Organisms associated with deep-sea hydrothermalism: Fauna from Pacific and Atlantic vent communities

Hydrothermal vents were discovered at the end of the ‘70s in the Pacific and haven’t ceased to amaze us since. A flourishing and unusual thrives over a small surface around sources of warm fluid. These communities comprise a limited number of species that, throughout their evolution, have adapted to this environment. 

How life is possible in these hostile environment ?

These oases of life in the abyssal desert rely entirely on bacterial production that uses the chemical compounds contained in the fluid to produce organic matter. The light  does not penetrate that far from the surface and plants (the energy source at the base of food webs everywhere else on the world) cannot live there. Although very rich, this type of environment is also very challenging because of the toxic compounds found in the fluids. The temperature can reach 350˚C in the pure fluid but the animals live in a zone where it ranges from 3 to 50˚C according to the distance from the fluid source (typical deep-sea water temperature is roughly 2˚C at these depths).

Larvae, preservation of the communities of hydrothermal vents

To date, only 600 species have been captured and described from the roughly 100 hydrothermal fields explored in the world’s oceans. The big groups of animals encountered near hydrothermal vents are also found elsewhere in the ocean. For instance, the bathymodiolin mussels (related to our mussels) are common around hydrothermal vents but the species are peculiar and found nowhere else, not even in the non-hydrothermal abyssal environment. 

Despite the -sometimes great- geographic distance separating them, hydrothermal species are similar from a vent site to another and even from one ocean to another. 

Adults of these species are attached or with reduced ability to move and cannot travel great distances and the perpetuation of these populations along the ridges can only be done by their larvae that are released in the water column. IN addition, the vents are ephemeral and their disappearance causes the extinction of the communities that are associated to them. Survival therefore mainly depends on the ability of the larvae of these species to reach other active sites through the water currents. The colonization of a ridge is done from vent to vent along the axial valley, as you would cross a river using stepping stones. Physical obstacles (fractures interrupting the axial valley, continents, sills, water masses, …) can limit this dispersion. We understand then that the communities of two close vents sites will look more alike than those from vents that are far apart.

Obstacles to larval dispersal

Several phenomena can impede larval dispersal. For example, fractures that interrupt the ridges’ axial valleys are common and can disturb the circulation of the currents that carry the larvae. There also are sills that stop the circulation of deep-sea water masses. In addition, the movement of continental masses can interrupt the exchange of larvae. This can isolate populations that will then evolve independently, and may form new species.

For example, the North American plate divided the East Pacific ridge 28 million years ago. This lead to an interruption of larval exchange between the East Pacific Rise and the Juan de Fuca/Explorer ridges in the North. The communities comprise numerous closely related species that have diverged since the physical separation.

Similarly, the closing of the Panama Isthmus more than 5 million years ago has isolated the vent fauna from the Atlantic and the Pacific. This leaves only a path through the Indian and Austral oceans for the larvae to move between the Pacific and the Atlantic. 

Communities from Atlantic and Pacific Vents

The communities of these two ridges do not have common species but some species are closely related, such as the mussels Bathymodiolus thermophilus (Pacific) and Bathymodiolus azoricus (Atlantic). By comparing these communities to those of the Indian ocean hydrothermal vents, we were able to show that these latter were closer to those from the Atlantic, suggesting that the colonization pathway went through the Indian ocean rather than directly between the Atlantic and the Pacific.

The large siboglinid tubeworms are not found in the mid-Atlantic ridge communities but we find mussels of different species (Bathymodiolus azoricus and Bathymodiolus puteoserpentis) and the conspicuous swarms of blind shrimp (Rimicaris exoculata) around the chimneys.

The fauna from the East Pacific Rise vents is probably the best know to date. The communities are dominated large bivalves (mussels Bathymodiolus thermophilus and the giant clam Calyptogena magnifica reaching 30-cm long), the siboglinid worms among which the emblematic Riftia pachyptila (the giant tubeworm reaching 1.5 meter long and 5 cm in diameter) and the Pompei worms living on the surface of the chimneys. The bivalves and the siboglinid worms are symbiotic with bacteria that allow them to obtain a profuse nutrition and to grow fast. In such an unstable environment, a fast growth allows the species to rapidly reach sexual maturity, a capacity that is important for their survival.