Glabraster A.H. Clark 1916

Glabraster A.H. Clark 1916

Figure 5 A–F

Glabraster A.H. Clark 1916: 122

Type species: Porania magellanica Studer 1876 (a synoym of Porania antarctica Smith 1876)

Diagnosis. Body weakly pentagonal to strongly stellate (R/r= 1.51–2.85). Arms, triangular in shape, varying from well-developed and elongate, contiguous with body to short and emerging abruptly from well-developed disk. Body generally tumescent. Prominent skin overlying abactinal, marginal and actinal surfaces. Plate outlines obscured in wet specimens. Abactinal plates forming widely-spaced, reticulate pattern. Skin present between widely reticulate plates bearing small ossicles among many of the open spaces. Spines present or absent on abactinal plates. When present, spines are prominent and present individually on each plate. Spines, short with thorn-like or blunt tips tend to be most prominent on plates where confluences of bands are present but they are also present in several individuals on individual abactinal bands. Marginal plates imbricate, differing in size. Superomarginals 3x–4x larger than inferomarginals.

Superomarginal plates, polylobate in shape, plate boundaries obscured by thickened tissue in wet specimens. Superomarginals with no spines, but spinelets variably present. Inferomarginal plates, irregularly polygonal, rounded and pinch out two to four plates before terminus in larger specimens but form nearly complete series in smaller individuals (R> 1.5 cm). Inferomarginals form actinolateral fringe, with one or two flattened spatulate spines present on each plate.

Actinal plates, imbricate, forming transverse series between adambulacrals, plates skin covered, otherwise surficial accessories absent in most (i.e., no granules, no spinelets, etc.). Furrow spine (s) pointed, narrow, sheathed in skin, variably one or two, set within the furrow. Subambulacral spines, one or two, paddle shaped, larger than furrow spines (approximately twice as thick) with blunt tips. Subambulacral tips vary from more widely flattened to strongly cuved (u-shaped in cross section).

Taxonomic comments. Our molecular results (Fig. 1) support separation of the Antarctic Porania antarctica from the Arctic/subArctic Porania pulvillus. This is consistent with several significant morphological differences between the two species, which has historically been perceived as interspecific variation (e.g., Clark and Downey 1992).

A.H. Clark (1916) created Glabraster to accommodate the southern hemisphere Porania antarctica Smith 1876 and Porania magellanica Studer 1876 (later synonymized). However, according to A.M. Clark (1993: 229), Glabraster was synonymized by Fisher (1940) and the name was not used for other citations of “ Porania ” antarctica in the literature [e.g., Koehler (1920)]. There was apparently no objection to its use in the literature prior to Fisher’s synonymy. The diagnosis of Glabraster is consistent with morphological characters separating it from the northern Porania providing us with a basis for reinstating it herein.

Porania pulvillus possesses an abactinal skeleton composed of tightly reticulate, semi-imbricate, plates when compared against Glabraster, which has a well-defined widely spaced reticulate abactinal skeleton (Fig. 5 A–F). Glabraster antarctica also has adambulacral furrow spines with flattened and blunt tips (Fig. 5 B), as opposed to those P. pulvillus which are blunt and pointed, but round/oval in cross section, sometimes with cone-like shape (Fig. 6 B)

The separation between Porania and Glabraster implies that characters which have been historically been used as support for the two species within a single genus should be attributed either to plesiomorphy (within the Poraniidae), such as thick skin, smooth skin, distinctive primary plates, or alternatively to convergence (e.g., ventrolateral flange formed by the inferomarginal plates). Both taxa occur in cold-water, low-latitude settings, which provides a plausible setting for similar environmental influence that could result in parallel morphological expression.