METHODS
Daya Bay is a shallow embayment with depths ranging from 5-20 m and an area of 650 km2, located in the northern South China Sea. On June 12, 2020, a huge mass of C. acicula was spotted in the waters close to the southwest shore of Daya Bay. Afterward, biological and environmental surveys in the entire Daya Bay were conducted from July 4 to 18, 2020. C. acicula was collected using vertical hauls of a plankton net (mouth diameter of 0.5 m and mesh size of 505 μm) from 1 m above the bottom to the surface. The filtered water volume was determined by the rope length multiplied by the net mouth size. Collected C. acicula was immediately taken back to the laboratory for subsequent abundance counting and biological parameter measurements, which included shell length, diameter at the aperture, and body weight. Environmental factors of surface water temperature, salinity, pH, DO were measured in situ using a YSI 6600 multi-parameter water quality monitor (Yellow Springs Instrument Co., Ohio, USA). Daily mean surface water temperature, salinity, and chlorophyll a (Chl a ) in May-July were obtained from a maritime buoyage system located in southwestern Daya Bay (see the buoy’s location in Fig. 3). The buoyage system record these data at an interval of half-hour. Historical data of C. acicula abundance in Daya Bay is from Xu (1985) and from an ocean observatory program of South China Sea Fisheries Research Institute (SCSFRI), which has been carried out every year since 2011.