Chitonaster Sladen 1889

Chitonaster Sladen,1889

Sladen, 1889: 282; Spencer & Wright, 1966: U58; A.M. Clark, 1993: 250; (as Chitonaster) H.E.S. Clark, 1971: 545; A.M. Clark, 1993: 275 (as Pentoplia)

Diagnosis. Body stellate. Abactinal plates flattened, scalar, overlapping and forming relatively thin body wall. Fasciolar channels weakly present to absent. Plate surface bare in most or with single large granule, blunt cylindrical and/or pointed spines, peripheral accessories absent. Marginal plates face laterally. Large pedicellariae, either bivalve or with enlarged flange-like valves (in C. felli, Fig. 5 B, D). Furrow spines one to three, typically two.

Comments. Chitonaster occurs only in the Southern Ocean and adjacent waters and was among the rarest taxa encountered by Manjon-Cabeza et al. (2001). Chitonaster Sladen 1889 has not been reviewed since its original description and significant collections of new material have yielded further insight into classification and diversity within Chitonaster. This includes the synonymy of Pentoplia with Chitonaster, the discovery of two new species, and a summary of character-based boundaries for Chitonaster species (Figure 3).

Several characters in Chitonaster are similar to those in Eratosaster and Calliaster, including the elongate furrow spines, the relatively small actinal intermediate plate area, the presence of spines on the abactinal and marginal plates as well as resemblance in arm and disk shapes.

Synonymy of Pentoplia. Pentoplia felli was described by H.E.S. Clark (1971) as the monotypic member of the Pentopliidae, a family that has since been synonymized into the Goniasteridae by Blake (1987).

H.E.S. Clark (1971: 546) indicated that Pentoplia was reminiscent of Hippasteria and Chitonaster but did not elaborate on specific character comparisons, save for the absence of “characteristic” pedicellariae on Hippasteria. Pentoplia shares several characters with Chitonaster, including flat scalar, overlapping, abactinal plates, short blunt conical spines on the abactinal and marginal plate surface, two to three furrow spines and a single large subambulacral per adambulacral plate, and the presence of prominent pedicellariae on the actinal surface. Pentoplia lacks the abactinal and marginal plate morphology as well as the large spines that are present in Hippasteria. Clark’s description emphasizes Pentoplia ’s enlarged pedicellariae as the primary feature distinguishing it from other asteroid taxa. Although other species of Chitonaster lack identical pedicellariae, they do possess enlarged pedicellariae that are very similar in appearance. Specimens present in CASIZ 162495 possess enlarged bivalve pedicellariae similar to those in Pentoplia in each interradial region suggesting an intermediate status for these taxa. Pentoplia felli ’s enlarged actinal pedicellariae is a unique autapomorphy for the species. However because several characters are shared between Chitonaster and Pentoplia it is argued here that the latter is a junior synonym of the former. Pentoplia should be entered into the synonymy of Chitonaster, retaining Chitonaster felli as a valid species.