modiolus thermophilus. The clams live on the outer perimeter of the vent site, commonly found in small crevices where low-temperature vent fluids are coming out of the fractured volcanic terrain. They have a long foot that aids them in moving as well as feeding, primarily for the uptake of vent fluids while their gills absorb oxygen and inorganic carbon from the circulating bottom waters.
The mussels, on the other hand, are commonly found in high-temperature settings near the vent opening and apparently ingest bacteria directly through filter feeding. Such direct ingestion of food is thought to be secondary to their primary source of nutrition from symbiotic bacteria living within their bodies. Other mollusks include limpet-like gastropods and whelks.
The most spectacular organisms associated with many hydrothermal vents are the large white, red-tipped vestimentiferan tube worms, Riftia pachyptila. Living in a highly precarious setting of varying levels of oxygen and temperature, this organism is truly unique. It lacks a mouth, gut, and digestive system and relies upon the symbiotic bacteria that make up half its body weight to feed it. Since these tube worms live where reduced vent fluids mix with the oxygenated bottom water, they need to withstand prolonged periods of time when anoxic conditions prevail. As a result, their blood includes human-like hemoglobin, which stores oxygen within their body.
Another fascinating worm living under an even harsher vent setting is the Pompeii Worm, Alvinella pompejana. These live in a mass of honeycomb-like tubes near high-temperature vents that they freely move in and out of. Their tubes have even been seen attached to sulfide chimney walls of 350°C black smokers, although they must live in the highly mixed waters having a lower temperature.
Scavenging and carnivorous brachyuran crabs are also associated with the vent communities, as well as numerous other organisms including anemones, siphonophores, fish, shrimp, and so forth, too numerous to describe in any detail here.
In 1984, an entirely different geologic setting was found in which similar organisms are living. Cold water seeps on the West Florida Escarpment in the Gulf of Mexico were discovered that support sulfide-oxidizing benthic communities. Groundwater flowing through porous limestone releases sulfide and methane-enriched water that leads to the growth of chemosynthetic bacterial mats and symbiotically supported communities of large mussels and vestimentiferan worms. These communities also include galatheid crabs, gastropods, sea anemones, serpulid worms, and other organisms typical of warm water vent settings.
Further to the west in the Gulf of Mexico, cold water seeps of hydrocarbons including methane were found to support similar benthic communities. More recently, hydrocarbon seeps off the west coast of California, in the North Sea, and the Sea of Okhotsh have been found to support a similar assemblage of organisms. Even the oily bones of a decomposing whale off California provide a home for this unique biological ecosystem. The investigation of seamounts was also expanded to include the investigation of craters, calderas, and pyroclastic deposits on seamounts in the Pacific.
Clearly in years to come, chemosynthetic animal communities will be found throughout the world's oceans and lakes wherever the conditions arise to spawn this unique symbiotic relationship. As this paper is being written there are those who are turning their thoughts to the volcanic terrains of Mars or the ice-capped ocean of Europa. Such thoughts include the continuing debate dealing with the very origin of life on our planet.
Following the excitement of the later 1970s and early 1980s, Alvin's annual diving program pushed north from the East Pacific Rise off Mexico to include regular visits to the Juan de Fuca, an isolated segment of the Mid-Ocean Ridge, connected millions of years ago to the East Pacific Rise. Dives in the Juan de Fuca and Gorda Ridges off the coasts of Oregon and Washington in 1984 resulted in the discovery of hydrothermal vents and high-temperature black smokers.
With increased funding from the National Science Foundation, the engineers supporting the Alvin program now were able to "harden" its capability and make major improvements in its propulsion, electrical, and instrumentation systems. Returning to the Mid-Atlantic Ridge after its lengthy overhaul in 1986, Alvin was able to investigate newly discovered hydrothermal vents and a unique benthic animal community dominated by shrimp.
In 1987, Alvin crossed the Pacific Ocean for the first time in its history, stopping in the Hawaiian Islands. There scientists investigated Loihi Seamount along the volcanic ridge extending southeast from the big island of Hawaii before continuing west to the Mariana Islands. A team of scientists had discovered hydrothermal vents in the back-arc basin west of the Mariana Islands, and Alvin was used to document and sample their chemistry and unique biology. Following what would be its only expedition to the western Pacific, Alvin and its support ship Atlantis II returned to San Diego for maintenance and repairs.
For the next 10 years, Alvin's diving schedule became fairly routine, journeying back and forth along the West, East, and southern coasts of the United States with frequent visits to the Juan de Fuca Ridge and Oregon coast, the California continental borderland, the East Pacific Rise, Guaymas Basin, Galapagos Rift, Mid-Atlantic Ridge, Bermuda, and the East Coast.
The investigation of hydrothermal vents including coldwater seeps continued to dominate Alvin's use, but other programs emerged as well. These included the continued investigation of seamounts and the investigation and instru