Phantoms in the Deep (and how they help mussels get on)

Deep-sea vents are colonized by highly specialized marine invertebrates that cannot move very far once they are adults. Only their larvae can drift. It is not well understood how these animals travel between different vents, which are often hundreds of kilometers apart. A new publication by an international group of scientists headed by Corinna Breusing from the GEOMAR Helmholtz Centre for Ocean Research Kiel and the Max Planck Institute for Marine Microbiology (MPI Bremen) provides new insights into the tricks of deep-sea travel.

The scientists used mussels of the genus Bathymodiolus, one of the most dominant fauna at vents, to investigate larvae dispersal and host genetic connectivity between vents along the Mid-Atlantic Ridge. The mussels thrive in these ecosystems thanks to their association with chemosynthetic bacteria that provide nutrition to their host. “We faced three major challenges during the development of this study”, co-author Lizbeth Sayavedra from the Max Planck Institute for Marine Microbiology in Bremen (MPI Bremen) explains. “First, direct larvae dispersal is extremely difficult to measure. Second, mussel collections of deep-sea vents are difficult to obtain, and third, the current genetic resolution of host-markers is limited.” Through an innovative combination of genetic methods and high-resolution ocean modeling, Breusing and her colleagues came to the conclusion that larvae cannot simply drift from one vent to the other. Their results show that larvae would hardly make it any further than 150 kilometers from the source site. Thus, dispersal between populations requires intermediate habitats – the so-called “phantom” stepping stones. This assumption was supported by contemporary migration rates based on neutral genetic markers.

“For most of the history of vent research, we’ve thought about these ecosystems as isolated island-like habitats”, Jillian Petersen from the MPI Bremen explains. “Our study questions this long-standing assumption: through population genetics and ocean current modeling, we showed that there must be undiscovered sites between the known vent fields to explain the dispersal of animals between vents.”

The oceanographers at GEOMAR had a tough job modeling ways the larvae can travel in the deep sea. “There exist virtually no data on current patterns of the deep sea”, says Arne Biastoch from GEOMAR. But after a lot of effort and adjusting, they managed to realistically simulate larval drift. The physical models of larval dispersal showed that it is extremely unlikely that larvae can disperse more than 150 kilometers. This was backed by a population genetic study of the mussels: “To explain the current genetic structure of mussel populations along the mid-ocean ridge in the Atlantic”, Petersen continues, “one has to assume many ‘stepping-stone’ habitats in between the known vents, which are hundreds of kilometers apart.”

 

“We call these habitats „phantom“ stepping stones as we do not know where they are and how they look like”, adds Thorsten Reusch from GEOMAR, senior author of the publication. This opens up the exciting possibility that there are many more hydrothermal vents out there to be discovered.

 

“In fact, we did find one such stepping stone with Bathymodiolus mussels on a previous cruise ”, Nicole Dubilier, director at the MPI Bremen, adds. “In 2010, we travelled to the Menez Gwen vent on the northern Mid-Atlantic Ridge and discovered a new hydrothermal vent at one thousand meters water depth, with chimneys as high as one meter and fluids with temperatures up to 300 degrees Celsius. The discovery was remarkable because the area in which it was found has been intensively studied during previous research cruises.”

 

“Investigating the dispersal of vent fauna is important, for example to understand the environmental impact of deep-sea mining”, Liz Sayavedra explains. If deep-sea mining was to be conducted around hydrothermal vent sites, which can be rich in massive sulfide, it is important to establish protected areas and corridors for the vent fauna. “Thus, it is crucial to know the time, distance, and generations that would take the larvae from one vent site to another.”

New methods for finding and mapping deep-sea vents using Autonomous Underwater Vehicles (AUVs) are starting to reveal the locations of many more of these stepping-stones. Early indications suggest that there might be hundreds of sites of diffuse flow, which can be more hospitable for animal life than the black smokers typically seen at hydrothermal vents. So far, it is not clear if all of these diffuse flow sites are colonized by animals. “But based on our results, I think there is a good chance we will also find vent animals such as Bathymodiolus at many of these”, says Petersen.


Written using material from the GEOMAR Press Release.

Breusing, C., A. Biastoch, A. Drews, A. Metaxas, D. Jollivet, R. C. Vrijenhoek, T. Bayer, F. Melzner, L. Sayavedra, J. M. Petersen, N. Dubilier, M. B. Schilhabel, P. Rosenstiel, T. B. H. Reusch (2016): Biophysical and Population Genetic Models Predict the Presence of “Phantom” Stepping Stones Connecting Mid-Atlantic Ridge Vent Ecosystems. Current Biology, 26.

DOI: http://dx.doi.org/10.1016/j.cub.2016.06.062