A chrysogorgiid octocoral seen with an ophiuroid brittle star associate on bare coral skeleton, which is very unusual as brittle stars are usually associated with healthy coral tissue.

A chrysogorgiid octocoral seen with an ophiuroid brittle star associate on bare coral skeleton, which is very unusual as brittle stars are usually associated with healthy coral tissue. Image courtesy of the NOAA Office of Ocean Exploration and Research, 2017 American Samoa. Download larger version (jpg, 1.0 MB).

Dive 06: "Utu" Seamount
February 21, 2017
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Dive 6: Cosmic Jellyfish

This spectacular little jelly was imaged during our first dive on 'Utu' seamount, on February 21.

The jelly (hydromedusa) is in a family of hydromedusae called Rhopalonematidae, which is known for the canals running vertically on the inside of the bell, gonads attached to these canals, and sometimes having two sets of tentacles.

Scientists have seen this species on previous Okeanos Explorer expeditions. In this video, you can see the perfectly relaxed arrangement of the two sets of tentacles; scientists think this is a position that allows for optimum feeding in the midwater environment at ~3,000 meters. Through remotely operated vehicle video observations such as this, we can learn much about the animals in the midwater and what they are up to when we can catch them in an undisturbed manner. Video courtesy of the NOAA Office of Ocean Exploration and Research, 2017 American Samoa. Download (mp4, 18.2 MB)

A previously unexplored seamount, tentatively called "Utu," was the target for Dive 06 of the expedition. Located in the northern region of the American Samoa Exclusive Economic Zone, the seamount appears to be composed of two distinct structures – a pancake-shaped structure at the base and a steeper-sided volcanic structure on top – which are consistent with two stages of volcanic activity on the seamount. The summit of the seamount is host to a clearly developed volcanic crater. During the dive, we explored along the crater wall of the shallower volcanic structure, starting at a depth of ~ 3,040 meters.

For the initial part of the dive, the bottom was predominantly sediment, as well as loose pebbles, cobbles, and boulders; few animals were observed. As the remotely operated vehicle moved closer to the crater wall, the landscape became increasingly dominated by large blocks and boulders (all ferromanganese encrusted), with light-colored sediment infilling the spaces between the blocks and cobbles. The loose rock, which is likely from volcanic material that had broken off the crater wall and rolled down hill, hosted a wide range of animals. Corals observed during the dive included numerous chrysogorgiid octocorals, whip and branching primnoids, and Iridogorgia. Moving along the dive track, we also observed glass sponges, sea stars, carnivorous tunicates, crinoids, and anemones, as well as several halosaurs and two cusk eels. Near the summit of the crater wall, the dive ended in spectacular fashion, with large outcrops and a large abundance of stalked glass sponges (Bolosoma) over a meter tall, several large octocoral fans (likely bamboos or primnoids), Iridogorgia, and large bamboo whips over 1.5 meters tall.