Tubeworms at the cold and calm hydrocarbon-seep sites in the Gulf of Mexico have surprisingly long life spans, especially compared to their cousins living at the hot and dynamic hydrothermal vents, according to a paper published in the 3 February 2000 issue of the journal Nature by a Penn State research team.
"The hydrocarbon-seep tubeworms we studied take from 170 to 250 years to grow two meters long, while hydrothermal-vent tubeworms grow well over a meter in just one year," says graduate assistant Derk C. Bergquist, an author of the research paper along with Frederick M. Williams, associate professor emeritus of biology, and Charles Fisher, professor of biology.
"The hot hydrothermal vents are a much more vigorous, variable, and ephemeral environment than the cold hydrocarbon seeps," Fisher says. Both the seeps and vents are regions on the ocean floor where fluids rising from the Earth's crust are breaking through and mixing with the seawater. But hydrothermal vents have a wide range of shifting environments including "black-smoker" chimneys belching out material as hot as 400 degrees Celsius, whereas seeps release the hydrocarbon fluid slowly and dependably over a long period of time at seawater temperatures.
"The chemical content of the fluid is similar but it's cooled down and calmed down a lot at the seeps," Bergquist explains. Tubeworms live on the sulfide and other nutrients in the fluids at both the seeps and the vents, but they grow at much different rates in these two very different environments, according to the researchers.
The scientists say the tubeworms they studied at the cold seeps are the most long-lived noncolonial animals without backbones currently known. "The tissues of colonial animals like coral and anemone are continually recycled by the community, so it is tough to say what a single individual is in a colonial animal," Bergquist explains. "Entire colonies can have life spans over 1,000 years, but no individual polyp in the colony lives that long."
To study the tubeworms' growth, the team used the Johnson Sea Link submarine, equipped with a special staining device they designed in collaboration with the Harbor Branch Oceanographic Institution, to reach the tubeworm sites 1800 feet below the ocean's surface about 120 miles off the Louisiana coast. They first used the "stainer" to color the white tubeworm casings a robin's-egg-blue, then returned to the site a year later to collect and study the animals. By comparing the overall length of each tubeworm with the length of its new, unstained, growth, the researchers were able to determine the average growth rate for animals of different lengths and ages.
Tubeworms, which grow in spurts as they lay down the successive rings of their external skeleton, grow more when they are young than later in their life span. "The trend for a decreasing growth rate with increasing size is typical of most animals, including humans," Bergquist explains. "The most realistic age estimate for an average 2-meter-long tubeworm, taking into account the changing growth rates over the animal's life, is 170 to 250 years old," Fisher says. "Since we collect animals much longer than 2 meters, we know that this is a minimum estimate and that some of these individuals live much, much longer."
The researchers now are trying to understand why these animals live so long and grow so slowly. Bergquist speculates it is possible that their long life results partly from the shortage of solid growing locations on the mostly muddy bottom of the Gulf of Mexico. Animals with shorter life spans in this environment are more likely to die off before any of their larvae could find a place to grow--leaving no trace of their genes in future tubeworm populations. "Natural selection would seem to prefer long-lived tubeworms in the hydrocarbon-seep environment, but it's going to take a lot of really careful hypothesis testing to say exactly why they live so long," Bergquist says.
The team's work in the Gulf of Mexico is largely driven by a Mineral Management Service policy designed to protect the tubeworms from the potentially damaging effects of oil drilling. The policy requires oil companies to demonstrate before they build a drilling platform on a particular leasing area that the site contains no high-density chemosynthetic communities--or that measures will be taken to prevent harm to these communities. One of the research sites, called Bush Hill because it is covered with bush-like clusters of tubeworms, is about 500 yards from one of the deepest oil-drilling platforms in the world. "Technology now is making drilling possible at deeper and deeper sties, so these deep-water communities are no longer safe from human activity," Bergquist says.
"Our research has shown that these tubeworms grow very slowly over a long time and that their larvae need to settle on exposed substrates, which may not be easy to find on the muddy floor of the Gulf," Bergquist explains. "If you destroy one of these communities, it likely would take a long time to come back if it comes back at all."
This research was supported by the Mineral Management Service of the U.S. Department of the Interior.
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