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Description

"Chrysogorgia antarctica, new species
Figs. 1-3

 

Description.—The type material consists of a large colony and several dozen smaller branches, presumably fragments of the same colony. The colony consists of a main stem 23 cm in height and 5.94 X 5.41 mm in diameter at its broken base, the holdfast not being present. A cross-section through the base reveals approximately 277 smooth, concentric lamellae, occurring at an average distance of every 10.7 μ.m. These smooth lamellae become slightly undulating near the surface of the axis, resulting in shallow longitudinal grooves on the surface of the axis. At a height of 7 cm the main stem bifurcates, the angle between the two re­sulting secondary stems being only 200; at a colony height of 19 cm (and stem diam­eter of 3.1 mm) both of these stems also bifurcate, resulting in four stems. Based on the numerous broken branches it would ap­pear that another two or three generations of dichotomous stem branching occurs. The thick, lower stems arc straight (not cork­screw or spiral in shape, as in most chry­sogorgiids), bearing short, often broken, di­chotomously branching side branches that are rarely more than 1 cm (or two inter-nodes) in length. These short side branches do not bear polyps, and those in the vicinity of the first stem bifurcation anastomose with the other secondary stem, producing a semi-enclosed cavity for a polychaete worm tube. The branching sequence of these short branches from the thick main stems is not clear. However, the upper, more slender stems, near and above the second stem bi­furcation, ascend in a spiral corkscrew man­ner which is more typical of the genus, and the branching sequence is predominantly 1/ 4R, each successive branch originating from the stem being offset by 900 to the right (viewed from above) from the preced­ing, resulting in roughly four longitudinal rows of branches. The I/4R branching se­quence is not as clear-cur as in most other congenerics, but is the predominant pattern. The distance between branches is 2.0-2.5 mm, resulting in an orthostiche interval be­tween aligned branches of 7-10 mm. The dichotomously branched side branches in the distal portion of the colony are bushy and often anastomose with one another, have up to 17 nodes, and are up to 60-65 mm in length. The internodes are 1-3 mm in length, but the slender (0.1 mm in di­ameter) terminal twigs may be up to 10 mm in length. Branching is highly asymmetric, usually only one internode of each dichot­omy continuing to branch again. The stems and branches have a golden-yellow luster. Polyps are sparse, occurring predominantly on the terminal twigs: usually one occurs at the tip of each terminal twig, sometimes two occur on the terminal twig, and rarely three polyps will occur on a long terminal twig. Rarely a polyp will also occur at the node that leads to a terminal twig.

 

Polyps are elongate and cylindrical, the distalmost polyp on a twig up to 2.7 mm in height and 1.2 mm in diameter, whereas more proximal polyps are usually smaller. about half this size. The base of some pol­yps is slightly enlarged into a brood cham­ber, containing 6-8 yellow developing oo­cytes, each about 0.35 mm in diameter.

 

All sclerites are unornamented (smooth) scales with extremely finely serrated edges, their size and shape differing slightly within the colony. The edge serration is more of a pleating than a true serration, each small ridge being oriented perpendicular to the sclerite edge and only 2.5-3.0 μm apart (Fig. 2A, right). Coenenchymal sclerites consist of small, elliptical scales 0.10-0.13 mm in length and 0.025-0.040 mm in width, the smaller value of the width cor­responding to the medial constriction. The coenenchyme of both stems and branches is covered by numerous, closely-spaced cnidal papillae (nematozooids), each 0.15-0.20 mm in diameter and height. The sclerites of the brood chamber and/or polyp wall are larger and broader scales, 0.27-0.37 mm in length and 0.07-0.12 mm in width, also usually slightly medially constricted, the lower value of the width corresponding to the constriction. They are transversely to obliquely arranged on the body wall and slightly overlapping. The base of each of the eight tentacles contains a conspicuous, elongate, biconvex region devoid of scler­ites, about 0.5 mm long and 0.1 mm wide, which allows for unimpeded outfolding of tentacles when expanded (Bayer & Stefani 1988). Each naked region is flanked by a pair of elongate, longitudinally-arranged scales up to 0.48 mm in length. one end being slender (0.05 mm) and rounded, the other end wider (up to 0.12 mm), thicker, and angular in shape (Fig. 2E). There is no edge serration on these sclerites. The nar­row portion of these sclerites is quite thin (about 0.012 mm), the wider end, which is arranged in the proximal position of the ten­tacle, is thicker, and is slightly convex on its outer surface and slightly concave on its inner surface. The remainder of each ten­tacle is covered with roughly rectangular to ellipsoidal scales 012-0.17 mm in length and 0.04-0.05 mm in width: pinnular scales are equal in length, but medially constrict­ed, reduced to a medial width of 0.022­-.025 mm.

