Nymphaster diomedeae Ludwig, 1905

Mah, Christopher L., 2016, Deep-sea (> 1000 m) Goniasteridae (Valvatida; Asteroidea) from the North Pacific, including an overview of Sibogaster, Bathyceramaster n. gen. and three new species, Zootaxa 4175 (2), pp. 101-141 : 123-126

Nymphaster diomedeae Ludwig, 1905

Figure 7 B–C, 8A–E

Ludwig, 1905: 128; Fisher 1928: 490; Macan, 1938: 374; Maluf, 1988: 34 (table), 118 (list); 1991: 349 (list); A.M. Clark, 1993: 265; Maluf & Brusca, 2005: 331 (list); Hendrickx et al., 2011: 809.

In situ observations. MBARI has observed this species in situ (dive 344, Feb. 2012) at 1252.94 m in the Gulf of California on a muddy substrate adjacent to what appeared to be a dead gonatid squid as well as other scavenging megafauna, including a lithodid crab and a second sea star resembling the astropectinid Dytaster ( Fig. 7 C). Nymphaster diomedeae was shown on the sediment surface with inflated, convex radial regions displaying extended papulae with upturned armtips and tube feet extended. A second image (same dive) shows approximately eight to nine individuals clustered around some type of organic debris ( Fig. 7 B). Two groups within the cluster each including three to four animals, with swollen disks, appear to be feeding or scavenging. Living color of these individuals was straw-yellow to white.

Although there have been no explicit feeding studies of Nymphaster , the Atlantic Nymphaster arenatus has been classified in megafaunal analyses as both a scavenger and a predator (daCosta et al., 2015; Wagstaff et al., 2014) which is consistent with these observations.

This observation is one of the first published accounts of this species in situ and a new significant observation for this widely occurring but poorly known genus. Observations of the Atlantic N. arenatus by the ROV Okeanos Explorer show it sitting with arms flattened and extended out on bottom sediments (Mah, pers. obs.).

Occurrence. Gulf of California , Mexico, Coiba Island ( Panama), Cocos Island ( Costa Rica) , Galapagos Islands ( Ecuador), 702–1618 m .

Description. Body strongly stellate, flattened. Disk broad with arms triangular in outline, tapering at arm tips ( Fig. 8 A), trapezoid in cross-section. Abactinal surface flattened, sunken relative to plane of superomarginal plates.

Abactinal plates irregularly rounded to variably polygonal, largest along radial regions proximal on disk ( Fig. 8 B). Carinal plate series largest, widest with adjacent radial series smaller, with sides having similar length and with. Interradial abactinal plates smaller, more quadrate and irregular in shape. Interradial abactinal plates largest proximally on disk becoming smaller and more crowded distally adjacent to contact with superomarginal plates ( Fig. 8 A). Abactinal plates covered with granules, four to 12, roughly hemispherical in shape, evenly distributed on plate surface ( Fig. 8 B). More widely distributed on radial and proximal plates but becoming smaller and more crowded distally, adjacent to superomarginal plate contact. Papular pores present only on radial regions of disk, absent interradially with fewer pores present on primary circlet. Pores, four to six per plate, decreasing in number and absent adjacent to superomarginal arm plates. Madreporite, round to polygonal with deep sulci, flanked by five abactinal plates. Fasciolar well-developed along radial regions, but weak to absent interradially.

Superomarginals, ~36 per interradius (armtip to armtip-but tips are broken) ( Fig. 8 A). Superomarginals offset from inferomarginals. Superomarginals wide (W>L) becoming more equivocal distally. Distalmost 10–16 superomarginals abutted over midline ( Fig. 8 A). Inferomarginals, ~32–34 per interradius (armtip to armtip, but tips are broken). Marginal plates in cross-section with rounded edges with abactinal superomarginal surface and actinal inferomarginal surface weakly convex. Granules, evenly spaced, similar to those on abactinal, actinal surface completely cover marginal plate surface. Granules, 80–100 cover superomarginal and inferomarginal plate surface ( Fig. 8 C). Granules on inferomarginal surface, less crowded but more similar in distribution and shape with those granules on actinal plate surface. Approximately 45 granules form periphery of marginal plate surface.

Actinal intermediate plates arranged into approximately three chevrons ( Fig. 8 D, E). Chevron adjacent to adambulacral plate series best developed extending to base of arm. Remaining chevrons on actinal intermediate region irregular, shortened. Actinal plates quadrate to irregularly polygonal; largest proximally, but becoming smaller, crowded and more irregular in shape adjacent to inferomarginal contact. Granules six to 15 cover each actinal plate; slightly pointed relative to those on abactinal and superomarginal surface ( Fig. 8 E). Shallow fasciolar grooves present among actinal plates and between actinal and adambulacral plates.

Furrow spines, two to six, blunt, slender but round to quadrate in cross-section ( Fig. 8 E). Longest furrow spines proximal becoming shorter distally. Furrow spines proximally two to four becoming higher distally. Subambulacrals separated by discrete space, spines short and granular but angular to round in cross-section, approximately twice as large as granules present on actinal surface. Adambulacral plates elongate longer distally. Oral plate with four furrow spines per side, similar in length and shape to furrow spines elsewhere. Oral plate surface covered by pointed granules, blunt, angular to round in cross-section becoming more spine like adjacent to mouth and furrow spines pointing into mouth ( Fig. 8 E).

No pedicellariae observed on abactinal, marginal actinal, or adambulacral plate surfaces.

Color in life of this species is white to straw-like whitish-yellow.

Material examined. USNM E 26330 . NE of Galapagos Islands, 4º50’N, 87º00’W, 1143 m . 1 dry spec. R=4.4, r=1.3; SIO-E4386 Costa Rica Mound11, 8.9208 N, 84.305 W, 1040 m. Coll. G. Rouse et. al. R/V Atlantis & HOV Alvin. 1 wet spec. R=8.1, r=2.8 (but arms broken at tips)

Figures

FIGURE 7. In situ images of M. tenellus and N. diomedeae. A. Mediaster tenellus present on molt or moribund crab. B. Nymphaster diomedeae in feeding aggregation. C. N. diomedeae approaching dead gonatid squid.

FIGURE 8. Nymphaster diomedeae USNM E 26330. Scale bar = 5.0 mm. A. Abactinal surface. B. Closeup of abactinal surface. C. Lateral view of interradial marginal plates. D. Actinal surface. E. Close up of actinal surface showing granulation, furrow spination and tube feet.