In the Ebro Delta, the abundance of burrowing bivalves has dramatically decreased, with the blue crab, Callinectes sapidus, being blamed by shellfish collectors. Trends from 2010 evidence a decrease in the capture of clams (Ruditapes spp.) before 2016 (start of blue crab fisheries), although further decline of both clam and cockle (Cerastoderma glaucum) occurred in 2018. In contrast, captures of razor clam (Ensis siliqua) have increased by 3.6-fold since 2016. Predation risk for these taxa with contrasting burrowing capacities (1.7 ± 0.3cm, 0.4 ± 0.2cm, and 26.3 ± 0.1cm, respectively), was assessed using predation preference (N= 5 tanks; 5 individuals of each species) and no-choice experiments (N= 5 tanks; 15 individuals) in the absence and in the presence of sand. Results showed that in the absence of sand, the razor clam, was fully preyed in 24h, clams in 96h, and cockles reached 60% after 144h. Conversely, when sand was present, only 4% of razor clams were predated, while clams and cockles reached 60-100% in 120-144h. No-choice results featured similar patterns depending on substrate availability. Overall, clams and cockles appear to be greatly vulnerable to blue crab predation, whereas the razor clam may escape thanks to its deeper burrowing capacity.

The intestinal microbial community of aquatic animals is closely related to the environment and thus affects the health of aquatic animals. Here we investigated annual variation of intestinal bacteria community structures of Ruditapes philippinarum cultured using two different methods, bottom sown and raft hanging. Bacterial diversity of seawater and sediment in different seasons was examined as well. Correlation between R. philippinarum intestinal bacteria community structures and environmental factors was analyzed. The results showed that in the two aquculture areas where R. philippinarum was differentially cultured, bacteria community structure of seawater and sediment both altered with seasons. However, neither seasonal changes nor difference in culture methods could significantly affect the intestinal bacteria community structures of R. philippinarum. Additionally, in terms of the alpha diversity of intestinal bacteria communities, there was no significant difference in R. philippinarum cultured using the two methods. For benthic cultured clams, alpha diversity of the intestinal bacterial communities was often higher than that of the environmental bacterial communities. For raft hanging cultured calms, Alpha diversity of the intestinal bacterial communities was normally comparable with that of the environmental counterparts. Beta diversity analysis showed that the intestinal bacterial community structure of R. philippinarum was mainly determined by both deterministic and stochastic processes. Correlation analysis showed that the intestinal bacterial community structures of R. philippinarum were determined by a variety of environmental factors. For clams collected from the benthic culture area, the intestinal bacterial structure was more correlated with the physicochemical properties of sediment in comparing with those of seawater. For clams collected from the raft-culture area, the intestinal bacterial structure was mainly determined by the phsicochemical characteristics of seawater. This study helps us to better understand the bacterial community structure in complex aquaculture ecosystems, providing a reference for healthy shellfish aquaculture and ecosystem protection in aquaculture area.