RESULTS
The shell length of C. acicula collected in Daya Bay ranged from 4.13 to 14.72 mm (averaged 9.60 ± 2.17 mm), similar to that observed in Indian seas and Japan seas (ranged from 9 to 12 mm) (Nishimura, 1965; Pillai and Rodtigo, 1984). The diameter at the aperture ranged from 0.25 to 1.27 mm (averaged 0.86 ± 0.25 mm). The body weight ranged from 2 to 3.8 mg ind.-1 (averaged 2.8 ± 0.6 mg ind.-1). On July 5, it was observed that the highest proportion in the population was from individuals with a shell length of 9 to 10 mm, and the accumulated percentage of individuals with shell length ≤10 mm was 72%, indicating that the majority of the population were juveniles (Fig. 2). Such a population constitution suggested that the C. acicula bloom would keep going on after July 4, as long as environmental conditions (e.g., water temperature, salinity, and food) are favorable. Indeed, the bloom lasted in the subsequent 10 days and finally faded on July 13, when the population abundance dramatically decreased (<33 ind. m-3), and lots of empty shells of C. acicula were observed in the sediments, as after the animals’ death their empty shells settle to the sea floor.
The bloom peaked from July 4 to July 8, according to our field measurements. Higher abundance was found in the waters along the southwest coast of the Daya Bay and the highest record of abundance occurred on July 5 (Fig. 3). The average abundance of C. aciculawas 1851 ind. m-3 alongshore the southwest Daya Bay, i.e., Stations S2-S7, more than half of which were of an abundance higher than 1000 ind. m-3, and Station S5 is exactly the site recorded the highest abundance (5595 ind. m-3) throughout the entire survey. For other stations in southwestern Daya Bay (i.e., other than Stations S2-S7), the abundance of C. acicula ranged from 328 to 2278 ind. m-3, with the highest value found at Station S17. In contrast, no C. acicula accumulation was observed either in the northern and eastern areas, the mouth of Daya Bay, or in the adjacent Mirs Bay (see the green squares in Fig. 3).