Dive 4: St. Croix Amphitheater

Date: November 4, 2018
Location: Lat: 17.58917°, Lon: -64.88913°
Dive Depth Range: 450-564 meters (1,476-1,850 feet)

During Dive 4, we explored an amphitheater-like feature at 450-564 meters (1,476-1,850 feet) depth, which included dramatic vertical walls that were good habitat for a variety of corals and sponges. Image courtesy of the NOAA Office of Ocean Exploration and Research, Exploring Deep-sea Habitats off Puerto Rico and the U.S. Virgin Islands. Download larger version (jpg, 407 KB).
The exposed carbonate walls with undercut ledges provided good habitat for a diversity of deep-sea corals. Image courtesy of the NOAA Office of Ocean Exploration and Research, Exploring Deep-sea Habitats off Puerto Rico and the U.S. Virgin Islands. Download larger version (jpg, 1.3 MB).
This deep-sea anemone was filmed in the process of eating a small midwater fish, a valuable observation since very little is known about the basic biology and ecology of most deepwater species. Image courtesy of the NOAA Office of Ocean Exploration and Research, Exploring Deep-sea Habitats off Puerto Rico and the U.S. Virgin Islands. Download larger version (jpg, 1.1 MB).
This toadfish (Chaunax sp.) is an ambush predator that lies on the seafloor waiting for prey to pass by, then opens its mouth to suck in its meal. Toadfish use modified fins to walk along the seafloor, but can also swim using their caudal fin. Image courtesy of the NOAA Office of Ocean Exploration and Research, Exploring Deep-sea Habitats off Puerto Rico and the U.S. Virgin Islands. Download larger version (jpg, 1.9 MB).

After a delay due to high winds, seas, and lightning, weather conditions finally improved and we were able to deploy the remotel operated vehicles (ROVs) around 11 AM AST. The purpose of the dive, located south of St. Croix, was to explore seafloor habitats for deep-sea corals, sponges, and associated communities. The dive site was chosen because multibeam bathymetry data revealed an intriguing amphitheater-like feature with steep walls, which typically provide good habitat for various benthic communities.

The dive turned out to be very exciting and delighted biologists and geologists alike. The terrain included dramatic vertical walls of carbonate, deep gorges that extended into the wall, as well as a series of terraces. Glass sponges and unbranched black corals were the most abundant animals on the dive, which also included many observations of other corals, including branched scleractinian corals, cup corals, stylasterids, primnoids, plexaurids, and a bamboo coral. Additionally, the dive included records of at least 10 different species of fish, as well as multiple species of sea stars, urchins, and sea cucumbers. Furthermore, the dive included multiple records of the commercially valuable queen snapper. After the dive, the mapping team conducted mapping operations focused on filling in mapping data gaps west of St. Croix.

This video shows exposure of layered sedimentary deposits of the submerged carbonate platform south of St. Croix, where erosion acting on layers of different strength has created a pronounced stepped texture. Here, harder material is resistant to erosion and forms thin benches and overhangs, while the softer material in between the harder layers is eroded away. An abundance and diversity of sessile fauna attached to the harder rocks suggests that the rocks remain stable for an extended period of time, even as strong currents slowly erode the weaker rocks and weaken the wall as a whole.

Text contributed by Jason Chaytor, U.S. Geological Survey.

Video courtesy of the NOAA Office of Ocean Exploration and Research, Exploring Deep-sea Habitats off Puerto Rico and the U.S. Virgin Islands. Download larger version (mp4, 80.4 MB).

Location of Dive 4 on November 4, 2018. Download larger version (jpg, 2.2 MB).

The remotely operated vehicle track for Dive 4, shown as a white line. Scale is water depth in meters. Download larger version (jpg, 324 KB).