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JULY
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This little atolla jellyfish can generate an impressive bioluminescent display.
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Dr. Tammy Frank (left) and Dr. Edie Widder review the animal counts from this evenings submersible dive.
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Ultraviolet light is highly damaging to living cells; planktonic distribution (with increasing numbers of animals found deeper) reflects their avoidance of UV exposure.
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Dr. Sonke Johnsen calibrates his ultra-violet camera system.
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The JOHNSON-SEA-LINK I submersible is on deck ready to go to work.
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Sergestids like this Sergestes corniculum are migrating in the water column.
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Click below to learn a bit more about this advanced and versatile deep-sea research tool...
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Accomplished videographer Brian Cousin is doing double duty on the Gulf of Maine cruise. He's gathering footage for a video about bioluminescence, and he's bringing the Gulf of Maine to your desktop as our @Sea correspondent. Click below to learn more about Brian...
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Is there a question you'd like to ask our intrepid correspondent? Send us an email at
AskAtSea@hboi.edu.
Selected questions will be forwarded to Brian, and we'll post the answers online.
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DISPATCH 11: Sergestids as Good as Mine!
@Sea correspondent/photographer, Brian Cousin
WHOI's Sonke Johnsen collecting a salp on our third blue-water dive.
July 17 --
It's another good day for operations in the Gulf of Maine despite yesterday's marine reports predicting overcast conditions, rain and rougher seas. It's really pleasant on the water, and a bit startling to hear that Boston is on it's way to record-breaking high temperatures.
For the second day in a row the science team was able to complete two dives in the JOHNSON-SEA-LINK I, a blue-water scuba dive and a successful Tucker trawl. Life is good!
Chief scientists Dr. Tammy Frank and Dr. Edie Widder have made enough submersible transects and collected enough data to make some prelimary observations. There are distinct population layers, or sonic "scattering layers" of animals in the water, and they are migrating. Krill seem to be at the leading edge of the vertical movement. Salps are next.
"The salps are actually migrating", reports Tammy. "We're not even sure what they're using as photoreceptors."
They don't have eyes, but they could have shadow receptors, as some other deep-sea animals without optic organs do. The sergestids, which reside in the deepest, dimmest water during the day, haven't been seen by Tammy or Edie during the evening submersible dives. That doesn't mean they are not migrating. They believe these little crustaceans are the most sensitive to the light - that may be one reason they live deeper than the other animals in the scattering layer.
"They're probably not getting as high up in the water column as we are by the time we have to end the dive. We're going to have to dive deeper and miss the krill to witness the sergestids migration."
The dives are conducted differently than those made earlier to characterize the transparency of the water. Rather than both scientists being in the sub, one remains on the ship while the other dives in the sphere of the JSL (they take turns, of course!). The shipboard scientist reports from the bridge the depth of the leading edge of the rising scattering layer, visible on the ships fathometer. As the layer passes by, the sub does horizontal transects, and the scientist identifies and counts the animals in a specific area in front of the sphere.
"That's how we know who's migrating first and to what depths. The krill are migrating first, up from between 1,200 and 1,700 feet during the day, to about 200 to 500 feet at night.
Without a mid-water submersible like the JOHNSON-SEA-LINK, qualitative work like this would be extremely difficult to accomplish, if not impossible
There was something different about the ocean today that bolsters Tammy and Edie's hypothesis about light response in mid-water animals even further.
Tammy says, "We just got an influx of colder, fresher water. It's murkier."
Tammy consults her binder of dive notes.
"I'll be darned. Sure enough, it was definitely dimmer today." She consults further, flipping through the pages comparing notes with previous dives. "We usually see sergestids around 600 meters (about 1,800 feet). Today we saw a lot of them at 500 meters (about 1,500 feet). So it looks like they have a light level they prefer to remain below."
If the water changes back to the way it was before today the sergestids may move back down to their previous daytime level.
Sonke Johnsen, the scientist from Woods Hole and former post-doc in Edie Widder's lab at Harbor Branch, is also making progress. Using ultraviolet light, he is measuring the transparency of gelatinous organisms collected by the JSL in deep water. He is comparing these to animals collected in shallow water during scuba dives. Sonke is observing that many of the shallow water organisms have UV blocking pigment to keep them from burning in the sun-lit surface layer where UV levels are still high. Water does not immediately filter out ultraviolet wave lengths, in fact low levels of UV light have been detected as deep as about 600 feet using instruments Tammy and Edie take aboard the submersible. Below that depth, instrument readings become hard to rely on.
So shallow-living gelatinous creatures have protection from the burning rays of the sun. The downside is that it makes them more opaque than transparent, so they're easier for predators to see.
"I've never heard of it (UV-protection) in these kinds of animals.", says Sonke. "Other researchers have found it in animals that are exposed to greater extremes of UV light, like you'd find in tidal pools."
What's important about it?
"More scientists are interested in making good optical measurements. One driving force is the change in the ozone layer in the atmosphere. Animals may not respond well to large increases in UV light. It may not be a big issue right now, like it is in Antarctica, but work now could help assist in policy matters down the road. Aside from it being really interesting work."
Sonke will analyze his data when he gets back to Woods Hole. He'll also be thinking of ways that he can do things better when he comes back out to sea with Tammy and Edie in September. Perhaps some modifications to the UV video system.
At the frontiers of science, there aren't many off-the-shelf solutions scientists can use to collect data and make hard-and-fast conclusions about the natural world - especially in the ocean. Sure there's a lot of technology out here, but these modern-day pioneers are always looking to make the next step: to go where no one has gone before and to understand what was previously unknown.
Says Sonke, "Biology is one of the most challenging sciences to tackle. There's so much diversity in the ocean and we know so little about any of it. Every time the sub comes up there's something someone onboard has never seen. That's why everyone's so excited."
"I'd like to see 200 years from now when the next generations of scientists look back at our archaic techniques and some of the results and say "Wrong!".
I work on the assumption that something wrong will be found with everything I do. Some of what we're doing is pretty primitive - swimming around in the ocean with jars, collecting things. But we have to start somewhere. If no one did anything, we wouldn't know anything at all."
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