Sibogaster bathyheuretor, Mah, 2020

Mah, Christopher L., 2020, New species, occurrence records and observations of predation by deep-sea Asteroidea (Echinodermata) from the North Atlantic by NOAA ship Okeanos Explorer, Zootaxa 4766 (2), pp. 201-260 : 234-237

Sibogaster bathyheuretor n. sp.

Figures 15 A–E, 16A–D

Etymology

The species epithet is a combination of the Greek bathys and heuretor for “deep discoverer” and honors the ROV Deep Discoverer which collected this and many other Okeanos Explorer species.

Comments

This species is similar to a widely distributed species described by Mah (2016) from abyssal settings (> 2000 m) in the Atlantic and the North and Central Pacific. This represents the first occurrence of Sibogaster from the Gulf of Mexico. Although Sibogaster nieseni has been imaged in the Pacific, this represents the first in situ observation of this genus from the Atlantic.

Ecological Observations

This species was observed feeding on a wide range of possible food and prey items, primarily from the West Florida Escarpment. One observation at 2192 m showed an individual perched on a denuded black coral stalk, having apparently digested the living tissue ( Fig. 16C ). A subsequent observation at 2138 m shows this species present between sargassum and elongate glass sponge spicules ( Fig. 16D ). This latter observation likely represents the animal feeding either on the sargassum or epizoic on the sargassum. The collected specimen from the West Florida Escarpment (USNM 1507293) was observed elevated above the sediment surface and feeding on unidentified subject below it ( Fig. 16A ). Following collection of the specimen however, no evident prey was observed attached to the mouth and the substrate the animal was sitting on showed no evident prey item. Its possible the animal was feeding on sediment, or possibly a sedimentary structure such as the test from a xenophyophoreans (Foraminifera).

The Perdido Canyon observation showed the specimen with its lower surface buried into what appeared to be flocculent sediment suggesting deposit feeding by the animal ( Fig. 16B ).

Occurrence: Perdido Canyon and West Florida Escarpment, Gulf of Mexico. 2597–2777 m.

Description

Body stellate (R/r=2.07) with broadly curved to straight interradial arcs. Arms triangular with pointed tips. Specimen with one arm regenerating.

Abactinal plates flat, round to polygonal in outline with smooth, bare surface. Plates with one to four small crystal bodies centrally on each plate. Each plate with 10–50 small quadrate granules, forming a wide periphery around each plate. Granules taking about 10% of the total diameter. Granules centrally on disk becoming more abundant especially within the primary circlet. Plates larger and more circular proximally becoming smaller, more ovate and irregular in shape distally adjacent to superomarginal plate contact. Abactinal plates extend from disk along arm where distalmost superomarginal plates are abutted becoming weakly convex on arms. Madreporite strongly convex with well developed sulci. Madreporite with two to four series of granules forming periphery around it, flanked by six to eight abactinal plates. Marginal plates extending away from abactinal surface. Anus located centrally on disk but surrounded by approximately 10 granules closely arranged around it, but more open when observed in situ.

Superomarginals 20 per interradius, from arm tip to arm tip, 20–21 inferomarginals, slightly offset from superomarginals distally but with 1:1 correspondence proximally. Superomarginal surface with mostly bald, smooth quadrate surface, remaining lateral region on superomarginal plate with 10–30, mostly ~20, round, evenly distributed granules. In situ observations show interradial superomarginals with widely spaced 40 to 200 round granules with pointed tips becoming bare and smooth distally. Inferomarginals with 100–400 round to pointed granules, widely distributed. One or two interradial inferomarginal plates with small paddle-like pedicellariae (about 1.0 mm in length). Small peripheral granules, approximately 20 per side, total, approximately 80–150 in total. Distalmost two or three superomarginals abutted over midline. Terminal plate triangular in shape, similar in size to one adjacent superomarginal plate.

