Physiological and Transcriptomic Response of Asiatic Hard Clam Meretrix meretrix to the Harmful Alga Heterosigma akashiwo

The ichthyotoxic raphidophyte Heterosigma akashiwo is associated with harmful algal blooms (HABs). Although the influence of H. akashiwo on fish has been reported, little is known of its effects on other marine organisms, such as shellfish. In this study, the physiological and transcriptomic responses of the Asiatic hard clam Meretrix meretrix to H. akashiwo were investigated. The results showed that clam survival was not influenced by H. akashiwo (at a concentration of 2 × 104 cells/mL), and M. meretrix eliminated the alga effectively after 48 h in co-culture. H. akashiwo did not alter the antioxidant capacity of the clams. However, 544 unigenes were found to be differentially expressed between the control and H. akashiwo-treated clams. Interestingly, the significant up-regulation of lysosome-related genes and transport proteins suggested remodeling of the clam metabolism possibly for digestion of H. akashiwo. In summary, our results indicate that the clam M. meretrix can effectively feed on H. akashiwo, and intensification of shellfish farming may be helpful to control and mitigate H. akashiwo blooms.