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Description

 

"Sporadopora dichotoma; Moseley, 1879, pp. 429-440, pl. 34, figs. 1, 2, pl. 35, figs. 1, 2, 9, pl. 36, p1. 43, figs. 1-9, 12, pl. 44, figs. 13, 14; 1881, pp. 36-47, 83, pl. 1, figs. 1, 2, pl. 2, figs. 1, 2, 9, pl. 3, pl. 10, figs. 1-9, 12, pl. 11, figs. 13, 14.--Weill, 1934, p. 484.-- Boschma, 1957, pp. 60-61; 1964, pp. 61, 62.--Squires, 1962, p. 136.--Lowe, 1967, pp. 102-104, pl. 9, figs. a-c.--Boschma and

 

Lowe, 1969, p. 15, pl. 5, map 5.

 

 

? Sporadopora dichotoma; Hall, 1898, pp. 177, 178.

 

 

Description. Colonies are large, robust, and flabellate, with regular, dichotomous branching. In larger colonies, the sides of adjacent

 

branches sometimes anastomose for a short distance, later becoming independent again. Terminal branches are round in cross

 

section, gradually tapering from thick branches to cylindrical, blunt, rounded tips, 4-20 mm in diameter. Basal branches are

 

us­ually elliptical in cross section, the greater axis of the ellipse being directed in the plane of the fan. The first dichotomous

 

branching does not occur until the colony has reached 4-6 cm in height, this basal branch usually being devoid of any pores.

 

Branching axils are usually a gentle U shape. The tal­lest colony known [Moseley, 1881, Plate 1, fig. 2] is 14 cm tall with a basal

 

branch diameter of 28.3 x 25.8 mm. The thickest basal branch (syntype: BM 1957.2.28.42) is 4.45 cm in diameter. Colonies are

 

attached to small rocks and scleractinian corals (e.g., Flabellum thouarsii) by a nonexpansive base.

 

 

The colony surface is smooth, composed of a white, porous, vermiculate coenosteum (Fig­ure 2A). The coenosteum of older, thicker

 

branches is slightly more solid, dissected by a reticulate maze of coenosteal grooves. Small, round to elliptical, shallow pits

 

up to 0.15 mm long (Figures 2B and 2C) are scat­tered over the coenosteum. Each pit usually bears a centrally placed tubercle. The

 

func­tion of these pits is unknown.

 

 

Gastropores are round and usually flush with the surface but are sometimes raised on a mound, up to 0.25 mm high in exceptional

 

cases. Gastropores are quite variable in size, 0.20-0.62 mm in diameter. Some gastro­pores are surrounded by a ring of 10-15

 

shal­low pits, each about 40-70 hm in diameter. These may be an expression of the radial coenosteal canals mentioned by Moseley

 

[1879, p. 433] or simply nematopores. Gastropore tubes are usually oriented perpendicular to the surface, but toward the center of

 

the branch they curve downward and run longitu­dinally for quite a distance. In any branch cross section there will he a central core of

 

both gastropore and dactylopore tubes. The base cf a colony detached from its substrate often reveals its founder gastrozooid as a

 

small gastropore penetrating the center of the base.

 

 

Because of the length and curvature of the gastropores the gastrostyles often are not visible from the surface. Castrostyles are long, slender, and needle shaped, with a del­icate, pointed tip. Styles may be extraordi­narily long, particularly on branches of great diameter; however, usually just the distal part is ornamented. For example, on a style 3.9 mm long and 0.19 mm wide (H:W = 20.5), only the distal 1.47 nun is ornamented, the basal shaft being irregularly cylindrical and smooth. Styles are deeply grooved by longi­tudinal furrows, the grooves being separated by five to seven thin ridges,bearing slender, pointed spines or tufts of spines. Sometimes, as the style tapers distally, each ridge suc­cessively becomes free and bears a separate pointed tip. Thin tabulae, about 4-5 ;Am thick, traverse the gastropore at irregular intervals, providing support for the elon­gate, slender styles. Some gastropores have 10-15 tabulae; others, of equal length, have none.

