Annual Variation of Shellfish Community Structure in Intertidal Zone of Changshan Islands

To understand the seasonal variations in shellfish community structure, the composition and diversity of shellfish in the intertidal zone of the Changshan Archipelago, China, were investigated. We investigated the composition and diversity of shellfish species in the intertidal zone (including NCD1, NCD2, BCD1, BCD2, TJD1, TJD2, DQD1, DQD2, NHD1, and NHD2) of five representative islands and 10 survey stations in the Changshan Archipelago during the low tide period in July and October 2021 and February and May 2022, and the water quality parameters of point water and interstitial water were analyzed. The results showed that the bottom material of the Changshan Archipelago was generally rock or gravel and the salinity fluctuated with the seasons. The salinity was high in summer and fall while low in spring and winter, with an average of 30.92±0.84. Compared with spring, summer, and fall, the temperature changed significantly in winter, with an average of (4.51±0.32) ℃ and the highest temperature being 23.48 ℃ during spring, summer, and fall. In general, the dissolved oxygen in the main islands of Changshan Archipelago (North and South Changshan Islands) was relatively low, with an average of (8.59±0.38) mg/L, while that in Tuoji Island, Daqin Island, and Nanhuangcheng Island was relatively high, with an average of (9.95±0.51) mg/L. A total of 12 species of shellfish were collected in this survey, including Crassostrea sp., Monodonta labio Linnaeus, Littorina brevicula, Mytilus galloprovincialis, Patelloida pygmaea, Chlorostoma rustica, Reishia clavigera, Thais luteostoma, Ocinebrellus inornata, Cellana toreuma, Ruditapes philippinarum, and Rapana venosa. The number of shellfish species in each season was the highest in spring, with an average of eight species, followed by in fall, with an average of six species. Winter had the least number of shellfish species collected, with an average of four species. In addition, the North and South Changshan Islands presented the most shellfish species at eight and nine species, respectively. The number of shellfish collected in DQD was the least (six species). There were differences in shellfish density at different survey sites, among which DQD2 had the highest density, with an average density of 41.26 ind./m2. The lowest density was in NCD2, and the average density was 13.71 ind./m2. In the same season, the shellfish density at different survey sites was different, the proportion of dominant species at different survey sites was also different. In spring, the density of Crassostrea sp., Cellana toreuma, and Chlorostoma rustica was high, while in summer, the density of Crassostrea sp., Mytilus galloprovincialis, Littorina brevicula, and Cellana toreuma was high. In fall, the density of Crassostrea sp., Littorina brevicula, and Mytilus galloprovincialis was the highest, while in winter, the density of Crassostrea sp., Patelloida pygmaea, and Cellana toreuma was the highest. The species diversity (H´) of shellfish in each tidal area of the survey site is shown as middle tidal zone (1.44) > low tidal zone (1.40) > high tidal zone (0.44). Overall, the average shellfish species diversity in the 10 survey sites was spring (1.26) > winter (1.21) > fall (1.11) > summer (0.95). The average evenness index of species (J´) in the 10 survey stations was 0.55, among which the evenness index of DQD2, NCD2, NHD2, and TJD1 shellfish was 0.67, 0.64, 0.64, and 0.55, respectively. The evenness index of shellfish in each tidal area of the survey site was in order of middle tidal zone (0.67) > low tidal zone (0.59) > high tidal zone (0.29). The seasonal change was not obvious. The average shellfish abundance at the survey site was 0.533, including 0.82, 0.68, 0.60, and 0.53 shellfish abundance at NCD2, NHD2, BCD2, and NCD1, respectively. The abundance index of shellfish in each tidal area of the survey site was in order of middle tidal zone (0.56) > low tidal zone (0.52) > high tidal zone (0.17). Overall, the average shellfish abundance in the 10 survey sites was spring (0.681) > fall (0.573) > summer (0.458) > winter (0.418). Seasons (temperatures) have the most obvious impact on the diversity and abundance of intertidal shellfish resources. Anthropogenic activities, such as aquaculture and tourism, also affect the distribution and quantity of shellfish species. The results of this study not only enrich the data of intertidal shellfish resources on islands in China but also provide basic information and reference for biodiversity and environmental protection of intertidal shellfish on the Changshan Islands.