“Adelopora pseudothyron, sp. nov.
(Figs. 1-22)
Description
Corallum usually uniplanar; however, some colonies maintain a loose, bushy branching (figs. 1-4). Largest colony 3.8 cm tall but other fragments with greater basal branch diameters indicate that larger colonies do exist. Branches tend to avoid coalescing; however, in densely branched colonies the branches sometimes anastomose, forming a reticulate fan (fig. 3). Branches round in cross section except toward the base of the colony, where the thick basal branches are slightly elliptical in cross section. Distal branches about 0.9 mm in diameter, largest basal branch up to 1 cm in greater diameter. Small diameter branches result from gradual tapering of larger branches or grow directly from the sides of larger diameter branches. A cross section of a distal branch reveals three to four large coenosteal canals of 50-90μm diameter and numerous smaller canals of 15-20 μm diameter (fig. 22). Axils of branching usually U-shaped. Colonies firmly attached by a thin, encrusting base to objects such as medium pebble-sized rocks, the scleractinian coral Solenosmilia variabilis, and the stylasterine Errina sp.
Coenosteum smooth, dense, and porcelaneous; always white. Discontinuous rows of coenosteal slits define longitudinal, parallel strips of coenosteum , often bifurcating and rejoining adjacent strips in their progression toward the branch tip (fig. 5). Coenosteal slits 3-4 μm wide, 10-100 μm long, and up to 50 μm deep. Larger, more continuous slits in thicker, older branches. The slits define strips of coenosteum between 53-70 μm wide. Texture of coenosteum composed of imbricated platelets, the free edges of which are directed distally (fig. 6). Width of each platelet is width of coenosteal strip; approximately 75-90 leading edges occur per mm. There are no coenosteal granules. On older branches of greater diameter, the imbricated platelets are often worn down, the platelets appearing as transverse rugae without the overlapping effect.
Gastropores occur only on branch tips (figs. 11-12) and at branching axils. The presence of a gastropore on a branch tip seems to cause the branch to bifurcate, which establishes it at this axil. Gastropore tubes cigar-shaped: straight, cylindrical, about 1 mm in diameter, and abruptly truncated in a gently rounded cul-de-sac (fig. 18). There is no gastrostyle. Small diameter coenosteal canals penetrate the wall and base of the gastropore.
Each gastropore is covered by a freely hinged operculum (figs. 7-14). Opercula are variable in shape but generally are round to elliptical, with one edge of the ellipse modified into a straight articular edge. The length of an operculum (measured from the hinge to the free end) varies from 0.39-0.58 mm, the width from 0.35-0.64 mm. Usually the length is greater than the width of any given operculum (i.e., length : width ratio about 1:1); however, this ratio varies from 1.34-0.71, with both extremes occurring on the same branch. Opercula are approximately 70 μm thick at their edges but considerably thinner toward the centre because of the great concavity on the lower side. The straight, articular edge of the operculum is modified into a rod, which projects beyond the ellipse as a short nub on either side (figs. 7-8). The rod fits into a coenosteal groove (fig. 16) and the nubs are freely retained within coenosteal cavities (fig. 17), allowing an articulation of the hinged operculum. When closed, the top of the operculum is flush with the branch surface and the free edge rests on a small ledge encircling the upper gastropore tube. Opercula can swing open approximately 90° before being checked by the coenosteum behind the hinge. A linear depression along the surface of the articular edge of the operculum allows a later checking contact and therefore a slightly greater opening of the operculum. A slit, occupying the middle third of the lower side of the operculum, runs parallel and adjacent to each edge (figs.7-8). These slits are presumed to be the scars of opercular retractor muscles.
Dactylopores, shaped as perforated tubercles, occur primarily on distal branches. The pores measure between 23-35 μm in diameter, whereas the tubercles may be up to 0.1 mm wide and 0.15 mm tall, the tallest dactylopore tubercles occurring on branch tips (figs. 11-12). Tubercles away from the branch tip are usually only 0.02-0.03 mm tall.
Ampullae of two different shapes occur on most colonies. One form is hemispherical and quite prominent, 1.06–1.22 mm in diameter (fig. 21). These often occur in clusters. The other kind is similar in size and shape but augmented by a small lateral extension, which is abruptly truncated (fig. 19). The truncated face, about 0.23 mm in diameter, is concave (fig. 20) but was never observed to be ruptured or to have any pores. The orientation of the extension is random.
The tissue was studied by gross dissection and histological sections of decalcified branches, and by thin sections and acetate peels of epoxy-embedded whole branches. The complex, anastomotic coenosteal canal network remained intact in the decalcified branches, and even the opercular shape is maintained by its canal system; however, the preservation was not adequate to examine the polyp in any detail. All that was visible in the gastropore was a thin band of tissue (opercular retractor muscle?) extending from the base of the pore, continuing along the side of the pore away from the hinge, and joining with the free edge of the operculum.
At least two types of nematocysts are present. Near the coenosteal surface are elongate but slightly oval nematocysts measuring 17.2-21.9 μm x 4.1-5.3 μm. Deeper in the branch (within a gastrozooid), a smaller rod-shaped form predominates, measuring 11.2-14.1 μm x 2.9-4.2 μm.
Remarks
Two of the stations at which A. pseudothyron was collected (Eltanin-1343, 1345) were made on a seamount in the Subantarctic South Pacific, which supports a flourishing deep-water coral bank (Cairns, in press). The framework scleractinian coral, Solenosmilia variabilis, supports a wide diversity and great concentration of animal life, including 32 groups of benthic invertebrates representing 13 phyla. Ordinarily, deep-water corals are found in low density and in areas of low faunistic diversity. The enormous diversity and abundance of life on a deep-water bank probably exerts a much higher predation pressure on the coral and it. is hypothesized that the evolution of a tightly fitted, defensive operculum is an adaptation to this predation pressure. It is ironic, therefore, that the operculum of Adelopora, perhaps the best defended stylasterine known, is sometimes penetrated by circular bore holes (figs. 9-10). These holes were originally thought to be produced by gastropod radulae but the small diameter of the pores (0.12 mm) argues against a molluscan predator (pers. comm., E. Yochelson and J. Rosewater).
Etymology
The specific name pseudothyron (Greek: secret door), refers to the gastropore opercula, which, when closed, are usually quite inconspicuous.
Types
HOLOTYPE: Eltanin sta. 254, 59°49'S, 68°52'W (seamount in Drake Passage), 512-622 m, 10 Oct. 1962, USNM 60198.
PARATYPES: Eltanin sta. 254, USNM 60128, BMNH 1981.8.1.1, ROMCN-B 102, RMNH Coel. 14114; Eltanin. sta. 1343, 54°50'S, 129°50'W, 567-824 m, 7 Nov. 1964, USNM 60131; Eltanin sta. 1345, 54°50'S, 129°48'W, 915-1153 m, 7 Nov. 1964, USNM 60132: Eltanin sta. 1521, 54°09'S, 52°08'W, 419-483 m, 30 Jan. 1966, USNM 60130; Eltanin sta. 25-325, 46°00'S, 83°59'W, 742m, 9 Oct. 1966, USNM 60129; Eltanin sta. 25-326, 46°04'S, 83°55'W, 298m, 9 Oct. 1966, USNM 60133.
Distribution
Known only from four Subantarctic seamounts from the Scotia Ridge, Drake Passage, off Chile (Chile Rise), and South Pacific (Heezen Fracture of Eltanin Fracture Zone). Bathymetric Range: 298-915 m.”
(Cairns, 1982: 71-81)
