Bivalve mollusk shells were collected in 2350 m depth in the Beaufort Sea, Arctic Ocean off northern Alaska. Initial identification suggested the specimens were a member of the bivalve family Thyasiridae, but no known eastern Pacific or Arctic living or fossil thyasirid resembled these deep-water specimens. Comparisons were made with the type of the genera Maorithyas Fleming, 1950, Spinaxinus Oliver & Holmes, 2006, Axinus Sowerby, 1821, and Parathyasira Iredale, 1930. We determined the Beaufort Sea species represents a new genus, herein described as Wallerconcha. These specimens also represent a new species, herein named Wallerconcha sarae. These new taxa are compared with known modern and fossil genera and species of thyasirds.

Thyasiridae , Beaufort Sea, Alaska, Mollusca , Bivalvia , Maorithyas , Wallerconcha , Spinaxinus , Axinus , Parathyasira , chemoautotrophic, endosymbiosis, taxonomy, Arctic Ocean

In an effort to understand the tectonic and sedimentary history of the Arctic Ocean between Canada and Alaska, a joint US-Canadian ice breaker expedition working under the sponsorship of the Extended Continental Shelf Project conducted operations in the Canada Basin during August 2010. The primary mission of the expedition was to collect seismic-reflection and high-resolution bathymetric data. Occasionally there was an opportunity to collect gravity and piston core samples throughout the basin. One of these cores was collected on a mound previously identified on seismic records. Bivalve mollusk specimens were collected in some of these samples and have provided the material for this paper.

The informally named “Canning Seafloor Mound” (Hart et al. 2011; hereafter referred to as the Canning Seafloor Mound), a probable cold seep, overlies the crest of a buried anticline in a region of sub-parallel compressional folds beneath the eastern Beaufort Sea outer slope. The collecting locality is adjacent to the frontier oil and gas regions offshore of Prudhoe Bay. The basin is host to extraordinarily deep sedimentary sections about 10 km thick with high organic matter content from river discharge, enhancing the probability of oil and gas generation at depth (Grantz et al. 2011, Grantz and Hart 2012).

Bernard (1972) reviewed the thyasird bivalves in western Canada. He examined specimens from throughout the northeast Pacific as well in the Arctic. In this treatment he synonymized a large number of genera into Thyasira Lamark, 1818, including Axinus G.B. Sowerby I, 1821 and Conchocele Gabb, 1866. Both genera are now known to be distinct (Oliver and Holmes 2007; Coan and Valentich-Scott 2012).

The only systematic treatment that included deep-water Beaufort Sea bivalves was presented by Bernard (1979). In the Beaufort Sea he documented four species of Thyasiridae between the intertidal zone and 2560 m, including a minute deep-water species Axinulus careyi Bernard, 1979.

Kristofovich (1936) reported on the Thyasira of Tertiary deposits on the western coast of Kamchatka, Russia. Fossil and modern species of Thyasira from northeastern Honshu, Japan, were detailed by Yabe and Nomura (1925).

Considerable research has been published in the last 15 years on thyasirds from cold seeps and hot vents (Oliver and Sellanes 2005, Oliver and Holmes 2006, 2007b, Oliver and Levin 2006, Taylor et al. 2007, Zelaya 2009, Oliver et al. 2013, Hryniewicz et al. 2014, Oliver 2014) and their associated with chemosynthetic communities, which are now known to be distributed widely throughout the world’s oceans (Sibuet and Olu 1998, Fujikura et al. 1999, Fujiwara et al. 2001). The Canning Seafloor Mound probably represents a cold seep habitat (Hart et al. 2011).

Here we describe a new genus and new species of thyasirid bivalve from a deep-water seafloor mound in the eastern Beaufort Sea, off northern Alaska and compare it to other thyasirid genera and species.

Bivalve specimens were examined from cores collected by the USCGC Healy (cruise HLY1002; USGS Station FAID H–3–10–AR; 71.3176°N, 143.9982°W) from the Canning Seafloor Mound, at a depth of 2530 m in the Beaufort Sea off northern Alaska (Figure 1). The Canning Seafloor Mound is conical and approximately 1200 m diameter and 180 m high (Figure 2).

Figure 1. 

Base map adapted from Jakobsson et al. (2008) showing the location of the Canning Seafloor Mound off the coast of northern Alaska.

Figure 2. 

Shaded seafloor bathymetry showing shape and approximate dimension of the Canning seafloor mount. The core sites are shown as red dots.

The bivalve specimens were recovered in two cores (1P–1 and 1GB–1) from 0.02 to 4.65 meters below the seafloor. The greater depth indicates long-term residence of the bivalves (see below under Age), whereas the shallow depth and assumed young age could indicate that this species might still be living on or near the mound. The shells are associated with gas hydrate, methane saturated sediments and authigenic carbonates (Edwards et al. 2011, Hart et al. 2011, Lorenson et al. 2011, Pohlman et al. 2011). This strongly suggests that the bivalves had chemotrophic endosymbionts similar to other bivalves that inhabit active cold vents (Roberts and Carney 1997, Fujiwara et al. 2001, Oliver 2014).

For Figure 7, diagrammatic line drawings were made from digital images of the holotypes of each species. Outlines of each type specimen were made in Adobe Photoshop by selecting all space outside of the shell, inversing the selection and creating a clipping path along the shell edge. With the clipping path selected, we processed the “stroke path” command.

