Exploring Vents: Vent Biology 

Hydrothermal Vent Biogeography

Since the discovery of animal communities thriving around seafloor hydrothermal vents in 1977, scientists have found that distinct vent animal species reside in different regions along the volcanic 40,000-mile Mid-Ocean Ridge mountain chain that encircles the globe. Scientists are investigating clues to explain how populations are connected, how they diverged and evolved separately.

To date, more than 590 new animal species have been discovered living at vents, but fewer than 50 active vent sites have been investigated in any detail. Scientists currently recognize six major seafloor regions—called biogeographic provinces—with distinct assemblages of animal species.

In the eastern Pacific, tubeworms, clams and mussels dominate vent sites. In contrast, tubeworms are notably absent at vents in the Atlantic. Instead, billions of shrimp swarm at vents along the Mid-Atlantic Ridge, which bisects the Atlantic Ocean floor. There are two biogeographic provinces in the North Atlantic. Different species of shrimp and mussels predominate at vent sites that are at different depths. The deeper ones are south of about 30°N and shallower vent sites occur to the north. Both Pacific and Atlantic vents have mussels, but not the same species.

The fourth province is in the northeast Pacific, off the U.S. Northwest coast, which shares similar types of animals (clams, limpets, and tubeworms) with the eastern Pacific province, but markedly different species of each. In the western Pacific Ocean, at spreading ridges west of the Mariana Islands, vents in the fifth province are populated by barnacles, mussels, and snails that are not seen in either the eastern Pacific or the Atlantic.

Scientists got their first chance to search for vents in the Central Indian Ocean in 2001 and found the sixth province. These vents are dominated by Atlantic-type shrimp, but also had snails and barnacles resembling those in the western Pacific.

Until 2005, all known Atlantic vent sites were north of the equator. Preliminary results from recent discoveries in the Atlantic south of the equator (5°-9°S) suggest these sites host similar but distinct species from known Indian Ocean and East Pacific Rise vents. Thus, the vents in the South Atlantic may represent a seventh biogeographic province.

The southernmost known chemosynthetic community in the Pacific is a vent site near 37°S on the Pacific-Antarctic Ridge. It includes Pacific-“like” fauna (bathymodiolid mussels, vesicomyid clams, and lepetodrillid snails).

Missing Pieces

The largely unexplored oceans in the Southern Hemisphere and the Arctic are critical regions where the missing pieces of the biogeographic puzzle may be found. Strategic exploration for hydrothermal vent and other chemosynthetic fauna in remote regions of the Mid-Ocean Ridge system will lead to discovery of new biogeographic provinces and fundamental insights into evolutionary relationships among the global deep-sea faunas.

All these regions contain the same basic ingredients to support life—chemical nutrients generated by geothermal processes at hydrothermal vent sites. So why do vent fauna differ in the Atlantic and Pacific, or in the eastern and western Pacific? How do we assemble these puzzle pieces to explain the diversity and evolution of vent species throughout the world’s oceans?