Nymphaster arenatus

Nymphaster arenatus (Perrier 1881)

Figure 13 A–E

In situ observations of this species showed it with its arms splayed out on the surface with its body slightly entrenched within the sediment (Figs. 13 A–E). There is morphological variation of this species throughout its range in the Atlantic. Observations of this species within the Gulf of Mexico suggest, broader, blockier arms in the Horne Dome observations from 1050 m (Fig. 13 A–C), but more slender arms from the “Archeology site” dive at 1880 m (Fig. 13D). A smaller individual (Fig. 13E) shows arms with less taper and a more planar surface.

This genus is known for having broad basal arm regions and long, inflexible arm tips resulting from having superomarginals in contact across the midline. Although this character is frequently used as a taxonomic character, the abutted superomarginals and wide arm morphology are poorly understood from a functional perspective. Their broad, flattened arms observed in situ, especially on flocculent sediment suggest that they help distribute the animal’s weight allowing it to adjust its position on the unstable sediment bottom.

Ecological Comments

One observation from Horne Dome in the Gulf of Mexico at 1049 m showed this species partially extended over what appeared to be a bivalve in the sediment (Fig. 13A). Although the angle makes a positive determination of predation difficult, the animal is tilted towards the shell with the tube feet in motion, as if to extend its cardiac stomach into the shell. Megafaunal analyses by daCosta et al. (2015) and Wagstaff et al. (2014) have classified Nymphaster arenatus as a scavenger and a predator which would be consistent with the in situ position observed for this species. Mah (2016) has reported on the eastern Pacific Nymphaster diomedeae, which is similar in appearance, as scavenger on the remains of a gonatid squid and other moribund tissue.

Associated Polychaete

The feeding observation of Nymphaster arenatus from Horne Dome also shows a small purple polynoid polychaete with gold colored appendages (Fig. 13B) in close contact along the basal arm region within the star’s interradius. Polynoids are familiar associates of asteroids, and show behavior ranging from commensalism (e.g., Wagner et al. 1979) to something more approaching kleptoparasitism (Freeman et al. 1998). Freeman et al. (1998) reported on a polynoid whose head was frequently found in the stomach of its host, Astropecten irregularis, suggesting it was feeding on stomach contents. Although Nymphaster is not an astropectinid it occurs in a similar habitat and the observed polychaete herein may exploit food from prey caught by the star during feeding.

Images Observed

Horne Dome, Gulf of Mexico, 28.96147, -88.19492, 1049 m EX1711_IMG_20171220T172643Z_ROVHD.jpg

EX1711_IMG_20171220T172806Z_ROVHD.jpg

Horne Dome, Gulf of Mexico, 28.95849, -88.1946, 1052 m

EX1711_IMG_20171220T 190940 Z_ROVHD.jpg

Gulf of Mexico, coordinates redacted (archaeology site), 1880 m EX1803_IMG_20180413T195359Z_ROVHD.jpg

“Unnamed Ridge on GC973” 27.00951, 91.67517, 1528 m

EX1803_IMG_20180416T162332Z_ROVHD.jpg