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ARS Home » Pacific West Area » Corvallis, Oregon » Forage Seed and Cereal Research Unit » Research » Publications at this Location » Publication #221909
Title: The Kumamoto oyster Crassostrea sikamea is neither rare nor threatened by hybridization in the northern Ariake Sea, Japan

Author
item Camara, Mark
item DAVIS, JONATHAN - TAYLOR SHELLFISH, INC.
item SEKINO, MASASHI - TOHOKU NTL FISH RES INST
item HEDGECOCK, DENNIS - USC
item LI, GANG - USC
item LANGDON, CHRISTOPHER - OREGON STATE UNIVERSITY
item EVANS, SANFORD - OREGON STATE UNIVERSITY

Submitted to: Journal of Shellfish Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/29/2008
Publication Date: 4/30/2008
Citation: Camara, M.D., Davis, J.P., Sekino, M., Hedgecock, D., Li, G., Langdon, C.J., Evans, S., Hedgecock, D. 2008. The Kumamoto oyster Crassostrea sikamea is neither rare nor threatened by hybridization in the northern Ariake Sea, Japan. Journal of Shellfish Research.27(2):313-322.

Interpretive Summary: The Kumamoto oyster Crassostrea sikamea is listed as a threatened species in its native Japan, and cultured breeding stocks used for aquaculture in the US have been inadvertently hybridized with C. gigas and inbred. As a result, Japanese conservationists and U.S. oyster growers share an interest in the status of this species in the wild. We collected wild oysters from three sites in Saga Prefecture located in the northern portion of the Ariake Sea, Kyushu, Japan, and used molecular markers to identify them to species. C. sikamea proved to be the dominant organism comprising 91% of the oysters sampled and typed. C. ariakensis was present (8% of typed oysters) but only at the lowest intertidal levels, and C. gigas was rare (1%) at these sites. We found no evidence of hybridization between any of the species and were unable to repeat a previous study, which suggested hybridization between C. sikamea and C. gigas based on sharing of a calmodulin allele. We also conducted gamete compatibility tests among all combinations of Japanese (Ariake Sea) and U.S. C. sikamea and C. gigas broodstocks and found strong one-way gamete incompatibility (male C. sikamea x female C. gigas) between species of Japanese stocks, supporting the molecular diagnosis of C. sikamea. However, this one-way incompatibility was less evident in U.S. stocks, indicating lower barriers to potential hybridization in commercially cultured stocks.

Technical Abstract: The status of the Kumamoto oyster Crassostrea sikamea in its native Japan is uncertain because of a lack of information about its abundance and distribution and a suggestion that C. sikamea and the Pacific oyster C. gigas hybridize in the northern Ariake Sea. Furthermore, broodstock populations on the U.S. Pacific coast have been hybridized with C. gigas in the past and may suffer inbreeding depression from multiple generations of hatchery-propagation. As a result, Japanese conservationists and U.S. oyster growers share an interest in the status of this species in the wild. We collected wild oysters from three sites in Saga Prefecture located in the northern portion of the Ariake Sea, Kyushu, Japan, in September 2006 and used molecular methods (species-specific PCR of the mitochondrial COI gene and PCR-RFLP of the nuclear ribosomal ITS1 gene) to assign 628 sampled oysters to one of three species found in this region. C. sikamea proved to be the dominant organism on artificial hard substrates, comprising 91% of the oysters sampled and typed. Many individuals confirmed as C. sikamea by diagnostic DNA markers had C. gigas-like phenotypes, such as striped shells. C. ariakensis was present (8% of typed oysters) but only at the lowest intertidal levels, and C. gigas was rare (1%) at these sites. We found no evidence of hybridization between any of the species and were unable to repeat a previous study, which suggested hybridization between C. sikamea and C. gigas based on sharing of a calmodulin allele. We conducted gamete compatibility tests among all combinations of Japanese (Ariake Sea) and U.S. C. sikamea and C. gigas broodstocks and found strong one-way gamete incompatibility (male C. sikamea x female C. gigas) between species of Japanese stocks, supporting the molecular diagnosis of C. sikamea. However, this one-way incompatibility was less evident in U.S. stocks, indicating lower barriers to potential hybridization in commercially cultured stocks.