The anemone living on this parapagurid hermit crab (likely Strobopagurus gracilipes) actually secretes a “shell” for the crab, which it inhabits instead of a gastropod shell (e.g., snail) that most hermit crabs call home.

The anemone living on this parapagurid hermit crab (likely Parapagurus sp.) actually secretes a "shell" for the crab, which it inhabits instead of a gastropod shell (e.g., snail) that most hermit crabs call home. Image courtesy of the NOAA Office of Ocean Exploration and Research, 2016 Deepwater Exploration of the Marianas. Download larger version (jpg, 902 KB).

Dive 13: Kunanaf Hulo Mud Volcano
May 4, 2016
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Jellyfish

Wait for it! This unidentified jellyfish was seen floating through the water column, before quickly retracting its tentacles and swimming away. This is likely a defensive response to the presence of the Deep Discoverer remotely operated vehicle. Video courtesy of the NOAA Office of Ocean Exploration and Research, 2016 Deepwater Exploration of the Marianas. Download (mp4, 71.2 MB)

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Pagurid (Hermit Crab)

This hermit crab, likely Parapagurus sp., appears to be missing a pair of legs, but in fact, the legs are instead modified to hold this anemone in place. Most hermit crabs have a shell that they inhabit; however, this species uses an anemone instead. Video courtesy of the NOAA Office of Ocean Exploration and Research, 2016 Deepwater Exploration of the Marianas. Download (mp4, 58.1 MB)

Dive 13 took place on a unique geological feature called a serpentinite mud volcano. Mud volcanoes in this region form along faults associated with the subduction of the Pacific Plate at the Mariana Trench, where serpentine mud rises to the surface from depths as great as 18 kilometers. Fluid is released from the downwelling plate, which can create active springs at the summit. This particular mud volcano had not been explored before, so we went searching for signs of active venting and associated benthic fauna. We began our transect at a depth of 3,662 meters and spent the dive transiting along a sedimented ridge that ran between two peaks of the mud volcano. While we did not find any of the expected active seeps or carbonate chimneys, we still saw a lot of novel biology and interesting geology. At the start of the dive, the surface was covered in sediment and manganese-encrusted rocks. This encrustation indicates that the seabed is relatively old, and we collected one of the rocks for age-dating. As we moved, we saw more cemented, platy-looking sediment and some more outcrops. We also saw small manganese nodules in some areas. The fauna of this dive was typical of sedimented abyssal plains elsewhere in the Pacific. There was evidence (e.g., lebensspuren: burrows and tracks) that there were sediment-dwelling fauna present, although few were actually seen. Organisms that we saw included stalked sponges (Caulophacus sp.), some of which had commensal Relicanthus (a relative of anemones) living on the stalks. The tentacles of these commensals can trail out for up to eight feet. Other interesting fauna observed included acorn worms (enteropneusts), primnoid corals, benthic ctenophores, and a parapagurid hermit crab (likely Parapagurus sp.) with a commensal anemone (actinarian). We collected a sponge (Caulophacus sp.) that had an unusual root structure that acted as an anchor in the sediment, rather than the more conventional attachment base to hard substrate. We also saw cusk eels (Ophidiidae), a lizardfish (Bathysaurus mollis), and the unusual fish Ipnops meadi – a relative of the tripodfish.