 

 

Dactylopores are round and approximately 0.12 mm in diameter; however, the upper size range is unknown and may overlap that of the smaller gastropores. The only way to distin­guish the two types of pores in a dried corallum is to search for the presence of a style or tabula in the gastropores; unfortu­nately, the delicate gastroscyles often are lost in sectioning, and they sometimes fall out during bleaching. Dactylopores are always flush with the surface.

 

 

Ampullae are always internal, with no sur­face relief, even in the distalmost branches. They are elliptical in shape, up to 1 mm long, and oriented with their greater axes perpendicular to the surface. A small ef­ferent duct links the cavity to the surface. This tube produces an irregularly shaped pore at the surface, 60-70 um in diameter.

 

 

Remarks. Judging from the maximum basal branch diameter, S. dichotoma is probably the largest of the Antarctic Stylasterina.

 

Regeneration of branches from broken tips is common.

 

 

Discussion. With the exception of Hall's [1898) doubtful report of S. dichotoma and Lowe's [1967] unpublished records, this spe­cies was known only from the type-locality. This report represents the second published record and includes 16 new localities. Hall's specimens, from the Miocene of South Hamilton, Victoria, Australia, were not exa­mined by the author but were assumed by Boschma [1957] to be a different species.

 

 

Sporadopora dichotoma is similar to S. mortenseni Broch, 1942, and Squires [1962] implied that they may be morphological vari­ants of the same species. However, examina­tion of two syntypes of S. mortenseni con­vinced me that [hey are separate species for the following reasons: S. mortenseni has (1) a more delicate colony with more slender branch tips, (2) a more solid coenosteal tex­ture, never porous like that of S. dichocoma, even at branch tips, (3) incomplete tabulae, and (4) smaller gastropores. Furthermore, the species are substantially separated geo­graphically and bathymetrically.

 

 

Eguchi [1964] reported S. mortenseni from 830 m off the Riiser-Larsen Peninsula (Cape Cook), Antarctica. These small, worn speci­mens are of the correct branch diameter for this species, but the very poor condition of the specimens does not make species identifi­cation probable. The deposition of these specimens is unknown, and therefore they have not been examined by the author, but judging from the description and illustration, it is premature to list S. mortenseni from the Ant­arctic coast. It appears to be endemic to the New Zealand region at depths of 122-282 m (H. Zibrowius, personal communication, 1980).

 

 

Material examined. Eltanin sta. 339, USNM 60298; sta. 684, USNM 60101; sta. 970, USNM 52647; sta. 992, USNM 52652; sta. 1088, USNM 52651, 52644; sta. 1089, USNM 52712; sta. 1536, USNM 60102; sta. 1593, USNM 60098. Islas Orcadas sta. 575-34, USNM 60100; sta. 575-82, USNM 62574; sta. 575-91, USNM 60097. Hero sta. 715-895, USNM 60099. Vema sta. 15 (PD) 10, USNM 52646; sta. 17-59, USNM 52645; sta. 17-65, USNM 52643. WH sta. 19-76, ZIZM. Challenger sta. 320, syntypes. Other materi­al: Two syntypes of S. mortenseni, Zoological Museum, Copenhagen.

 

 

Types. Holotype not designated by Moseley; therefore all specimens from Chal­lenger sta. 320 considered as syntypes: BM 1880.11.25.167 [Moseley, 1881, Plate 1, fig. 2]; BM 1880.11.25.166, 168, 192; BM 1957.2.28.42 (basal fragment of very large diameter). Type-locality: 37°17'S, 53°52'W (off Rio de la Plata, Uruguay), 1097 m.

 

 

Distribution. Off Uruguay; Falkland Plateau; Scotia Ridge from Tierra del Fuego to South Georgia; South Shetland Islands (Map 1) Depth: 250-1498 m."

 

 

(Cairns, 1983)

 

 

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Creative Commons Attribution Non Commercial 3.0 (CC BY-NC 3.0)

© National Museum of Natural History, Smithsonian Institution

Source: Antarctic Invertebrates Website (NMNH)

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