The Mysterious Movements of Deep-Sea Larvae
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Stay down and go far
McGillicuddy embedded in his models a certain degree of what he termed “sensitivity analysis” to account for larval physiology and behavior that Mullineaux and other biologists still don’t understand. McGillicuddy can opt to change different interconnected variables every time he runs the model to analyze how various factors affect how larvae are transported.
Even with these variables, the models produced some surprising results. One of the early hypotheses was that larval movements could be influenced by plumes of fluids emanating from the vents. The hot plumes rise into cold seawater until they cool down and achieve what is called neutral buoyancy, then they turn and drift horizontally, much the way smoke from a smokestack does.
(Click to enlarge image) In the first experiment of its kind, a specially designed injector system on the submersible Alvin released a jet of sulfur hexafluoride(SF6), a harmless chemical tracer, from a 50-millimeter orifice. The SF6 dissolved into the seawater, and scientists tracked where it flowed over the next several weeks to learn how oceanic flows might disperse the larvae of deep-sea organisms. (P.R. Jackson, J.R. Ledwell, and A.M. Thurnherr)
If this hypothesis held, larvae that manage to swim a few hundred meters off the ocean floor would be the ones dispersing long distances. McGillicuddy’s models, however, suggest that precisely the opposite may be true.
“According to these models, the flows are very energetic right near the bottom,” Mullineaux said. “It appears that those larvae which get into these bottom currents get advected longer distances than the ones that get up off the bottom.” Those larvae might get carried too far away from vent sites where they could resettle.
Follow the currents
Guided by these surprising findings, WHOI researchers launched the third phase of the LADDER plan: to test the model results in the field with a help of sulfur hexafluoride, or SF6, a harmless compound that scientists can track as it goes with the flows, as larvae do.
For this experiment, the team turned to Jim Ledwell, another scientist in the Applied Ocean Physics & Engineering Department at WHOI, and an expert on tracers. After studying McGillicuddy’s model results, Ledwell went aboard WHOI’s research vessel Atlantis to apply his expertise in the laboratory of the real world.
On Nov. 12, 2006, Alvin dove down into what’s known as the “axial trough” on the East Pacific Rise—a 100-meter-wide chasm on the ridge’s summit with steep cliffs tens of meters high on each side. It extends for kilometers along the spine of the ridge. In the first experiment of its kind, a specially designed injector system on Alvin released 6.6 pounds (3 kilograms) of the SF6 along a ¾-mile (1,200-meter) section of the ridge 20 miles south of the vent site at 9o50’ N.
Would the summit trough confine the flow of water and larvae and act as a conduit between habitable vent sites? Are some larvae swept up and away from the ridge, never to find a suitable vent to settle? What other factors determine the trajectories and distances larvae can disperse from their natal vents to remote vents?
Thirty-two days after the dye was released, Ledwell and company began methodically collecting water samples to find the tracer, using a sampling device called a CTD rosette, which is lowered on a wire from Atlantis. Over the next 20 days (bracketing the average 40-day larval lifespan), they sampled within the axial trough north and south of the release point. They found the tracer at the 9o50’ N site, demonstrating that larvae could make the 20-mile trip along the ridge axis within a larval lifespan.
Most of the tracer, however, was found in a patch about 50 kilometers west of the ridge, though it appeared to be heading back toward the ridge. Currents moved the tracer westward, away from the ridge crest where vents form. But then, about 25 days after release, the currents swung the tracer back toward the same ridge it came from.
Unexpected pathways
Lavelle’s model and the tracer results, as well as theoretical work, are all in agreement that larvae could get from one vent site to another is by traveling north along the western flank of the ridge. The tracer experiment also illustrated another pathway suggested earlier by MIT/WHOI Joint Program graduate student Diane Adams, working with Mullineaux. This second pathway involves eddies, or swirling currents, that may have carried the tracer or larvae away from the ridge at first, but then transported them back to the ridge.
“As these eddies come by, they make the tracer go one way for a while, and the other way for a while,” Ledwell said. “It’s just like the weather systems that come across in the atmosphere—the wind blows from the north for a while, and then a storm comes through and blows from the south.”
Whatever explains why the tracer did what it did, results of the test surprised Mullineaux, who for years suspected that mixing processes in the water would prevent larvae from getting back on the ridge once they veered too far off. Now, she says, the data suggest that it’s at least feasible for larvae to travel along previously unsuspected ocean pathways to colonize new sites farther along the ridge.
Ledwell agrees. “They might take circuitous routes,” he said, “but the larvae certainly can use the currents for resettlement and getting where they need to go.”
(Click to enlarge image) Between 32 and 52 days after the tracer was released at about 9°30'N (around No. 2 on the left map), researchers took 100 samples of water in various locations (or "stations," indicated by numbered circles) to see where the tracer dispersed. White circles=no tracer found; pink=a little tracer; red=a lot of tracer found. Stations 2 through 53 of the tracer survey (left map) showed the tracer had moved to 9o50’ N, demonstrating that larvae could make the 20-mile trip along the ridge axis within a larval lifespan. Stations 54 to 100, sampled subsequently, showed that the tracer moved about 50 kilometers west of the ridge, before heading back toward the same ridge it came from. (Courtesy of Jim Ledwell, Woods Hole Oceanographic Institution)
Beyond the LADDER project
As is usually the case, successful experiments provide answers—and even more questions, which LADDER project scientists are looking to follow up on. One factor they plan to closely examine is how seafloor topography influences current flows. In this case, a range of undersea mountains (or “seamounts”)—northwest of the site where the eruptions occurred and the tracer was injected—may have play a role in steering current patterns. The seamounts may turn flows 180 degrees, steering the flow around them and returning larvae to their native ridge, Ledwell said. Or, the seamounts could trap larvae drifting through from other places and redirect them to vent sites on the ridge.
For his part, McGillicuddy has more questions about how larvae actually behave. So he will continue to incorporate a greater number of realistic larval attributes into his model to see how they influence how and where they are transported.
As for Mullineaux, she said she would like to further investigate the snail and limpet species that colonized the areas depopulated by seafloor eruptions but that had not been seen there before.
“Are these species pioneers that researchers hadn’t been early enough to catch previously?” she asked. “Or are they indicative of a ‘regime shift’ ” in which new species take over sites previously dominated by different species? She’s also interested in learning more about the conditions and cues that prompt larvae to detect suitable sites and settle down. “These are the kinds of questions we hope to be able to answer.”
