Accumulation of Paralytic Shellfish Toxins in Mytilus unguiculatus and Its Effect on Filtration Rate

Paralytic shellfish toxins (PSTs) are a class of neurotoxic marine biotoxins that are widely distributed and cause more than 2, 000 poisoning events worldwide each year, with mortality rates of up to 15%. PSTs can accumulate through the food chain and are mainly distributed in marine organisms such as bivalve mollusks. Several countries and regions, including the European Union, China, and the United States, have established strict regulatory limits (400 MU/100 g or 800 μg STXeq/kg) for PSTs and implemented monitoring programs. As reported previously, factors such as filtration rate, selective feeding, and the efficiency of the organism in absorbing toxin-producing algae can significantly affect the accumulation of PSTs in bivalve mollusks. Mytilus unguiculatus is one of three major commercial mussel species in China, with important economic value and social impact. Due to its high nutritional value, it is extensively cultured as an important shellfish species in the Zhoushan archipelago of Zhejiang Province in China. Alexandrium spp. are the main toxin-producing algae in the area. PSTs have been detected in mussels between May and July after harmful algal blooms. Research into the elimination characteristics of PSTs accumulation in M. unguiculatus is urgently needed to establish a monitoring and control program.In this study, 760 mussels were randomly selected and fed A. catenella at different cell densities, with a high-density group (7.00×105 cells/d), a low-density group (2.80×105 cells/d), and a control group. The experimental period lasted for 30 d, during which the accumulation period approximately represented days 1–7 and the elimination period days 8–30 d. A total of 14 sampling points were set up on days 0.5, 1, 2, 4, 6, and 7 of the accumulation period and days 1, 3, 5, 7, 11, 15, 19, and 23 of the elimination period. Six mussels were randomly collected at each sampling point and dissected into soft tissues, hepatopancreas, and edible tissues. Liquid chromatography-tandem mass spectrometry was used to determine the content of PSTs. During the accumulation period, 5 mL culturing seawater was collected during or 1 h after feeding, and Ruge's solution was added. The filtration rate of the mussels was determined by counting the quantity of A. catenella cells in the water. The results showed that the toxins in M. unguiculatus were not equally distributed. The highest PST content in hepatopancreas tissues was 7, 458.2 μg STXeq/kg (high-density group) and 2, 555.9 μg STXeq/kg (low-density group). The highest PST content in edible tissues was 108.6 μg STXeq/kg (high-density group). The hepatopancreas was identified as a target organ for toxin accumulation. From day 3 to day 7, the filtration rate of mussels decreased, eventually reaching 30% of its initial value. The filtration rate of M. unguiculatus in the high-density group was not significantly different from that of the low-density group during days 1–5 and was significantly lower during days 5–7. During the elimination phase, the PST elimination rate in mussels was 18.4% (hepatopancreas), 18.1% (soft tissues), and 13.1% (edible tissues). At day 30, the residual content of PSTs in the hepatopancreas of mussels was approximately 1 400 μg STXeq/kg in the high-density group and 600.0 μg STXeq/kg in the low-density group. Changes in the proportion of each PST component were transferred from A. catenella to M. unguiculatus. The proportion of C2 was significantly reduced from 74.1% (A. catenella) to 22.6% (high-density group) and 17.1% (low-density group) (P < 0.05); the percentage of C1 increased from 10.6% (A. catenella) to 54.1% (high-density group) and 54.0% (low-density group) (P < 0.05). No significant difference was observed in the percentage of GTX5 between A. catenella and mussels in the different density groups (P > 0.05). No significant conversion was observed between the PST components in the hepatopancreas of mussels throughout the experiment.Our data indicate that the daily accumulation rate of PSTs in M. unguiculatus was lower than that in other mussels. Moreover, the toxin elimination rate was higher than that of other mussels. A negative correlation was observed between the filtration rate of M. unguiculatus and the PST content of each tissue type. These results show that M. unguiculatus is more sensitive to PSTs than other mussels. During the stage of PST transfer from A. catenella to M. unguiculatus, a high proportion of C2 toxin was converted to C1 toxin. After accumulating in the hepatopancreas, the PST profile exhibited relatively stable performance. In summary, we conclude that, due to higher susceptibility to toxins and lower conversion rates, a lower risk is associated with the consumption of M. unguiculatus than with that of other mussels. Our findings will contribute to improving our understanding of the mechanisms underlying the PST accumulation risk in M. unguiculatus and provide valuable scientific insights for developing prevention and risk management strategies concerning PSTs.