The Mysterious Movements of Deep-Sea Larvae
Filed under: Discovery,News & Resources
(Click to enlarge image) A larva of a deep-sea bivalve called Bathypecten vulcani collected on the East Pacific Rise. Its diameter is 840 microns (the width of a human hair is about 100 microns). (Photographic Identification Guide to Larvae at Hydrothermal Vents in the Eastern Pacific)
How do the tiny progeny of seafloor animals disperse through the ocean?
The marvelous migrations of fish and whales through the deep sea have been hard enough for us humans to follow. But what about tiny organisms—many smaller than the dot beneath this question mark? How they move from one spot to another in the depths has long remained beyond our grasp.
(From WHOI / by Matt Villano) — The enigma deepened in 1977, when scientists discovered spectacular and strange communities of animals clustered near vents on the seafloor. These so-called hydrothermal vents spew chemical-rich fluids that sustain clams, mussels, tubeworms, snails, and other species. Like shellfish in shallow waters, most of these relatively sedentary deep-sea animals reproduce by releasing eggs and sperm into the water. These develop into tiny floating larvae—the aquatic animal equivalent of seeds—that disperse, settle at vent sites, and grow.
Here’s the catch: The vents are distributed intermittently along mid-ocean ridges—the long volcanic mountain chains that bisect the seafloor throughout the globe. These vents “turn on” and “turn off,” fueled by the ebbs and flows of hot magma beneath the seafloor.
(Click to enlarge image) A larva of a deep-sea bivalve called Bathypecten vulcani collected on the East Pacific Rise. Its diameter is 840 microns (the width of a human hair is about 100 microns). (Photographic Identification Guide to Larvae at Hydrothermal Vents in the Eastern Pacific)
So how do the larvae, tinier than specks of dust, maintain their populations in such a patchy, transient environment? How do they get transported from one active vent site to another that might be tens of miles or more away?
These questions are keys to understanding how life has evolved on the seafloor and how it survives. Answering them requires a blend of biology, oceanographic physics, geology, and chemistry. So in 2006, scientists from Woods Hole Oceanographic Institution (WHOI) led a collaboration of researchers from different disciplines who investigated the broad range of factors that could influence larval movement, focusing on one particular mid-ocean ridge called the East Pacific Rise.
“There’s so much happening down there that we simply needed to take an interdisciplinary approach,” said WHOI biologist Lauren Mullineaux, “and I think everyone is glad we did.”
The research project, dubbed LADDER (LArval Dispersal on the Deep East pacific Rise), is prompting scientists to rethink much of what they suspected about how deep-sea larvae disperse.
The seafloor repaved
In the late 1990s, Mullineaux and colleagues from Pennsylvania State University and University of North Carolina launched a project that began to make inroads into the lives of larvae. They developed high-pressure systems to culture deep-sea larvae in the lab and study the larval stages and life cycles of several species. During multiple visits over a few years to vent sites on the East Pacific Rise, the scientists used the submersible Alvin to place and later retrieve experimental blocks of rock around seafloor vents to collect larvae and learn how they settle.
The researchers discovered that the larvae of one tubeworm species contain enough stored energy, in the form of lipids, to survive for 30 to 40 days. Some larvae had hairlike cilia for swimming, but they wouldn’t take the tiny larvae very far. They needed to get swept into big ocean flows or currents. But what kind of flows and how far could they take the larvae?
“We thought we had a pretty good sense of how the larvae got into the flows,” Mullineaux said, “but we weren’t sure what happened once the larvae were in the flows.”
Then in 2006, an unanticipated event gave researchers a rare opportunity. A series of excursions with Alvin showed that seafloor eruptions over several months on the East Pacific Rise had wiped out well-studied vent communities of organisms at 9°50’ N, south of Mexico.
Until then, it had been hard for scientists to distinguish whether oceanic flows were bringing in larvae to 9°50’ N from distant sites, or taking larvae from 9°50’ N to other distant sites, or preventing larvae from leaving 9°50’ N and forcing them to settle near their birthplace. Any of those were possible. New vents formed in the area, but the eruptions that paved over previous communities with lava presented the researchers with a clean slate and a chance to observe how larvae colonize an area from its beginning.
(Click to enlarge image) WHOI biologists used stacked plastic plates, called "sandwiches" (left), as artificial substrates for larvae of vent animals. The sandwiches were deployed on the seafloor near a hydrothermal vent site with Alvin and later recovered (center) to see how larvae (tubeworms, in the right photo) settled and grew on them. (Skylar Bayer/Brown University; WHOI Alvin Group/LADDER III cruise)
What would larvae do?
Mullineaux jumped into action, teaming with Jim Ledwell and Dennis McGillicuddy from WHOI, Andreas Thurnherr from the Lamont-Doherty Earth Observatory at Columbia University, and Bill Lavelle from the National Oceanic and Atmospheric Administration. They set out on a series of cruises to capture as much information as possible about where new animals colonizing the clean-slate site came from and how they got there. Seafloor experiments in 2007 and 2008 showed that while some of the same animals that had been there before (such as tubeworms) came back, newcomer species showed up, too, including snails and limpets.
(Click to enlarge image) WHOI scientist Dennis McGillicuddy used a computer model developed by NOAA scientist Bill Lavelle in a series of experiments that simulated where particles—representing larvae released at a mid-ocean ridge—would end up. In the experiment above, light blue dots represent starting points of particles released 10 meters above the ridge crest, spaced 1 kilometer apart and spanning 20 kilometers across the ridge crest. Dark blues lines indicate their pathways; red dots represent where they ended up 30 days after release. (Courtesy of Dennis McGillicuddy, Woods Hole Oceanographic Institution)
The researchers’ next goal was to learn more about the oceanic flows that transport larvae. But it isn’t easy to track larvae at the bottom of the sea. They are too small to affix with acoustic tags, too small to follow around in a submarine, and too diffuse to sample. So, the team embraced a tripartite plan to measure deep-sea currents, to use computer models that would simulate larval transport in oceanic flows, and to re-enact larval dispersal in the field with the help of a special dye-like tracer.
The process began with data collected by Thurnherr, who deployed current meters on the ridge to record how currents fluctuate. These data formed the basis of a computer model that Lavelle developed to simulate hydrodynamics on and around the ridges.
Next, McGillicuddy, a scientist in the Applied Ocean Physics & Engineering Department at WHOI, used Lavelle’s hydrodynamic model to (virtually) “release” particles representing larvae. He ran a series of computer simulations to see where the larvae would end up.
“The fundamental issue is that the interactions between the physical circulation and biological processes are so complex that it’s hard to take a pencil and paper out and work out exactly what’s going to happen,” McGillicuddy said. “We use numerical models as a way to simulate the environment and explore how organisms may respond to physical forcing of various types.” (See a simulation at right.)
