Crassostrea ariakensis (Fujita, 1913)
Crassostrea ariakensis resembles other oysters in having unequal valves and an irregular shape. The right (lower) valve is thinner, flatter, and smaller than the left. Both valves are covered with concentric growth layers (lamellae) on the outer surface, but with fewer and stronger ridges on the left (upper) side. The edges of the lamellae are flat and not strongly rippled, as in C. gigas (Langdon and Robinson 1996). Colors of the lamellae on the outer surface vary from gray and yellowish brown to purple, while the inner surface of the valves is smooth and grayish white, with purple on the edges. The muscle scar on the inner surface of the valves is large and purplish (Coan et al. 2000). Substantial geographical variations were noted in different areas of Japan and China (Zhou and Allen 2003). Crassostrea ariakensis is reported to grow to 200-240 mm (Carriker and Gaffney 1996).
Crassostrea ariakensis appears to be part of a species complex, including the closely related C. honkongensis and C. nippona. It is less closely related, but still quite similar to at least nine other Crassostrea species from Indo-West-Pacific waters. A recent genetic study indicates that populations of C. ariakensis from northern and southern China constitute two separate sibling species. Hatchery stock in the US, including that of the Virginia Institute of Marine Science, used for Chesapeake Bay and North Carolina trials, include both species, although they are dominated by the northern genotype (Zhang et al. 2005). Molecular tools are needed for accurate identification of the species in this group (Reece et al. 2008).