Effect of modified clay on the transition of paralytic shellfish toxins within the bay scallop Argopecten irradians and sediments in laboratory trials

Shellfish aquaculture is an important marine industry that has been threatened by harmful algal blooms (HABs) and, in recent years, toxic HABs in particular. The modified clay (MC) treatment is the most promising method to mitigate HABs and has been frequently applied in the field. To determine how the application of MC affects the accumulation, transformation, and detoxification of paralytic shellfish toxins (PSTs) in the bay scallop Argopecten irradians and surrounding sediments, a series of experiments over 16 days in a laboratory simulated Alexandrium tamarense bloom were conducted. After 3 h of exposure to A. tamarense at 2070 (+/- 120) cells mL(-1), the toxicity level in the tissues of the scallops increased to 105.9 (+/- 16.6) mu g STX eq. 100 g(-1) in the control group (group A, with no addition of MC), while the toxicity levels in the scallops of groups B and C (0.1 g L-1 and 0.5 g L-1 MC added, respectively) remained at relatively low levels. During the 3 h, the scallops in group A had incorporated 73.2 mol% of the toxin contained in the initial A. tamarense algal culture, the scallops in group B had incorporated 13.5 mol% of the toxin, and the scallops in group C showed almost no elevation in tissue toxin content. Meanwhile, 56.8 mol% and 107.1 mol% of the PSTs contained in the initial algal culture were settled in the sediments along with the MC-algal cells combination in group B and C, respectively. The results indicated that the application of MC can effectively prevent filter-feeding bivalves from ingesting toxic algal cells and decrease the accumulation of toxins by settling the toxic algal cells in the sediment. It was also indicated that the toxins accumulated within the sediments were more rapidly detoxified than that in the scallop tissues. Toxins accumulated by the scallops persisted in the tissue for approximately 16 days, while in the sediments, all of the PSTs contents were below the HPLC detection limits 4 days after the addition of MC.