Pythonaster atlantidis A. H. Clark 1948

Pythonaster atlantidis A.H. Clark 1948

Figure 20 B–E

Pythonaster atlantidis is one of three species within Pythonaster, a rarely encountered genus in the Myxasteridae. Each of the known Pythonaster sp. is poorly understood and relatively few specimens are available for each species. Some species, such as the Atlantic Pythonaster murrayi are known only from the holotype. This account and the upcoming summary of the Okeanos Explorer CAPSTONE expedition will be the first published accounts documenting in situ observations of Pythonaster as well as observations of its feeding behavior. The observations herein document new occurrence of this species from the Gulf of Mexico.

Pythonaster is recognized by the very distinct swollen proximal arm region, linear rows of ensacculate spinelets present along the surface of the arms, the thin body wall, and the continuously webbed actinolateral spine series present along the furrow of the arms. Details such as the number of spine clusters, 4 to 8 were counted from imagery, are consistent with descriptions of this species from trawled collections.

Based on other observations of Pacific Pythonaster spp. individuals are mostly observed perched on vertical wall faces, especially hard rock surfaces. However, one specimen of Pythonaster, thought to be P. atlantidis was observed on soft substrates on the northwest Florida Escarpment at 1970 m. This specimen (Fig. 20B), at R=~15.0 (30.0 cm total diameter) had relatively swollen arms with a more cylindrical appearance with thicker, more homogeneous arms, but otherwise appeared consistent with other observed species, which tended to have more strongly swollen proximal arm regions and narrower more tapering distal arm regions. When observed in situ this species appears strongly inflated with seawater which apparently provides buoyancy to the ensacculate spines and other structures.

All observed individuals of this species were white with a translucent body wall. The ambulacrals were visible and appeared as a brighter white than the body wall. The proximal arm regions had a very weak purple to violet highlight. Tube feet were white with bright white suckers.

Feeding Observations

This species was observed feeding on two different types of glass sponges (Hexactinellida) (Figs. C-E), with one observation from DeSoto Canyon, Gulf of Mexico at 2604 m (Fig. 20 C-D) and a second on Jaguey Spur at 2666 m, off the coast of Puerto Rico (Fig. 20E). The Gulf of Mexico feeding observation showed tube feet cardiac stomach extended, the latter with elaborate frilled folds around the edges, extending from two sides around the base of the sponge. The second feeding image from Puerto Rico showed the animal on the sponge surface, but with its disk slightly sunken, presumably extending its stomach onto the sponge tissue.

Occurrence: Gulf of Mexico, Puerto Rico, South of Azores (35°37’N, 30°15’W), Hudson Canyon, North Atlantic (38°25’06”N, 70°52’, 30’W), Porcupine Abyssal Plain, 2604 –4877 m

Images Examined

Escarpment Canyon, Gulf of Mexico

EX1711_IMG_20171201T171312Z_ROVHD.jpg

DeSoto Canyon, Gulf of Mexico, 28.28388, -87.22355, 2604 m (both below) EX1803_IMG_20180425T155302Z_ROVHD.jpg

EX1803_IMG_20180425T155445Z_ROVHD.jpg (feeding on glass sponge)

largest specimen, Northwest Florida Escarpment, Gulf of Mexico, 28.1254, -86.65615, 1970 m EX1803_IMG_20180426T184110Z_ROVHD.jpg

Jaguey Spur, off coast of Puerto Rico 17.606345, -67.273602, 2668 m, EX1811_IMG_20181109T171906Z_ROVHD.jpg (on Rossellidae) glass sponge)