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Macrofaunal Characterization of Peridotite-Hosted Ecosystems
Associated with the Lost City Hydrothermal Field
Buckman and Shank


Abstract
In 2001, an off-axis peridotite-hosted hydrothermal system supporting up to 60 meter tall carbonate-brucite chimneys was discovered on the Atlantis Massif (30°07.4'N, 42°07.24'W; 700-800m depth). In 2003, a second interdisciplinary expedition returned to this site, the Lost City Hydrothermal Vent Field (LCHF) to understand the linkages among hydrothermal alteration of the mantle, geochemistry, and biological ecosystems within this environment.

The Lost City vent field features many components that offer opportunities to identify novel ecological niches and new species (with new adaptations) associated with potentially unique (yet prevalent) chemical habitats. These components include Lost City's close proximity (<15 kilometers) to, yet pronounced depth difference (~3000m) from the Mid-Atlantic Ridge floor, and unique fluid chemistry derived from serpentinization reactions and production of carbonate chimneys at the seafloor.

Lost City also offers the opportunity to identify the evolutionary relationships of chemosynthetic fauna in comparison to those endemic to vents and seeps. Preliminary characterization of actively venting carbonate habitats (~10-40?C) indicated they were dominated by several species of gastropods and amphipods, including Bouvierella curtirama and Primno evansi.

Also present were polychaetes, nematodes, euphausids, foraminifera, ostracods, bivalves (including mussels), stomatopods, and demosponges. Non-venting habitats (e.g., the sides of inactive chimneys, sedimented areas, and breccia cap rock) were occupied by Lophelia, Desmophyllum and gorgonian corals, galatheid crabs, mussel shells, gastropods, foraminifera, pteropods, urchins, asteroids, limpets, and ophiuroids.

The dominant mobile fauna near actively venting edifices was a purple-gray ~10 to 30cm-long cut-throat eel (Synaphobranchus kaupi). Wreckfish (Polyprion americanus) were also abundant throughout the field as well as number of other unidentified fish.

Preliminary observations indicate that the upper pelagic and mid-Atlantic deep-sea fauna (>200m) have a marked presence on Lost City faunal composition (e.g., the presence of pteropods, forams, and amphipods), though some vent fauna are also present. A cursory comparison of biomass between the dominant faunas of Lost City, Mid-Atlantic Ridge, East Pacific Rise, and Juan de Fuca, suggests that Lost City supports significantly lower biomass, with the large majority of the dominant fauna on the order of hundreds of microns or less in length.

Results from ongoing phylogenetic studies of Lost City species will be presented to shed light on the role of depth, fluid chemistry, and the presence of non-endemic species in controlling community composition. These findings will be used to examine the role of Lost City-type vent fields (a potentially ubiquitous style of venting along the Mid-Atlantic Ridge) as stepping stones or evolutionary refugia for the chemosynthetic fauna along the Mid-Atlantic Ridge.

The Lost City Ventfield Ecosystem
  • Annelida – Glyceridae and Amphinomidae
    A minimum of seven species of polychaetes were collected, including representatives of the Dorvelleidae (Ophiotrocha sp. and 2 other species), Ceratulidae (Aphlichaeta sp.), Glyceridae, Amphinomidae, and Polynoidae. The above polychaetes were all associated with actively venting habitats. Polychaetes living within corals were collected from non-active areas and from chaff beach. Serpulid polychaetes were observed around the periphery of the field.
  • Cnidaria – Desmophyllum
    Cnidaria were found mainly in non-active areas, and were the dominant fauna on the cap rock and field periphery. Which include anemones, gorgonians, Desmophyllum and Lophelia corals, and hydroids.
  • Porifera
    Five morphospecies of sponge were collected. Most sponges were associated with non-active habitats, though one morphospecies was prevalent on carbonate samples from active spires.
  • Mollusca – Marker H in situ Mussel in carbonate Gastropod sp. 1
    Small gastropods (at least three species) were found throughout the vent field. One morphospecies was particularly abundant on active carbonates. Empty mussel shells were found near the base of a slope, and live Bathymodiolus aff. azoricus (based on shell morphology and DNA sequence data) were found on the side of an active spire. This species of mussel is considered to be vent endemic and is found at other sites along the MAR. Numerous smaller bivalves (mm in length) were collected and may be juvenile B. aff. azoricus. Empty archaeogastropod limpet shells were also recovered. Shells of spiral gastropods and pteropods were very abundant at Chaff Beach.
  • Arthropoda – Primnos evansi Crab sp. 1
    Amphipods (Bouvierella aff. curtirama and Primno evansi) were the most abundant fauna at the Lost City vent field. These two species were found mainly in active areas, but were also collected off of carbonates structures in ambient water. B. aff. curtirama is also found at the Lucky Strike vent site on the MAR. P. evansi is found throughout the North Atlantic from the surface to a depth of 1700m. Calanoid copepods, ostracods, tanaidaceans, brachyuran crabs, galatheid (Munida) crabs, conchostracans, isopods, barnacles, euphausids (Nematoscelis sp.), and pycnogonids were collected from active and non-active carbonates.
  • Echinodermata
    Typical deep-sea echinoderms, including urchins, asteroids, and ophiuroids were collected at the Chaff Beach area and observed on the Breccia Cap. The urchin, Areosoma fenestratum, was collected from the side of an active spire, but was more commonly observed in inactive areas.
  • Osteichthyes – Synaphobranchus kaupi
    Wreckfish (Polyprion americanus) and cut-throat eels (Synaphobranchus kaupi) were prevalent surrounding the Lost City fields. Fish were abundant in the waters surrounding the fields.
Summary
  • Approximately 65 morpho-species, representing 13 phyla were sampled at the Lost City Hydrothermal Field.
  • Lost City appears to share at least two vent-endemic species with the northern Mid-Atlantic Ridge sites- Bouvierella aff. curtirama (amphipod) and Bathymodiolus aff. azoricus (mussel).
  • Lost City fauna is dominated by small crustaceans and gastropods, especially in actively venting areas.
  • Vent specific fauna such as Bouvierella aff. curtirama and Bathymodiolus aff. azoricus are found along with non-vent organisms such as Primno evansi, Lophelia, and Araeosoma fenestratum. This combination of vent fauna and Mid-Atlantic pelagic and deep-sea fauna is unique to the Lost City
  • Lost City's unusual vent fluid chemistry (pH ~11), the presence of carbonates and serpentenization, and the relative shallow depth of the site are considered to exert strong structuring forces on the community assemblages at this vent field.
  • Genetic studies to delineate the relationships between Lost City fauna and the fauna inhabiting other vent sites on the MAR are ongoing. These studies will examine the potential role of these potentially ubiquitous peridotite hosted systems as refugia/stepping stones for chemosythetic organisms in the Atlantic.
Acknowledgements
We extend our sincerest gratitude to the officers and crew of the RV Atlantis, the submersible pilots of DSV Alvin, Debbie Kelley, Jeff Karson and the members of the Lost City Scientific Party for their invaluable assistance. Many thanks to the ABE Group, and map-maker Mike Jakuba for their expertise.
Support was provided by NSF • OCE-OCE-0136871 to TMS.