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Finding a Home on the Ocean Floor
From: Woods Hole Oceanographic Institution
| By:
Edwin Schiele |
EDITOR'S INTRODUCTION |
Much like those above sea level, homes on the ocean floor don't last forever. At the bottom of the ocean, organisms such as tubeworms, giant clams and white crabs live around hydrothermal vents. They obtain energy through symbiotic relationships with microorganisms that produce sugars from the chemicals in hydrothermal fluid. After ten to fifty years, a hydrothermal vent will become inactive and the organisms that live on it will need to search for different homes.
Woods Hole Oceanographic Institute senior scientist Lauren Mullineaux and her colleagues are studying how the larvae of seafloor organisms discover new habitats and colonize them. Working thousands of meters below sea level, they sample the existing larval populations and set up experiments to determine what factors affect the survival of these organisms. Their research may also make the management of shellfish beds easier by revealing what factors impact the recruitment and endurance of shellfish larvae. |
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| Lauren Mullineaux in her lab at the Woods Hole Oceanographic Institution. | |
 auren Mullineaux's research group studies a side of benthic organisms (animals that live on the seafloor) that until recently has received little attention. These marine animals, which include tubeworms, shellfish, such as mussels and clams, and gastropods, such as snails and limpets, have two distinct life cycle phases. The adult phases are the most recognizable; the majority of people would be hard-pressed to recognize the equally important larval stages, which look nothing like the adults. |
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| Two stages of larval bivalves. | |
Unlike the adults, which lie on the seafloor, these microscopic larvae rise up into the water column and drift in the currents for weeks, months or even years. They eventually settle to the seafloor, and if the conditions are right, develop into adults. Mullineaux and her group are investigating how these microscopic larvae are able to find suitable habitats and then colonize them. Some of the most fascinating habitats they are investigating are the hydrothermal vents on the deep ocean floor. |
Life at the bottom of the oceans
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| Mullineaux enters the deep-sea research submarine Alvin. | |
Life on the deep-ocean floor must survive extreme conditions. Sunlight does not penetrate to the ocean floor, so it is cold and dark. The pressure from the weight of all the water is hundreds of times greater than what we experience at sea level. Until recently, scientists thought that although life existed on the seafloor, the abundance of organisms was low. |
These ideas changed dramatically in the 1970s. Scientists onboard the research submarine Alvin made a series of dives to the East Pacific Rise, part of the global mid-ocean ridge system. There they discovered scalding water billowing up from hydrothermal vents on the seafloor. Surrounding these underwater geysers were clusters of bizarre animals that were completely new to science, including tubeworms, giant clams and ghostly white crabs. |
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| An underwater view from inside the submarine Alvin. | |
Hydrothermal vents form in places where there is volcanic activity. They are most often found along the global mid-ocean ridge system where tectonic plates are spreading apart and new oceanic crust is being created. Seawater seeps down through cracks in the ocean floor and circulates deep into the ocean crust. Molten rock (magma) far below the seafloor heats the water to temperatures of up to 350 degrees Celsius (650 degrees Fahrenheit). The scalding water reacts with rocks in the ocean crust, picking up sulfides and dissolved metals, such as copper, zinc and iron. |
Because hot water is more buoyant than cold water, it rises up through the ocean crust and jets out the vent openings. When this hot hydrothermal fluid mixes with the cold oxygenated seawater, the metals carried up by the fluid react with sulfides to form black minerals. These black particles make the fluid look like smoke; hence these vents are called "black smokers." |
Chemosynthesis
The question of how organisms on the seafloor get enough energy puzzled scientists for a long time. Sunlight does not reach the hydrothermal vents, so organisms cannot manufacture sugars through photosynthesis. The answer is found in the hydrothermal fluids gushing from the vents. |
Microorganisms, including bacteria and Archaea, extract energy from chemicals in the hydrothermal fluid, such as hydrogen sulfide. Just as plants use energy from the sun to convert carbon dioxide into sugars, these microbes use chemical energy to manufacture sugars. This process is called chemosynthesis. |
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| A block covered by vestimentiferan tubeworms from the seafloor. | |
These microbes form the base of the food chain at hydrothermal vents. Some animals, including tubeworms, clams and mussels, have formed symbiotic relationships with these microbes. For example, a tubeworm's cavity is packed bacteria. The tubeworm provides the bacteria with hydrogen sulfide and oxygen then uses some of the sugars the bacteria produce as food. |
Clams and mussels wedge themselves into cracks in the seafloor around hydrothermal vents. Both types of shellfish have formed symbiotic relationships with bacteria that live in their gills. The clams and mussels use some of the sugars that the bacteria manufacture as food. |
When the clams and mussels spawn, the microscopic larvae move up into the water column and are carried away by the currents. Although the larvae are unable to fight the currents horizontally, they can move vertically through the water column by changing their buoyancy. Scientists hypothesize that the larvae find hydrothermal vents by detecting some sort of chemical signal from the vents. They then drop down to the seafloor. |
Homes on hydrothermal vents are only temporary. Hydrothermal vents remain active for a limited amount of time, perhaps ten to fifty years. When an old vent becomes inactive, the organisms must somehow find new vents to colonize. |
Studying larvae dispersion
To learn how the larvae disperse to new vent sites, Mullineaux and her group have dived in submersibles to vents thousands of meters deep. There they have sampled and collected larvae, measured the currents that carry the larvae away, and set up experiments on the seafloor where the larvae settle down in order to determine which factors may influence the larvae's survival. These experiments include transplanting substrates on the seafloor from one type of environment to another and putting down cages to exclude predators. They are also collaborating with colleagues who are placing the larvae in pressure vessels and studying the larvae's behavior and metabolism in order to learn how long the larvae can survive in the water column. |
Mullineaux and her group are also studying shellfish larvae in commercial shellfish beds along the coast. In many shellfish populations, the larvae only successfully settle down and develop into adults every two or three years. This unpredictability has made managing these shellfish beds difficult. Mullineaux and her group are trying to figure out what environmental factors affect the recruitment and survival of the larvae. This information should help managers predict how the shellfish populations will vary from year to year. |
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