The « Giant Cleftclam » is an extremely large (probably the largest extant) thyasirid very widely distributed in the northern Pacific Ocean, ranging from East China Sea to Russia to Oregon, USA. It is a chemosymbiotic bivalve hosting both methane and sulfur oxidising endosymbionts in a much enlarged gill, and relies on these for energy. Although it is a well-documented member of the Pacific methane seep comm.unity often co-occuring with other chemosymbiotic bivalve genera such as Calyptogena and Acharax, it is also found in reducing sediments rich in methane and sulfides outside seeps. It occurs across a wide bathymetric range from -50m down to more than -1500m deep. Both the specific epithet and common name comes from the deep ridge or cleft on the posterior end of the shell, a feature common in Thyasiridae but most prominent in genus Conchocele. The shell shape changes drastically throughout life stages; the anterior shell margin protrudes beyond the beak in young specimens (as shown here), but becomes flat to strongly concave in adults. Adult shells are also more elongated and much thicker. Typical shell length is around 90mm. but extremely large specimens exceeding 165mm. are known. Recent specimens are very rare due to its restricted habitat, especially large live-taken ones, although virtually identical Cenozoic fossils are not uncomm.on. Conchocele disjuncta Gabb, 1866 described from Pliocene fossil of California, USA is widely regarded as a synonym.