A field of small, sedimented balls that we have tentatively identified as the amoeba Gromia sphaerica or a close relative. We saw huge number of these throughout the dive.

A field of small, sedimented balls that we have tentatively identified as the amoeba Gromia sphaerica or a close relative. We saw huge number of these throughout the dive. Image courtesy of the NOAA Office of Ocean Exploration and Research, 2016 Deepwater Exploration of the Marianas. Download larger version (jpg, 1.4 MB).

Dive 4: "Enigma Seamount"
April 24, 2016
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Jellyfish

This stunningly beautiful jellyfish was seen during Dive 4 on April 24, 2016, while exploring the informally named "Enigma Seamount" at a depth of ~3,700 meters.

Scientists identified this hydromedusa as belonging to the genus Crossota. Note the two sets of tentacles — short and long. At the beginning of the video, you'll see that the long tentacles are even and extended outward and the bell is motionless. This suggests an ambush predation mode. Within the bell, the radial canals in red are connecting points for what looks like the gonads in bright yellow. Video courtesy of the NOAA Office of Ocean Exploration and Research, 2016 Deepwater Exploration of the Marianas. Download (mp4, 48.8 MB)

Dive 4 took place on "Enigma Seamount," which was given its informal name because we don't know much about it. Its morphology is quite different from other seamounts in the region, which generally have a flat top with steep, smooth sides radiating out into narrow ridges. By contrast, this one is more circular in form and the sides are much less smooth. We were interested in sampling here because we believe that this seamount, and others that lie along a northwest-southeast trend, may have formed on the boundary between the ancient Pacific Plate to the northeast and a much younger plate to the southwest. We began the dive at 3,778 meters in a sedimented area with loose volcanic rocks, where we collected a rock sample. As we transited upslope, we encountered a small outcrop of what were likely pillow basalts and collected a second rock there. We continued to move up the center of a small valley, encountering scree slopes, small to medium-sized talus that appeared volcanic, and sediment cover that ranged from fine to pebbly. We speculate that in the more pebbly areas, the fine sediments have been swept away by currents. Toward the end of the dive, we encountered several pillow mounds that were at least 10 meters high; we collected a third rock there. Throughout the dive, we saw high concentrations of small, rounded balls that looked like they had been constructed from sediment. These have been tentatively identified as a large species of single-celled amoeba, but biologists are still unsure. There is also a possibility that these balls might be sponges. Fauna was scarce throughout much of the dive, perhaps due to a lack of current in the low-relief areas. However, we still observed stalked crinoids and primnoid corals, swimming polychaete worms, a cusk eel, Caulophacus sponges, cladhorizid sponges, a Munidopsis squat lobster, a beautiful hydrozoan jellyfish, and at least two Nematocarcinus shrimp. We collected two primnoid corals and began our ascent at 3,615 meters after transiting ~900 meters.