Colonizing The Deep Sea: WHOI Scientist Helps Find Answers To Hydrothermal Vent Puzzle

In a paper published May 3 in the scientific journal Nature entitled "Larval Dispersal Potential of the Tubeworm Riftia pachyptila at Deep-Sea Hydrothermal Vents", Mullineaux and colleagues provide the first direct answer to the questions "How long can a vent larva live? And how far can it disperse?

Their work is part of the LARVE Project, funded by the National Science Foundation, to understand how vent species maintain their populations in changing vent environments, how they colonize new vents, and what controls their distributions over regional and global scales. The Nature paper is co-authored by Adam Marsh of the University of Southern California, Mullineaux, Craig Young of Harbor Branch Oceanographic Institution, and Donal Manahan of the University of Southern California.

Hydrothermal vents were discovered in 1977 by geologists in the eastern Pacific Ocean several hundred miles north of the Galapagos Islands. The unexpected finding of abundant marine life, including tubeworms several feet long and clams and mussels the size of dinner plates, in a place thought barren of most life forever changed views of life in the deep sea. These animals live in total darkness in near freezing temperatures, surviving through chemosynthesis, or chemicals from the earth's interior, rather than photosynthesis, or sunlight, as plants do on land.

"Larval dispersal has been one of the major questions in understanding how vent organisms colonize sites," Mullineaux says of the study. "Hydrothermal vents are constantly changing because of frequent volcanic and tectonic activity related to the formation of the earth's crust. Although we have known for a while that larvae can colonize new vent sites tens to hundreds of miles apart very quickly, we've never understood how the larva of these animals disperse to do that, or how long they can survive. Now we have some answers."

Mullineaux and her colleagues studied the larvae of the giant tubeworm Riftia pachyptila, a red-tipped worm that can grow several feet in length and which lives in a white plastic-like tube about an inch and a half in diameter. Specimens were collected from the eastern Pacific at several locations. The team then successfully reared embryos of the giant tubeworm to the larval stage under deep-ocean temperatures and pressures and followed their early development using custom-designed culture systems. They measured the metabolic rates of the larva at conditions like those at the deep-sea vents, 2¥ C (about 36¥ F) and 250 atmospheres of pressure.

They found that a typical larva of Riftia pachyptila could potentially survive for 38 days, just long enough it appears to get to another active vent many miles away and colonize it before running out of food.

But knowing the potential life span was only part of the puzzle. How far could the larva go and survive? The answer, it turns out, depends on where the vent is. At active vents, such as one of the experiment sites at 9¥50' North on the East Pacific Rise off Mexico, the hot waters mix with seawater to form buoyant plumes that rise until they cool to neutral buoyancy at a distance of 175-200 meters (about 575 to 650 feet) above the ocean bottom. Many of the early life stages of hydrothermal vent animals have been found trapped in this neutrally buoyant plume. Local currents may then carry the plume great distances, a sort of larval highway.

Since 1977 dozens of vent sites have been found in the Atlantic and Pacific Oceans. Animal communities differ from site to site, perhaps because of this larval highway and the nature of the currents at the different vent locations. More than 500 species of animals have been found at the known sites to date, many of them new species.

In August 2000, a Japanese team found the first vents in the Indian Ocean. A team aboard the Woods Hole Oceanographic Institution's Research Vessel Knorr has just concluded the first U.S. expedition to explore hydrothermal vents in the Indian Ocean. The Knorr team has reported finding species of animals that appear to be similar to those found in both the Atlantic and Pacific, but many others that may be new species, raising the question again about how these animals disperse or move around the globe.

Although no tubeworms have been found at the Indian Ocean vent sites, Mullineaux says that could be because the larvae can't get there, or the Indian Ocean sites aren't suitable, or perhaps that vents with tube worms just haven't been found there yet.

Animal populations at different vent sites, in appears, may have different dispersal limits based upon the local currents. Mullineaux's team says the life span of this species can now be used to predict dispersal under the current conditions at other hydrothermal vent sites.

"That's a major step forward," she adds." And it demonstrates the importance of scientists from many disciplines working together to answer some very basic but fundamental questions about how marine life on our planet survives."

The LARVE Project (Larvae At Ridge VEnts) is a component of the RIDGE (Ridge Inter-Disciplinary Global Experiments) Initiative, a coordinated, interdisciplinary program aimed at understanding the geology, physics, chemistry and biology of processes occurring along the global mid-ocean ridge system.