Desmophyllum dianthus ( Esper, 1794 )

Stolarski, Jarosław, 2003, Three-dimensional micro- and nanostructural characteristics of the scleractinian coral skeleton: A biocalcification proxy, Acta Palaeontologica Polonica 48 (4), pp. 497-530 : 508

Desmophyllum dianthus ( Esper, 1794)

Morphology.—The septal microarchitecture of D. dianthus is similar to that of S. paliferus . The distal septal margin is smooth in “macroscale” however, as viewed in SEM close−ups, its surface is composed of irregularly distributed rounded “patches”. They cover the entire length of the septal margin which is ca. 50 µm wide. The straight course of the distal margin may deviate in zigzag fashion towards granules on the septal flanks, however these are generally rare in the adult stage of corallum.

Transverse sections.—In TLM, three zones of different light−transparency properties and colors are distinguishable in transverse section of the S1 (made in half of its length) and part of the trabeculothecal wall ( Fig. 7C ): (1) dRAF zone ca. 15–20 µm wide, with a slightly wavy course and dark brown coloration (“dark line”), (2) zone (ca. 100 µm wide) of fibers with brownish zonal coloration (however, lighter than in dRAF zone), and (3) zone of transparent and nearly colorless fibers that form the main mass of the skeleton. The last two fibrous zones differ only in coloration, but show the same orientation of fibers or even their continuation. Generally, fibers are perpendicular to the dRAF zone, as in TD region in other corals discussed here ( Fig. 7C, D ). Fractures and/or very distinct natural borders occur between some bundles of fibers ( Fig. 7C , red arrows). In MFM, skeleton stained with acridine orange dye exhibits a threefold chromatic response ( Fig. 7E ): (1) bright green fluorescence of narrow dRAF zone, (2) slightly weaker than of the former zone and green−yellow fluorescence of zone with brownish fibers as seen in TLM, and (3) near orange fluorescence of fractures or natural borders between fiber bundles in zone of transparent and colorless fibers as seen in TLM ( Fig. 7E , red arrows). In SEM ( Fig. 7A ), polished and etched sections show distinct negative relief of the dRAF zone (ca. 15–20 µm wide, often differentiated into dCRA), whereas fibrous zones show generally positive etching relief, except for the borders between layers of fibers. Fibers enclosing the dRAF zone form a distinct region ca. 20 µm wide, beyond which layers of fibers show much smaller (ca. 5 µm wide) growth increments.

Longitudinal−radial sections.—In TLM, sections made in RAF plane show narrow dark brown “strands” (ca. 10–15 µm wide) arranged fanwise in the plane of the section ( Fig. 7F ). Similarly, as in the longitudinal sections of the above described corals, the undulating course of the RAF in radial and longitudinal directions makes the dRAF visible only in some surfaces. In higher magnification, extremely fine (ca. 2 µm long), regular constrictions of “strands” are visible; however, perhaps because of the standard (not ultra−thin) thickness of the section, it cannot be clearly stated whether constrictions correspond to regular alternations of transparent colorless and more opaque brownish layers as in other corals. Also inconclusive in this respect were observations in MFM, however, similarly as in other corals investigated, only the areas of brownish “strands” exhibited bright green−yellow fluorescence ( Fig. 7G ). In SEM, polished and etched sections show ca. 10–15 µm wide “strands” ( Fig. 7B ) that generally have a negative etched relief; occasionally individual “strands” are not clearly delimited from each other, but rather form broader areas of negative relief. In the longitudinal direction “strands” show the alternation of zones more and less susceptible to etching. In places this alternation is very dense and regular (every 2 µm), and occasionally are less regular with wider zones more susceptible to etching (ca. 10 µm and more).

Figures

Fig. 7. Desmophyllum dianthus (Esper, 1794). ZPALH.23/3. Recent, southern Indian Ocean (NE St. Paul Island), MD50 cruise, Stat. 32/CP 145, 38°40.66’S, 77°35.47’E, 825–1020 m. A, B. Polished and etched septum sectioned transversely (A) and longitudinally in RAF plane (B). dRAF form chain of dCRA (“centers of calcification”) (A), which sectioned longitudinally exhibit alternating etching relief (B). dCRA (A) are composed of apparently non−crystalline, “blurry” material. C–E. Transverse polished section of septum in TLM (C, D) and MFM (E) micrographs. In TLM (C, D) organic components differentiate into dark−brown zone of wall and septal dRAF and light−brown, banded zone enclosing dRAF; in MFM (E) they exhibit lighter, green−yellow, and darker, greenish fluorescence, respectively. Contact zone between bundles of fibers (also with apparent fractures) contain organic matter that forms a fluorescent network (E); examples highlighted with red arrows. F, G. Septum longitudinally sectioned in RAF plane in TLM (F) and MFM (G). Minute growth increments (less than 5 µm) appear as “denticulations” on fluorescent bars (G). Growth direction within septum indicated by black arrow.