Pathological survey of Manila clam Ruditapes philippinarum and Pacific oyster Crassostrea gigas in Garorim Bay on the west coast of Korea
Hye-Mi Lee, Hee-Jung Lee, Hyun-Sil Kang, Jae-Kwon Cho, Hee-Do Jeung, Young-Ghan Cho, Hyun-Ki Hong and Kwang-Sik Choi
School of Marine Biomedical Sciences (BK21 PLUS), Jeju National University, 102 Jejudaehakno, Jeju 63243, Republic of Korea
Shellfish production on the west coast Korea has been declined for the past two decades, and the parasite infection is considered to be one of the causes responsible for the decline of Manila clam Ruditapes philippinarum. In this study, we first investigated the pathologic condition of Manila clams and the Pacific oysters Crassostrea gigas in Garorim Bay on the west coast, where Manila clams and the Pacific oysters are co-cultured. Histology and Ray’s fluid thioglycollate medium assay (RFTM) revealed that most of Manila clams are infected by protozoan parasite Perkinsus olseni (98.0-100.0% prevalence). In contrast, none of the Pacific oysters diagnosed by the histology showed P. olseni in their tissue. The mean infection intensity of P. olseni in Manila clams was high, ranging from 0.796 (Ohiji-ri) to 2.058 (Dangsan-ri) × 106 cells/g wet tissue weight. In histology, metacercaria of Parvatrema duboisi was identified from the mantle tissue of Manila clam. Ancistrocoma-like ciliates (ALCs) were found from the digestive tubules of the oysters, with the prevalence ranging from 10.0 to 13.0 %. Rickettsia-like organisms (RLOs) were commonly identified from the epithelial cells of the digestive tubule of the Pacific oysters and Manila clams, with a prevalence ranging from 0.0 to 10.0% in Manila clams and 6.7 to 10.0% in the oysters. The mean condition index (CI) of Manila clams from Ohji-ri was significantly lower than CI of clams at Dangsan-ri tidal flat (ANOVA, P < 0.05). The mean digestive gland atrophy (DGA) of clams at Ohji-ri was significantly higher than the DGA of clams at Dangsan-ri (ANOVA, P < 0.05). The mean infection intensity of P. olseni in the host clams from Dangsan-ri was significantly higher than the clams in Ohji-ri (ANOVA, P < 0.05). The observed spatial variation in the CI and DGA was believed to be linked to a spatial change in the available food in the environment, although more investigations must be carried out to validate this hypothesis.
  
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