Styracaster yapensis Zhang & Zhou & Lu & Wang 2017, sp. nov.
Creators: name: Zhang, Ruiyan
name: Zhou, Yadong
name: Lu, Bo
name: Wang, Chunsheng
Publisher: name: Zenodo
Year: 2017
Subjects: Biodiversity
Taxonomy
Animalia
Echinodermata
Asteroidea
Paxillosida
Porcellanasteridae
Styracaster
Styracaster yapensis

description: Styracaster yapensis sp. nov.
(Figs.1–6)
Etymology. Name after the type locality Yap Trench.
Description. Rays five, short, thin, rounded, and of uniform thickness throughout. R/r = 2–3, 35/ 17 mm in the holotype and 30/ 10 mm in the paratype. Disk conspicuously convex. Interbrachial arcs broad (Figs. 2A, 2B).
Abactinal area uniformly covered with numerous, scattered granular plates, inclusive of the arm base, where they are obscured by a thin membrane (Figs. 2A, 2C). Plates spineless. Paxillae absent. In the holotype small projection present at disk center, surrounded by a small circle of enlarged or elongated granular plates which are almost pseudo-paxilliform (Fig. 2D). No projection in the paratype, there is a central pore at disk center (Fig. 2E). Madreporite large, sub-circular, located on the edge of the disk, in close proximity to the only fully developed cribriform organ and separated from it by a narrow groove. Madreporite of a diameter about the upper length of the adjoining cribriform organ (2.8 mm). The striae radiating from a proximal point (Figs. 6A, 6C).
Superomarginal plates 15–16 in number, counted from the midpoint of interradius to the arm tip, joining at the arm midline from about the sixth opposite pair. Superomarginal plates longer than higher on the disk and subquadrate on the arms (Figs. 3A–D, 6A–D). On the arm midline, 3–4 thin, tapering abactinal spines placed on the proximal five to six joining superomarginals (Figs. 3A, 3C). Distally occasionally some protuberances on one or both of the paring superomarginals (Figs. 3B, 3D). The abactinal spines rather transparent and delicate; the innermost one the largest and the most robust, the longest reaching 2.5 mm. The number and location of abactinal spines and protuberances varying in different arms (Figs. 3A–D).
Inferomarginal and superomarginal plates generally corresponding in number and length, only in one or two interradii where an additional inferomarginal or superomarginal plate appears in the middle of the arm interradius (Fig. 6B). The inferomarginals of the disk about as long and high as the accompanying superomarginals (Figs. 6A– D), but slightly lower than the superomarginals in the arm (Figs. 3A–B) (in the paratype the inferomarginals are almost twice or three times lower than the superomarginals) (Figs. 3C–D). All inferomarginal plates unarmed.
Only one fully developed cribriform organ in the madreporite interradius, in close proximity to the madreporite and separated from it by a narrow groove (Fig. 6A, 6C). Cribriform organ subtrapezoid in form, extending from the upper margin of the superomarginal to the lower margin of the inferomarginal, covering the whole height of the interradial margin, not visible from the abactinal side (Figs. 6A, 6C). The paratype has two more rudimentary cribriform organs on each side of the fully developed one. The rudimentary cribriform organs are of a triangular form, which are much smaller than the fully developed one and not extending to the low margin of the inferomarginal (Fig. 6C). No cribriform organ visible in other interradii in the holotype (Figs. 6B); 1–3 lines of small papillae between the central 2–3 superomarginals exist in some interradii in the paratype (Fig. 6D).
Terminal plate large, with 4–6 sharply pointed terminal spines. 2(–3) dorsal terminal ones placed on the end of the arm midline and 2(–3) distal ones placed on a lower level at the extremity of the furrow. The terminal spines are generally of varied size; the longest one almost as long as the first abactinal spine (Figs. 3E–G).
Ventrolateral areas expansive, covered with a thin transparent membrane (Fig. 4E, 5B). Ventrolateral plates naked, elongate and very delicate, not uniformly arranged in tangential and radiating series, not extending to the arms. In most interradii, the plates are fairly indiscernible.
Ambulacral furrows narrow and entirely closed in by the adambulacral spines (Figs. 2B, 4C, 5D). Tube feet in 2 rows, with no terminal suckers. Adambulacral plates 22–24 in number. The proximal ones bearing three pointed furrow spines, with the largest on the adoral side (Fig. 4D). The spines in the paratype slightly flattened (Figs. 5B– C). On arm each adambulacrals bearing 2–3 furrow spines (Figs. 4C, 5D). Only one subambulacral spine present on each adambulacral plate in the holotype; the paratype with 2–3 subambulacral spines on each proximal adambulacral plate and 1–2 on each distal plate (Figs. 4C–D, 5B–D).
