A brisingid sea star clings to a ferromanganese cobble. The cobble, and the associated sea star, were collected shortly after being imaged by D2.

A brisingid sea star clings to a ferromanganese cobble. The cobble, and the associated sea star, were collected shortly after being imaged by D2. Image courtesy of the NOAA Office of Ocean Exploration and Research, 2017 American Samoa. Download larger version (jpg, 1.1 MB).

Dive 05: "Leoso" Seamount
February 20, 2017
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Dive 5: Hydroid

A solitary hydroid seen while exploring 'Leoso' seamount, which straddles the boundary between the American Samoa Exclusive Economic Zone (EEZ) and the Cook Islands EEZ at a depth of ~3,770 meters. Video courtesy of the NOAA Office of Ocean Exploration and Research, 2017 American Samoa. Download (mp4, 13.2 MB)

During today's dive, we explored a seamount tentatively called "Leoso" seamount, which straddles the boundary between the American Samoa Exclusive Economic Zone (EEZ) and the Cook Islands EEZ. The seamount appears to be comprised of two distinct structures: a larger, flat-topped guyot at the base topped by a smaller, flat-topped volcanic structure that is consistent with being a rejuvenated (or post-erosional) stage of volcanism for the seamount. The dive started at a depth of ~3,770 meters, midway up a steep portion of the smaller, shallower volcanic structure. Deep Discoverer (D2) landed on a ferromanganese-encrusted surface, possibly a lava flow, partially covered with some loose cobbles. As D2 moved upslope, the terrain transitioned first from an increasing percentage of cobble cover, to being dominated by flow structures, and then to smooth, ferromanganese-encrusted pillow basalts. Throughout the dive, we encountered several glass sponges, including many stalked (Bolosoma) and non-stalked sponges, both dead and alive. We also observed black animal tracks on some rocks, possibly made by holothurians or snails. Other animals seen included small ophiuroid brittle stars, hydroids, asymmetric yellow urchins (possibly Pilematechinus; one was collected), barnacles, anemones, and a likely new species of cookie sea star. Only one coral was observed during the dive – the black coral Bathypathes, for which a sample was collected. The dive ended on the top of the seamount, where we observed a smooth surface of ferromanganese crust covered with a thin, rippled layer of sediment and very little to no fauna.