Paralophaster densus (Fisher, 1940) nov. comb.

FIGURE 19A – E

Lophaster densus Fisher, 1940: 173; A.M. Clark 1962: 50.

Diagnosis (based on USNM 10102 and Fisher 1940)

Body stellate, R/r=2.53, arms short, triangular, Disk thick with convex rays (Fig. 19A C). Interradial broadly curved. Abactinal paxillae abundant, close-set, each with 15–30, usually 20–30 spinelets, these forming a close-set canopy over the abactinal surface giving a hirsute appearance (Fig. 19A, B). Paxillar base with four lobes. Superomarginals alternate with inferomarginals, similar but larger than adjacent abactinals, approximately 10–18 spinelets, thick, capping each plate (Fig. 19B). Inferomarginals 20 per arm side, larger than superomarginals, approximately 10–18 spinelets, blunt, thick (Fig. 19C). Actinal interradial region with three chevrons of plates, each with spines, 1–9, variably showing higher numbers proximally then decreasing distally. Furrow spines 2–5 with highest number proximally (Fig. 19D). Subambulacral spines, 4–5 in curved, transverse series, increasing in size from outer to inner. Oral plates with furrow spines 6–8, suboral spines, 9–10 in weakly defined series adjacent to median suture.

Comments

Following placement of Lophaster densus within the Paralophaster clade (Mah & Foltz 2011b) species position as based on morphology is reviewed. Characters from Fisher (1940), H.E.S. Clark (1963) and Koehler (1912a), together with new specimens are in accord with the molecular treatment. On the paratype, superomarginal paxillae, if present, are consistent with those on the abactinal surface, but are apparently more evident on the holotype, which Fisher (1940) described as having a distinct set of superomarginal plates which intercalate between the larger inferomarginal plates.

As indicated earlier, Fisher’s (1940: 175) original diagnosis of Paralophaster only included those species without “differentiated superomarginal plates” (paxillae) which distinctly differs from later accounts (A.M. Clark, 1962) that re-interpreted this character to mean similar in size to abactinal paxillae (as discussed earlier). As such, this has come to include additional species which Fisher did not originally include in his taxonomic definition for Paralophaster . This appears to be the case for Lophaster densus . Fisher (1940) made note that this species was collected from the same station as Myoraster (= Paralophaster) antarcticus, commenting on similarities of the paxillae.

Characters diagnostic of Lophaster densus (Fisher 1940: 174) are similar to those of Paralophaster antarcticus (Fisher 1940: 182 as Myoraster) suggesting the two species are closely related, and possibly synonymous. This includes the high numbers of spinelets on the abactinal and marginal paxillae (15–30 versus 30–40, or “up to 35”, Fisher 1940: 174), overlapping or similar numbers of marginal plates per arm (20–30 versus 20–40) and overlapping or similar numbers of furrow and subambulacral spines (3 to 4 versus 2 to 5). Based on Fisher (1940) and examination of small sized specimens, body shape in L. densus is less stellate (R/r=2.53 at R= 1.9 cm) than that of Paralophaster antarcticus .

Some differences remain evident, such as the difference in paxillar spines on the abactinal surface and molecular position. Mah & Foltz (2011b) showed Lophaster densus occurring on a sister branch to the clade including Paralophaster godfroyi and Paralophaster antarcticus suggesting a more distant phylogenetic position to either of those two species. Further sampling and understanding of morphological variation are desirable.

Material Examined

USNM E 10102, Paratype, Bransfield Strait, South Shetland Islands, Antarctica, Southern Ocean. −63.2889, −59.8042, 200 m. Coll. R/V Discovery, 2 March 1927. 1 wet spec. R=0.75 r=0.3.