Dive 12: Unnamed Seamount
July 26, 2017
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Dive 12: Sea Cucmbers Make Everything Better

Holothurians, also known as sea cucumbers, play an important role in bottom ocean ecosystems, as they ingest and cycle sediment and nutrients through their systems for other animals on the seafloor. This sea cucumber, which measured about 10 centimeters (four inches) long, was seen while exploring at a depth of 2,325 meters (7,628 feet) on an unnamed seamount in the south central portion of the Johnston Atoll Unit of the Pacific Remote Islands Marine National Monument. Video courtesy of the NOAA Office of Ocean Exploration and Research, 2017 Laulima O Ka Moana.. Download larger version (mp4, 27.1 MB).

Deep Discoverer (D2) transited from a saddle on the seamount up the ridge to a crest. There was a strong north-to-south current on the sandy bottom that left evenly spaced ripples in the sediment. An array of rocks was widely spread around the sandy field. Sponges were largely absent in this area. Rock pens were present, but octocorals and black corals were not. The bottom composition changed at the base of a massive boulder cliff as D2 transited upslope to the crest, which had variable substrate that contained several platforms and valleys. There was sparse fauna compared to yesterday’s dense communities, although we did observe colonial corals and stalked glass sponges. Most glass sponges were solitary, few occurred in groups. The diversity of octocorals included chrysogorgiids, bamboo corals, precious corals, and mushroom corals. Alternapathes, Bathypathes, Heteropathes, Parantipathes, and Stauropathes were the black corals at this site. Anemones were observed, including a large dark purple cerianthid – apparently lacking a tube and residing on a rock face. A single benthic ctenophore was present on the surface of a glass sponge. Of the few echinoderms, feather stars and sea cucumbers were most abundant. Both types of snake stars present on host octocoral were observed on Hemicorallium and "normal" brittle stars were found wrapped around dead glass sponge stalks. Two species of stalked crinoids were also documented. Other fauna included swimming polychaete worms, chaetognaths, swimming shrimp, a large brown side gill slug, and large sea spider that was observed feeding on an octocoral. The few fishes present were grenadiers and a halosaur. Structures thought to be protists were abundant throughout the dive, including small treelike forms, larger fan-shaped structures, tiny round capsule-like forms, and short uneven branching forms.