Paralophaster Fisher 1940

Paralophaster versus Lophaster

Mah & Foltz (2011b) included three Paralophaster species, P. antarcticus, P. godfroyi, and P. lorioli in their molecular phylogenetic overview of the Valvatida. Although these three species clustered together on both two-gene and three-gene trees, as part of a single lineage, Lophaster densus was also supported. This has resulted in further study of the diagnostic characters that separate Lophaster and Paralophaster.

Lophaster has been historically characterized by the possession of distinct, more elongate, and larger marginal paxillae (i.e., marginal plates that are paxillae-like in shape), especially the superomarginal paxillae, which are clearly distinguished from the abactinal paxillae in Lophaster. This is contrasted with Paralophaster that was originally characterized by Fisher (1940: 175) as simply having “undifferentiated” superomarginals but has undergone redefinition to being similar in size or more precisely as being “…hardly if at all, larger than the abactinal paxillae” (e.g.,A.M. Clark 1962: 80). Fisher (1940) also described the genus Myoraster to accommodate Lophaster antarcticus which he argued was distinctive based on actinolateral muscle bands, in comparison to Lophaster densus which was lacking Myoraster ’s muscle bands. A.M. Clark (1962) synonymized the genus Myoraster transferring Lophaster antarcticus to Paralophaster. Paralophaster has been reported from further occurrence (e.g., Presler & Figielska 1997) and environmental accounts (e.g., McClintock et al. 2011) with no further taxonomic/systematic overviews other than the recently described Paralophaster from deep-sea North Pacific settings near Japan (Mah & Fujita 2020).

Phylogenetic analysis of the Valvatacea (Mah & Foltz 2011b), included three Paralophaster species, P. godfroyi, P. lorioli, and P. antarcticus as well as the Antarctic Lophaster densus. All four taxa were supported on a single clade, supporting the Solasteridae, by two-gene and three-gene trees.

The key diagnostic character for Paralophaster is based upon Fisher’s (1940) diagnosis which describes “undifferentiated” superomarginal plates. The description of this character has undergone modification by A.M. Clark (1962) and H.E.S. Clark (1963) as superomarginal plates which are similar or identical to the abactinal plates in size.

Based on observations of Lophaster specimens, the marginal plates show two relatively thick and elongate paxillar heads which are variably articulated or show fusion at the basal portion of each plate. This appears to be a consistent character of marginal plates among Lophaster species. Paralophaster in P. godfroyi, P. antarcticus, and P. densus when clearly present appear to be shorter and smaller overall than the inferomarginals, but closer in appearance to the abactinal paxillae. Marginal paxillae in Lophaster appear to be a function of size, length and wide spacing versus those in Paralophaster which are smaller, shorter and more crowded.

Marginal Plates in Paralophaster

Blake’s (1978) criterion defined the marginal plate series as extending from the primary on the disk to the terminal plate on the arm. Identification of plates as superomarginal versus inferomarginal plate series was based on their relative position, with the inferomarginal series identified based on its proximity to the adambulacral plate series. Marginal plate characters show two primary trends in Paralophaster, those with only a single prominent marginal plate series and those showing two, including clearly defined superomarginals. In both instances, it is assumed that both series are present but that there is some degree of character modification to the superomarginal and/or inferomarginal series.

In the former, there is only a single prominent series which, based on location and position is thought to be an enlarged inferomarginal series. A.M. Clark (1962) interpreted this as superomarginal plates which were essentially identical with the abactinal paxillae but very weakly expressed or jumbled, such as in Paralophaster lorioli. Paralophaster paucispinus n. sp. displays a more extreme case with a seemingly incomplete or possibly absent superomarginal plate series.

In the other Paralophaster, there is a well-developed, observable series of superomarginals that are present on the abactinal surface above and either adjacent or alternating with the inferomarginal plates, such as in P. godfroyi, P. hyalinus, P. antarcticus and P. densus. In these latter species, larger specimens (>R=3.0 cm) such as those of Paralophaster godfroyi, the paxillar base of the presumptive superomarginal plates were directly articulated on the surface of the trunk of the larger inferomarginal paxillae in a manner identical to those on other solasterids, such as Lophaster gaini. In Paralophaster antarcticus, superomarginal paxillae were observed as a separate series, intercalated above and between the larger inferomarginal series.