Actinal surface flat, composed of approximately 2 full series in chevron-like format with one or two irregular, incomplete series present distally adjacent to inferomarginals. Individual plates quadrate to irregular in shape. Surface with 10–50 widely spaced granules, ranging from round to angular and pointed. Smaller peripheral granules, approximately five to eight per side, totaling 20 to 40, central granules larger, more irregular. Granules similar in size and distribution with those on inferomarginal surface.

Furrow spines seven to eight in a straight to weakly curved fan, spines blunt-tipped. A single large paddleshaped subambulacral pedicellariae present immediately adjacent to furrow spines on first six adambulacral plates. Pedicellariae the size of two to three adjacent granules.Adambulacral plate surface around pedicellariae smooth and clear of accessories. First adambulacral plates with largest pedicellariae. Adambulacral plate surface with one or two regular series, each with three to five pointed or angular granules, widely spaced, separated from pedicellariae by discrete space. Distalmost six to eight adambulacral plates with prominent spine, blunt, cylindrical in cross-section becoming larger near am tip.

Oral plates with 14 furrow spines, flattened to quadrate in cross-section with a single prominent spine projecting into the mouth, one per plate (thus two per interradius). Oral plate surface adjacent to furrow spines bare and smooth. Fossae between paired oral plates with tissue, granules eight to ten, paired on each side. Prominent paired spines as part of granule series on each half of the oral plate series. Oral plates with five to seven widely spaced angular plates around periphery in contact with adjacent actinal plates.

Color in life, white on disk with central orange on each plate. Marginals and arm plates dark orange. Regenerating arm white to light orange.

Comparisons

This species is distinguished primarily based on the differences along the adambulacral plate accessories. Sibogaster n.sp. lacks the larger subambulacral spines observed in S. nieseni and possesses pedicellariae on the surface of the adambulacral plate which are paddle-shaped and different from the more tong-shaped pedicellariae in Sibogaster nieseni .

Images Observed

Perdido Canyon, Gulf of Mexico, 26.1466, 94.86662, 2777 m, EX1803_IMG_20180418T164814Z_ROVHD.jpg

DeSoto Canyon, Gulf of Mexico, 28.28367, -87.22315, 2629 m EX1803_IMG_20180425T151605Z_ROVHD.jpg

black coral. West Florida Escarpment, Gulf of Mexico 24.91709, -84.49052, 2192 m EX1803_IMG_20180430T161527Z_ROVHD.jpg

West Florida Escarpment, Gulf of Mexico 24.91523, -84.4933, 2138 m EX1803_IMG_20180430T183436Z_ROVHD.jpg

Material Examined

Holotype. USNM 1507293 West Florida Escarpment , Gulf of Mexico , 27.71516, -85.74739, 2597 m, Coll. D. Wagner, with ROV Deep Discoverer aboard NOAA Ship Okeanos Explorer. 1 wet spec. R=2.7 r= 1.3. 27 April 2018. EX1803_IMG_20180427T 210536 Z_ROVHD.jpg

Figures

FIGURE 15. Sibognaster bathyheuretor n. sp. Holotype. USNM 1507293 A. Abactinal surface. Scale bar=5.0 mm. B. Close up abactinal surface. Scale bar=8.0 mm. C. Oral surface. Scale bar=5.0 mm. D. Furrow spination. Scale bar=1.0 mm. E. Lateral view showing marginal plate surface. Scale bar=2.0 mm.

FIGURE 16. Sibogaster bathyheuretor n. sp. In situ. A. Holotype. USNM 1507293 West Florida Escarpment, Gulf of Mexico, 2597 m. B. Entrenched in sediment, Perdido Canyon, Gulf of Mexico, 26.1466, 94.86662, 2777 m, C. Feeding on antipatharian. West Florida Escarpment, Gulf of Mexico, 2192.9m. D. Feeding on sponges/sargassum West Florida Escarpment, Gulf of Mexico, 2138 m.