The following abbreviations are used in the text: ECS – Extended Continental Shelf; FAID – field activity identification; GNS – GNS Science, Lower Hutt, New Zealand; LACMIP – Invertebrate Paleontology section, Natural History Museum of Los Angeles County, California, USA; mbsf – meters below seafloor; SBMNH – Santa Barbara Museum of Natural History, California, USA; NHMUK – The Natural History Museum, United Kingdom; NMST – National Museum of Nature and Science, Tokyo, Japan; NMW.Z – National Museum of Wales, Zoology, Cardiff, Wales, UK; USGS – United States Geological Survey; USNM – National Museum of Natural History, Smithsonian Institution, Washington D.C., USA.

The new species has shell characteristics closest to “Maorithyas” hadalisOkutani et al., 1999 (Figures 6A–H), collected from over 7,000 m in the Japanese Trench. Wallerconcha sarae is much more inflated, has broader umbones, and a much longer ligament and nymph. When compared to M. hadalis, W. sarae has a much larger, broader, and more elongate anterior adductor muscle scar.

There are also similarities between Wallerconcha sarae and members of the genus Spinaxinus Oliver & Holmes, 2006. However, all of the currently described species in this genus have a minutely spinose periostracum. The eastern Atlantic S. sentosus (Figures 4A–D) is less inflated than W. sarae, has narrower beaks, and a smaller anterior adductor muscle scar (Figure 7C). Spinaxinus emicatus Oliver in Oliver et al., 2013, from the Gulf of Mexico is compressed and circular in outline, has narrow beaks, an evident radial sulcus, and a much shorter nymph when compared to W. sarae. The Fijian S. phrixicus Oliver in Oliver et al., 2013, is also compressed and circular in outline with narrow beaks, but it has a distinctive shell sculpture of commarginal ridges.

The minute deep-water Beaufort Sea thyasirid, Axinulus careyi is much smaller (maximum length 2.7 mm), has a more defined escutcheon, and lacks the broad posterior hinge fig. It also has a relatively short, narrow anterior adductor muscle scar when compared to the long broad scar of W. sarae.

Axinus grandis (Verrill & Smith in Verrill, 1885) and A. cascadiensis Oliver & Holmes, 2007 have a few external similarities to Wallerconcha sarae. Axinus grandis is an Atlantic and Mediterranean species, that is easily separated from W. sarae by its roughly diamond-shaped shell outline. Axinus cascadiensis is known only from a seamount off Oregon (Oliver and Holmes 2007) and the shell outline serves to separate A. cascadiensis from W. sarae. With A. cascadiensis being less inflated, having narrower, more prosogyrate umbos, and a strong anterior protrusion. In addition, the escutcheon of A. cascadiensis is larger and more deeply impressed.

The Cretaceous fossil Thyasira becca cobbani Kauffman, 1967 (pl. 5, f. 34, 35; 1969, pl. 127, f. 20) has a deep radial sulcus and narrow, strongly prosogyrate beaks. Thyasira becca cobbani is known from the western interior of the North America in the Pierre Shale, Upper Cretaceous (Campanian-Maastrichtian) of Pueblo County, Colorado and in the Riding Mountain Formation, Upper Cretaceous (Campanian-Maastrichtian) exposed along the Assiniboine River, Manitoba, Canada. Thyasira alaskensis Kauffman, 1969, described from the Miocene and (or) Pliocene Nuwok Formation Member of the Sagavanirktok Formation on the Alaskan North Slope is easily separated by its more rounded outline, smaller and less prosogyrate umbo, and in having a prominent sulcus, although it is reportedly closely related to T. becca cobbani (Kauffman, 1967). Both of these fossil species have narrow hinge fig, narrow, strongly prosogyrate beaks and a deep radial sulcus, all of which excludes them from Wallerconcha.

We thank the officers and crew of the U.S. Coast Guard Cutter Healy for their professionalism in ship handling and, together with members of the HLY1002 scientific party, for assistance with core recovery. In particular, we recognize the efforts of Commanding Officer Captain William (Bill) J. Rall (USCG), Andrew Stevenson (USGS retired), and USGS marine technicians Jenny White and Peter dal Ferro. John Pohlman, Brian Buczkowski (USGS Woods Hole, Massachussetts), and William Schmoker (Polar TREC Arctic Research Consortium) assisted in sampling the cores. John Taylor (NHMUK) alerted us to the possibility that this species represented a new genus. Marianna Terezow (GNS Science, New Zealand) provided photographs of the holotype of Maorithyas marama. Hiroshi Saito (NSMT) loaned us the holotype of Maorithyas hadalis.

In addition, we thank Diego Gabriel Zelaya (Museo de La Plata, La Plata, Argentina), P. Graham Oliver (NMW.Z) John D. Taylor (NHMUK), Lindsey T. Groves (LACMIP), Mary McGann (USGS), Paula Mikkelsen (Paleontological Research Institute), Richard C. Willan (Museum and Art Gallery of the Northern Territory), and Eugene V. Coan (SBMNH) for their helpful comments on the manuscript.

P. Graham Oliver and Anna M. Holmes provided the images of Spinaxinus sentosus. Daniel Geiger provided images of the periostracum of type specimens of Wallerconcha sarae and Maorithyas hadalis.