2.1. Sample collection and processing
In this study, 140 shellfish samples were collected between
February 2014 and February 2015 in retail market A and B, whose
seafood are both from the biggest aquatic wholesale market in
Shanghai. The samples include clam (Meretrix meretrix; N ¼ 43),
oyster (Crassostrea ariakensis; N ¼ 38), razor clam (Sinonovacula
constricta; N ¼ 35), and scallop (Chlamys nobilis; N ¼ 24). Before
being sold, these shellfish were stored in ice box. After collection, all
samples were placed on ice, immediately shipped to our laboratory,
and processed under sterile conditions within 3 h. Each sample
(25.0 g) was homogenized with 225 mL alkaline peptone water
(APW; Land Bridge Technology, Beijing, China) containing 3% (w/v)
NaCl in a sterile polythene bag (Whirl-Park®, Nasco, Shenzhen,
China), and samples were blended in a homogenizer (AE