 

The mineralogical composition of the axis is Mg-calcite (9.7 mole % MgCO3), which is similar to values obtained for two other chrysogorgiids by Bayer & Maclntrye (2001). There was no holdfast available to analyze, but the composition of the sclerites was also Mg-calcite (9.1 mole % MgCO3), which is consistent with the polymorph found in all octocoral sclerites samples to date (Bayer & Macintrye 2001).

 

Discussion.—Chrysogorgia is one of the most speciose of the approximately 280 al­cyonacean genera, its 60 species (see Cairns 2001) ranking it as seventh or eighth in spe­cies richness. Versluys (1902) facilitated comparison among species by dividing the genera into three groups based on the type of sclerites in the body wall and tentacles. C. antarctica clearly falls within Group C, also called the "Squamosae typicae," its 15 species being characterized by having only scales (no rods or spindles) in both body wall and tentacles. Versluys further subdi­vided this group into subgroupings based on branching sequence, i.e., 1/3L, 1/4L, and 2/5L. C. antarctica has a branching se­quence of 1/4R, which distinguishes it from all other species in its group. Several spe­cies in Group B have a 1 /4R branching se­quence (see Cairns 2001), but species in that group are characterized by having rods and/or spindles in their tentacles. C. antarctica also appears to be unique in the repeated dichotomous branching of its main stem, and in the unique shape of the ten­tacular scales that border the naked region. And, although not unique to C. antarctica, the presence of branch nematozooids is rare in this genus.

 

The concentric axial lamellae are as­sumed to be laid down with temporal reg­ularity, but the time interval between each lamella can only be speculated. The growth rate of only one chrysogorgiid, C. agassizii, has been measured by repeated in situ ob­servations (Vinogradova 2000). He found its annual growth in height to be about one cm. The holotype of C. antarctica is 23 cm in height but was probably at least another 10 cm taller if its distal branches were in­tact, resulting in an estimated height of 33 cm and a hypothetical age of 33 years. SEM of a polished and partially decalcified cross-section through the base of the axis (Fig. 3) shows that the width of a calcified growth band between lamellae varies from 5.2 to 15.5 μm. averaging 10.74 μm (σ = 2.92). If the greater radius of the axial stem is 2.97 mm, then approximately 277 growth bands could be expected to be present; these bands and thinner darker lamellae are visi­ble even with a dissecting microscope at x50. Using these approximations, this number of growth bands laid down over 33 years would yield an average increment of one band every 43.5 days, or a range of 31.8 to 55.5 days if the range of the stan­dard deviation is used. This roughly lunar periodicity is quite different from the an­nual banding found in shallow-water plex­aurid species (Grigg 1974, Opresko 1974).

 

Etymology.—Named for the general re­gion from which the type was collected.

 

Material eAatained.—One colony in nu­merous fragments (holotype), Eltanin 27­1901, USNM 82103. Type locality: 76°30'S, 174°54'E (off Cape Crozier, west­ern Ross Sea, Antarctica), 445-448 m, 20 Jan 1967.

 

Distribution.—Known only from the type-locality."

 

(Cairns, 2002: 217-222)

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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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