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March 19, 2003: Leg 3, Day #8

Farallon Basin 

We're back in this central location after the glitch gremlin re-appeared and cut short our northern excursion. Nothing serious, just a loose connection inside a camera housing that led us to believe there were bigger problems on the vehicle than there actually were.  

Yesterday we measured the abundance and diversity of animal life in the plume of a hydrothermal vent. No one has ever done this before with an ROV (at least not that we are aware of), so we had to "invent" some methodology on the spot. In the past, researchers addressed this problem by sampling along horizontal paths that ran over the vents and then compared these with runs made at a distance from the vents. Their results were always equivocal, with no apparent big differences.  

We decided to come at the problem from a different angle—literally. We figured that because the vents are point sources, horizontal sampling runs might not be best because they are only in the plume for a brief period. So our approach was to come down on the vent from above, in a vertical profile, and compare this with profiles made away from the vent. Well, it worked. In our first comparison, we ran right down the middle of the smoky plume and saw many more animals and more kinds of animals than in the profile we ran later about a kilometer away.  

We repeated the process several times on the following day and learned that currents will bend the plume, and thus, the picture isn't always as straightforward as it appeared to us initially. Nevertheless we have a great data set of quantitative video profiles that should answer the question, at least for this vent. Our experience also illustrates that the best expeditionary science is often done on the fly. In the middle of these operations, the pilots asked us, “What do we do next?" We could only reply with "Give us a minute, we're making this up as we go along!" 

Can you remember the first time you saw a giant redwood tree? Well, something like that happened to us today. We were descending through 1300 meters when a dark visual target appeared, out at some distance from ROV Tiburon. We stopped our descent and flew toward it. Looming up out of the dark distance appeared a fantastic creature. It was the biggest medusa that any of us had ever seen.

Mar19_stygio.jpg (42158 bytes)Stygeomedusa gigantea lives in cold, deep water and is only very rarely seen. Swimming across its dark brown bell and through its voluminous oral arms was a commensal fish that we did not recognize. Everyone gathered in the control room to watch.  

Using our lasers for a scale reference we estimated the diameter of the bell to be at least a meter and the length of the oral arms at two to three meters. That's a big jellyfish Jack! With skillful flying we examined the surface and structure of the thing from every angle. We found that the entire outer surface was dusted with batteries of nematocysts (stinging cells). The fish is apparently immune to them because it snuggles right in.  

Later, when the fish moved off a little, Buzz Scott used Tiburon like a cutting horse to isolate it and maneuver it into our suction sampler. Got it. Now we can measure the fish and use it as a reference for determining the size of the jelly. Back in the lab, after the dive, Jeff Drazen identified the fish as Thalassobathia.

Tomorrow we'll work again in the Farallon Basin, testing Tiburon's low-light SIT camera and surveying the water column for how vertical distribution patterns are influenced by low oxygen levels. We'll let you know how it goes... 

Bruce Robison

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