Dytaster insignis

Dytaster insignis (Perrier 1884)

Figure 4 A–B

Dytaster is identified based on the numerous and complete series of sharp spines present on the superomarginal and inferomarginal series. Most characters determining Atlantic Dytaster spp. require a specimen, however Clark and Downey (1992) illustrate the proximity of the madreporite to the superomarginal plates as a diagnostic character for Dytaster grandis and Dytaster insignis, which both occur in the Gulf of Mexico. Dytaster grandis possesses a madreporite in direct contact with the superomarginals versus D. insignis whose madreporite is located well away from the superomarginal plates. Both observations listed below show the latter character, identifying this species as Dytaster insignis.

Both individuals appeared to be the same species, but two colors morphs were observed, a deep orange form from the West Florida Escarpment (2987 m) and a white colored form from Period Canyon (2745 m). Based on the laser based scale bars projected by the ROV, the Perdido Canyon specimen was half the size (R=10.0 cm diameter) than the Florida Escarpment specimen (R=~20.0 cm diameter) suggesting that color could be correlated with size. The different localities and available food might also explain the variation in color.

Ecological Observations

Although well represented in museum collections, in situ video observations of this species, by Okeanos Explorer, are the first of this species from the Gulf of Mexico. Tyler et al. (1990) show a single photo of Dytaster grandis on bottom settings amidst phyotdetritus at 3500 m in the Northeast Atlantic. Tyler et al. (1990) noted that Dytaster grandis was omnivorous, summarizing gut contents of various prey items but also phytodetritus as food.

Most Paxillosida have been characterized as burying themselves in the subsurface of weakly consolidated sediment (e.g. Jangoux 1982, Clark & Downey 1992) based primarily on trawled sediment and gut contents as well as comparisons with similar shallow-water taxa. But first hand observations of many deep-sea Paxillosida, especially those at abyssal depths is lacking and suggests a more variable lifestyle than that of consistent, long-term subsurface burial.

Video of the individual from Period Canyon suggests at least partial burial with disk and arms emerging from very flocculent sediment. Arms were flush with the sediment with spines projecting upwards and away from the body. The disk was strongly swollen and appeared to be filled with sediment.

This is contrasted with the second image showing the individual which was observed moving across the sedimented surface of the West Florida Escarpment with arms lifted well above the bottom by its tube feet. This latter individual had a greatly enlarged disk, apparently filled with sediment and including what appeared to be a discrete turret on top of the disk, which gradually lost shape and seemed to return in shape with the top of the disk. Dytaster and other astropectinids lack an anus and discharge finished, digested sediment back through the mouth back out onto the sea bottom. This could explain one possible role of movement in this species, i.e., the need to discharge inorganic sediment to the environment following digestion.

Images Examined

white. Perdido Canyon, Gulf of Mexico, 26.1471, 94.86584, 2745 m EX1803_IMG_20180418T171538Z_ROVHD.jpg.

orange. West Florida Escarpment, Gulf of Mexico 27.70883, -85.74822, 2987 m. EX1803_IMG_20180427T153915Z_ROVHD.jpg