Living on the Seafloor

Life at the bottom of the oceans

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.

Drawing by Paul Oberlander.

Hydrothermal vents

These ideas changed dramatically in the 1970's. 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.

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 C (650 degrees F). The scalding water reacts with rocks in the ocean crust, picking up sulfides and dissolved metals, such as copper, zinc, and iron.

Photo by Dan Fornari.

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."


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.

Photo by Dan Fornari

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 with bacteria. The tubeworm provides the bacteria with hydrogen sulfide and oxygen, and then uses some of the sugars the bacteria produce as food.

Clams and mussels

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.

Photo by Dan Fornari.

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.