as groupings of the fine dense bands. The fine bands were clearly displayed because the colony grew unusually quickly, and because it maintained a relatively smooth outer surface. As a result, the fine bands were aligned, through the thickness of the skeletal slice, with the X-ray beam that displayed them. Fine bands were not so apparent in X-radiographs of the bumpy-surfaced colony because they were seldom aligned with the X-ray beam. The annual banding pattern in Porites, and in many, if not all, massive corals, appears to be composed of such fine bands. The fine bands may not necessarily represent lunar periodicity. The thickness of slices removed from colonies for X-radiography, together with the alignment and curvature of fine bands within the slices, tends to merge or overlap the X-ray image of individual fine bands and, as a result, emphasizes seasonal patterns in their width, frequency and, perhaps, density. The fundamental nature of the fine banding pattern was demonstrated by X-radiographs of the core-slice of P. solida taken at different angles. At certain angles, the fine bands were displayed in regions where, at other angles, only the familiar annual density pattern had been seen. Fine bands appear to be generated at the outer growth surface of colonies and may represent a direct and immediate response to environmental factors.