Sublethal and antioxidant effects of six ichthyotoxic algae on early-life stages of the Japanese pearl oyster

Several species of harmful algae form blooms that are detrimental to aquatic organisms worldwide with severe economic loss to several industries. The cosmopolitan ichthyotoxic dinoflagellates and raphidophytes Karenia spp., Chattonella spp., Heterosigma spp., and Margalefidinium (Cochlodinium) polykrikoides are known to cause mass mortalities of fish and invertebrates, and the dinoflagellates Heterocapsa spp. are known to cause mass mortalities of shellfish, notably bivalve molluscs. The species K. mikimotoi, K. papilionacea, H. circularisquama, H. akashiwo, M. polykrikoides, and C. marina form recurrent harmful algal blooms (HAB) in coastal aquaculture areas of shellfish, coinciding with the reproduction seasons of natural and farmed bivalve molluscs. In the present study, their effects on eggs, fertilization, embryos, and three larval stages (D-shaped, umbo and pre-settling larvae) of a model bivalve species, the Japanese pearl oyster, Pinctada fucata martensii, are reported. The harmful algae had differential negative effects on each developmental stage, and had differential effects on larvae depending on their growth stage. Eggs were more affected by M. polykrikoides, K. mikimotoi and H. circularisquama than H. akashiwo and K. papilionacea. Fertilized eggs and developing embryos were more affected by M. polykrikoides and H. circularisquama than K. mikimotoi, K. papilionacea and H. akashiwo. Mortalities as well as abnormalities were not observed in any larval stage; however, motility of d-larvae and umbo larvae was more reduced by H. circularisquama and C. marina, than M. polykrikoides. In elder, 16 day-old larvae, all harmful algae induced a significant decrease in motility with the most severe effect observed during exposures to H. circularisquama, C. marina, H. akashiwo and M. polykrikoides. The superoxidase dismutase activity in larvae was not affected by exposure to any harmful alga; however, 6- and 16-day old larvae experienced a significant increase in GST activity following 48 h of exposures, with higher sensitivity of the elder larvae to C. marina, K. mikimotoi and M. polykrikoides. These results indicate that all tested harmful algae are differentially detrimental to the early-life development of the Japanese pearl oyster, with involvement of oxidative stress. Both M. polykrikoides and H. circularisquama were the most toxic followed by C. marina, K. mikimotoi, H. akashiwo and K. papilionacea. In addition, more developed larvae were most sensitive to these harmful algae in terms of motility-avoidance behavior and oxidative stress response, suggesting that ingestion of the harmful algae might enhance the toxicity of contact-dependent effects and dissolved extracellular compounds. The results also showed that superoxide anions were not associated with effects observed in larvae. Instead cellular detoxification was induced. The differential, stage-specific and growth-specific sublethal effects on bivalve development and recruitment also warrant further investigations of the oxidative stress and antioxidant enzyme activities in larvae of bivalves, to better address the toxicity mechanisms of ichthyotoxic HAB and their impacts on the reproduction, recruitment, and fitness of bivalve molluscs. Summary: The harmful algae Heterocapsa circularisquama, Chattonella marina, Hetrosigma akashiwo, Karenia mikimotoi, K. papilionacea, Margalefidinium (Cochlodinium) polykrikoides differentially affect early life stages of Japanese pearl oyster and activate detoxification enzymes in feeding larvae.