Key to the Species of Paralophaster

(0) Abactinal, marginal paxillae with abundant fine spinelets, 20 to 40. Abactinal surface with close-set paxillae presenting an almost brushy, fuzzy appearance (Fig. 18A, B, D). Marginal plates number 20–45 per arm side, (40–80 per interradius) close-set. Superomarginal paxillae present intercalated between larger inferomarginals and distinctly larger (2–4×) than abactinal paxillae (Fig. 18D).................................................................................... (1)

(0’) Abactinal, marginal paxillae with fewer spinelets, 3 to 25, mostly 10–14 (e.g., Fig. 22A, C). Marginal plates 13–26 per arm side. Superomarginal paxillae either inconspicuous or absent, only a single prominent series, the inferomarginals, present along each arm................................................................................................ (2)

(1) Body more strongly stellate with elongate arms R/r= 2.0–4.0 at R=1.0–16.0 cm. Superomarginals similar or identical to abactinal paxillae (Fig. 18).................................................... Paralophaster antarcticus (Koehler, 1912a)

(1’) Body more weakly stellate with short, triangular arms R/r=2.53 at R= 1.9 cm (Fig. 19). Superomarginals present/alternating with inferomarginals...................................................... Paralophaster densus (Fisher, 1940)

(2) Subambulacral spines with serrations, either along the sides or the upper top of the spine (Fig. 22F, G). Marginal paxillae 9–18 per arm side at R=2.0–4.0.............................................................................. (3)

(2’) Subambulacral spines might have rough surface, but without serrations (Fig. 23F). Marginal paxillae 13–30 per arm side at R=1.7 to 4.1......................................................................................... (4)

(3) Subambulacral spines with 7–12 thorny serrations along the shaft (Fig. 22F, G). Abactinal paxillae relatively short, squat with spinelets, 10–25, emerging from plate near the body surface, each with 1–3 pointed tips (Fig. 22D, E). Superomarginal plates absent or weakly present. A single prominent series of marginal paxillae present (Fig. 22C, D, E). Individuals found to brood juveniles in coelom. Fig. 22B............................................... Paralophaster lorioli (Koehler, 1907)

(3’) Subambulacral spines more weakly developed thorny serrations, 3–5 at the tip of each spine. Abactinal paxillae with elongate shaft, hyaline spinelets, 8–25 with single to bifid-tipped (i.e., fork-like) points. Superomarginal and inferomarginal paxillae both evident, similar in scale, inferomarginals, thicker than superomarginals.... Paralophaster hyalinus H.E.S. Clark, 1970b

(4) Abactinal paxillae, relatively low and stout with spinelets 2 to 4 (Fig. 23B). Marginal paxillae 14–16 per arm side at R=2.6 (Fig. 23D, E). Each marginal paxillae with 4–10 spinelets (Fig. 23F). Known only from the holotype.................................................................................................. Paralophaster paucispinus n. sp.

(4’) Abactinal paxillae with numerous spinelets, 4–25, shaft variably elongate and narrow to short and thick. Marginal paxillae 13–23 per arm side, from R=1.7 to 4.1. Marginal paxillae with 8–30 spinelets..................................... (5)

(5) Abactinal paxillae slender and elongate with spinelets, 4–10 as long as the shaft on which they sit (Fig. 21B). Actinal plates 4–10, slender. No brooded juveniles. Arms long and strap-like, R/r=2.3–4.1. Historical depth range: 70−2040 m.............................................................................. Paralophaster godfroyi (Koehler 1912a)

(5’) Abactinal paxillae short and thick with spinelets 10–25, longer than the shaft on which they sit (Fig. 20B). Actinal plates one or two (Fig. 20C, F). Brooded juveniles enclosed by pouch in coelom (Fig. 20D, E). Arms short, triangular, R/r=1.7−3.0. Known from 2000−4000 m................................................................ Paralophaster ferax n. sp.