Mouth plates large and very conspicuous (Figs. 4A–B, 5A). Two proximal spines to each pair of mouth plates, pointing to the actinostome. Proximal spines relatively large, thicker than the furrow spines. Each mouth plate carrying four subequal spinelets on the margin and about six suboral spinelets or granules on the surface. The marginal spinelets conspicuously smaller than the furrow spines. Peristome wide and conspicuous.
Coloration. Color in life grayish white.
Distribution. Styracaster yapensis sp. nov. is known from two dives at the Yap Trench in the western Pacific. Both specimens are collected from fine-grained muddy sediments from a depth between 6377 and 6575 m.
Remarks. Madsen classified Styracaster species into three groups, represented by S. horridus Sladen 1883, S. chuni Ludwig 1907 and S. armatus Sladen 1883 respectively (Madsen 1961). The former two groups are characterized by the paxillae-covered abactinal surface, and the latter two by relatively small R/ r ratio (usually
3). Styracaster yapensis sp. nov. falls in the third group, which includes S. armatus (Sladen, 1883), S. monacanthus (Ludwig, 1907) and S. clavipes (Wood-Mason & Alcock, 1891), sharing the characteristics of a non-paxillae covered abactinal surface and small R/ r ratio. Styracaster yapensis sp. nov. is distinguished from the latter species by the unique form of abactinal armature and rudimentary cribriform organs. In S. yapensis sp. nov., granular plates are scattered on the whole abactinal surface with spines or spinelets absent, whereas in S. armatus and S. clavipes, the abactinal surface is covered by small rounded plates each bearing 1–2 spines, and in S. monacanthus, granular plates each bear a blunt spinelet; S. yapensis sp. nov. has 0–3 rudimentary cribriform organs in each interradius, which consist of only a few lines of small papillae, and one well-developed cribriform organ only at the madreporite interradius, which is large and extending across the whole height of vertical suture, whereas S. armatus, S. clavipes and S. monacanthus have 3, 5 and 5* well-developed cribriform organ in each interradius respectively. Mironov et al. (2016) suggested a pronounced trend to simplification of the morphology in Porcellanasteridae, including the decreasing in number of respiratory organs, cribriform organs and body size, as an adaptation to deep-sea environments. S. yapensis sp. nov. has the fewest number and the most degraded form of cribriform organ within the genus, which might as well be an adaptive strategy for its life in each interradius respectively. (* S. monacanthus was described to …having 7 cribriform organs.) Mironov et al. (2016) suggested a pronounced trend to simplification.
The type locality of S. armatus is close to the type locality of S. yapensis sp. nov., but much shallower (near the Caroline Islands at 3384 m depth). The similar adambulacral and mouth plate armature of the two species may suggest a close affinity. Styracaster yapensis sp. nov. is further distinguished from S. armatus by 1) abactinal surface that entirely covered by uniform granular plates, extending onto the arm bases (in S. armatus there are naked spaces at arm bases); 2) fairly conspicuous terminal plates with 4–6 spines (R=35 and 30 mm) (in S. armatus the terminal plates are inconspicuous, with 3 spines (R= 38–39 mm)); 3) the number and form of marginal plates. The number of superomarginal plates on each arm in S. armatus (R<40 mm, R/r = 2.2–4) is usually 9–12, joining at the arm midline from the 4th opposite pair (counted from the midpoint of interradius). Both specimens of S. yapensis sp. nov. (R=35 and 30 mm, R/r =2 and 3) have 15–16 superomarginal plates on each arm, joining at the 6th opposite pair. This indicates much shorter superomarginals in the new species, at least interradially, which is not likely to be a variation related to growth stage. In addition, the form of superomarginals in S. yapensis sp. nov.
changes from longer than higher on disk to subquadrate on arm, whereas in S. armatus, the superomarginals are subquadrate on disk and become lower and rectangular on arm. S. yapensis sp. nov. is the second porcellanasterid species from the Yap Trench. Previously Porcellanaster ivanovi was found from there at 8560–8720 m depth (Belyaev, 1985).
S. yapensis sp. nov. is readily distinguished from S. monacanthus by 1) subambulacral and suboral spines (present in the former, absent in the latter); 2) number of abactinal spines on arm (3–4 to 1); 3) number of and terminal spines (4–6 to 3).
Gene bank accession numbers: See Table 2.
description: Published as part of Zhang, Ruiyan, Zhou, Yadong, Lu, Bo & Wang, Chunsheng, 2017, A new species in the genus Styracaster (Echinodermata: Asteroidea: Porcellanasteridae) from hadal depth of the Yap Trench in the western Pacific, pp. 153-162 in Zootaxa 4338 (1) on pages 155-159, DOI: 10.11646/zootaxa.4338.1.8, http://zenodo.org/record/1035643