Travel to the middle of the Pacific Ocean, plunge two miles to the seafloor near an underwater volcano, release a strawberry seed in the pitch darkness, and figure out where it will land.
That sort of sums up the task that faced University of Delaware marine biochemist Adam Marshand his colleagues in their quest to determine how new tubeworm colonies are formed at hydrothermal vent sites. The findings are reported in the May 3 issue of Nature.
“Hydrothermal vents are very ephemeral. These underwater volcanoes appear as the ocean floor spreads, and then they rapidly disappear,” says Marsh. Scientists have often wondered how tubeworms, which are sessile creatures and can’t move about the seafloor, wind up at new vent sites.
“The key to solving the mystery is the dispersal of their young, which are about as big as strawberry seeds,” Marsh notes. “Where these tiny larvae go is determined by the deep-sea currents and the larvae’s life span — which sets a maximum time limit for them to travel between vent sites and survive.”
To find out how long a journey baby tubeworms can survive, the scientists first had to figure out how to culture them in the lab. Stainless steel chambers with continuously flowing seawater were specially designed to re-create the larvae’s natural environment of 2°C (35°F) and crushing pressure of nearly 3,700 pounds per square inch.
“With the exception of hydrothermal vents, most of the deep ocean is only a few degrees above freezing,” Marsh explains. “Tubeworm larvae must withstand these cold temperatures as they are dispersed by the currents, hopefully to a home in warmer water near a vent site.”
The scientists found that tubeworm larvae do not develop a mouth in the first 34 days of development, so they can’t obtain energy from food during this period to help them weather their ride on the currents. Instead, they must rely on an internal fuel supply. To determine how much energy the babies have, the scientists assessed the proteins and lipids in a tubeworm egg and the rate at which this fuel is used. From these studies, they determined that baby tubeworms have an average life span of about 38 days.
“We were surprised by this,” says Marsh. “Other creatures that live in extreme environments, such as Antarctic sea urchins, have much longer early life stages, lasting several months.”
How far can baby tubeworms travel in 38 days? To answer that question, the researchers needed to account for both vertical and horizontal movement. Through a combination of lab studies and field work at vent sites along the East Pacific Rise off Mexico, they found that the larvae are almost neutrally buoyant. Like most fish, they can maintain a steady position in the water no matter where the currents take them. In this case, the larvae are likely to first be entrained in the plumes of water that rise 175 meters (574 ft) above the vents as the super-heated water rockets out of the Earth and mixes with the cold seawater.
Then deep-sea currents move the larvae along. The scientists assessed the speed and direction of this flow by positioning current meters 175 meters above the ocean floor at their test site and on a nearby ocean ridge. To estimate the dispersal potential of the tubeworm larvae, they used computers to model the movement of neutrally buoyant particles released at hourly intervals into the observed flow pattern.
“We were at the point where we could say that if a tubeworm larva — this tiny speck — was released at 9 a.m. on a certain day, we could determine where it would be at 3 p.m.,” says Marsh.
After several months of studies, the scientists concluded that baby tubeworms rarely travel farther than 100 kilometers (62 miles). While this distance may seem pretty substantial to most of us, Marsh says it's not much when you consider that active vent sites are a fairly rare occurrence on the ocean floor.
"Even at the ridge segments where there is volcanic activity, a distance of 100 to 200 kilometers may separate areas of active venting," he notes. "Thus, the tubeworm larvae's dispersal range tells us that, on average, they do not routinely travel between active vent sites. Instead, the colonization of a new vent site is dependent on rare current conditions when a larva ends up traveling beyond 100 kilometers."
Marsh says this research to solve the mystery of how new tubeworm colonies are formed illustrates the interdisciplinary approach needed to tackle many marine questions.
“This study is truly integrative in that we had people working on the biochemistry of the tubeworm eggs, the physiology of the embryos, the buoyancy of the larvae, and the hydrodynamics of the ocean around deep-sea vent systems,” Marsh adds. “We had to put all of this together to determine how far the larvae would go.”
Marsh's co-investigators included Lauren Mullineaux, Woods Hole Oceanographic Institution; Craig Young, Harbor Branch Oceanographic Institution; and Donal Manahan, University of Southern California. The research was funded by the National Science Foundation.