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Shells from Deep Arctic Ocean Sediment Reveal a New Clam Species, Hint at Methane-Based Seafloor Ecosystem
A new type of bivalve mollusk (clams, mussels, oysters, and their kin) has been discovered more than 2.5 kilometers (1.5 miles) below the ocean surface off the coast of northern Alaska. Shells of the new clam were found in sediment cores that penetrated as much as 4.5 meters (15 feet) below the seafloor surface. (No living specimens were observed.) Methane gas recovered from the same cores suggests that the clams may have been part of a methane-based ecosystem. The recovered shells’ ages are estimated to range from a maximum of 1.8 million years old to near present, but scientists cannot discount the possibility that the new clam might still be alive today. The animal represents a new genus and species, and its discovery was reported in December 2014 in the open-access scientific journal ZooKeys (“A new genus and species of Thyasiridae (Mollusca, Bivalvia) from deep-water, Beaufort Sea, northern Alaska”).
The path to discovery is seldom simple or easy, and this one is no exception. It resulted from the collaboration over several years of four scientists: geologists Brian D. Edwards, Thomas D. Lorenson, and Charles L. Powell, II, of the U.S. Geological Survey (USGS), and bivalve mollusk specialist Paul Valentich-Scott of the Santa Barbara Museum of Natural History (California).
In the summer of 2010, Brian Edwards (now a USGS emeritus scientist) was the chief scientist on a joint U.S.-Canadian icebreaker expedition aboard the U.S. Coast Guard Cutter Healy (“Scientists Set Sail to Map the Arctic Seafloor”). The primary purpose of the expedition was to map the Arctic seafloor and the sediments beneath to help delineate the outer limit of the U.S. Extended Continental Shelf, where the nation can potentially exercise sovereignty over seafloor and sub-seafloor resources in accordance with the international Law of the Sea (“2010 Extended Continental Shelf Survey”).
Edwards took deep sediment core samples to further understand the geology of the region (“Mud, Glorious, Mud—and Gas Hydrate! A Photo Tour of Our First Day of Sampling”). The bivalve shells were discovered in sediment cores collected near the summit of an unusual seafloor mound, informally named the Canning Seafloor Mound, about 150 kilometers (90 miles) off Alaska’s north coast (see map below). The 130-meter (430 foot)-high mound was previously identified by USGS geophysicist Pat Hart while reviewing seismic-reflection data collected by the USGS in the 1970s. This site was targeted for coring because it lies above the crest of an anticline (an upward-bowing of the sediment layers) that brings older sediments closer to the seafloor. The clams were an unexpected bonus, as was a chunk of gas hydrate (an ice-like form of methane gas combined with water) found in one of the cores—the first methane hydrate sample reported from the deep Arctic Ocean basin. Additionally, residual gas recovered from the core liner contained a high percentage of methane.
After returning to his USGS laboratory in Menlo Park, California, Edwards worked with geochemist Tom Lorenson and other USGS scientists to open, examine, and sample the cores. They found numerous clam shells whose positions corresponded to depths ranging from 31 centimeters (1 foot) to nearly 4.5 meters (15 feet) below the seafloor surface. Many specimens were intact, with both top and bottom valves (shells) in place, indicating they had lived there instead of being carried there. The numerous shells, an odor of hydrogen sulfide, and the evidence of methane gas all suggest that the clams were part of a cold-seep ecosystem, in which the base of the food chain consists of bacteria that produce energy not from sunlight (photosynthesis) but from chemicals (chemosynthesis), such as hydrogen sulfide and methane seeping up and out of the seafloor. Like other cold-seep bivalves, the clams likely were nourished by chemosynthesizing bacteria living in their tissues.
Lorenson said: “We suspect the clams were part of a methane-oxidizing ecosystem occupying the mound, which is probably leaking a lot of methane from deeper sources.” He and USGS biogeochemist John Pohlman collected methane, authigenic carbonate (carbonate minerals precipitated in place, commonly associated with cold-seep communities), and pore water from the cores to explore this hypothesis; their analyses are still underway. (Learn more about the cores and how they were processed at http://www.polartrec.com/expeditions/international-continental-shelf-survey/journals/2010-12-23.)
The shells recovered from the cores were taken to USGS paleontologist Chuck Powell for identification. Powell was able to ascertain the higher level classification of the clam shells (Family Thyasiridae), but he was unable to determine the genus or species. Powell contacted Paul Valentich-Scott, a clam specialist from the Santa Barbara Museum of Natural History in California.
Upon examining these ancient shell specimens, Valentich-Scott was fairly certain that they were new to science. The hunt to validate the potential new species was on. Valentich-Scott contacted a number of thyasirid bivalve specialists around the world and all gave it a thumbs-up as a new species. Further, several scientists felt it also might be a new genus (the level above species).
“It is always exciting when you are the first person to be looking at a new creature,” declared Valentich-Scott. “While I have been fortunate to discover and describe many new species in my career, it is always exhilarating at the outset.”
Then the painstaking work began. Valentich-Scott contacted museums around the globe and requested to borrow specimens that were potentially related to the new species. Though he found many species that shared some characteristics, none matched the new Arctic specimens.
The four scientists have been writing up their findings for the past two years, and in December 2014, the work was published in the international science journal Zookeys (http://dx.doi.org/10.3897/zookeys.462.6790).
The new clam, Wallerconcha sarae, is named after two individuals. The genus is named in honor of Thomas R. Waller, a prominent paleontologist at the Smithsonian Institution. The suffix “concha” meaning shell, was added to create the name Wallerconcha. The new species is named after Sara Powell, the daughter of co-author Chuck Powell. Powell was quick to mention, “I want to name new species after all of my children.”
Although many of the specimens collected were clearly ancient, the scientists cannot discount the possibility that Wallerconcha sarae is alive today. Lorenson summarized it this way: “The likely collection of living specimens of this species awaits expeditions to come.” Who knows what other new creatures might be found in those expeditions?
The full citation for the new paper is:Valentich-Scott P., Powell, II, C.L., Lorenson, T.D., and Edwards, B.R., 2014, A new genus and species of Thyasiridae (Mollusca, Bivalvia) from deep-water, Beaufort Sea, northern Alaska: ZooKeys, v. 462, p. 11–26 [http://dx.doi.org/10.3897/zookeys.462.6790].
in this issue:
Shells from Deep Arctic Ocean Sediment Reveal a New Clam Species
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