High Survival with Fitness Cost in Marine Copepod Tigriopus japonicus Exposed to the Toxic Dinoflagellate Alexandrium pacificum

Blooms of the toxic dinoflagellate Alexandrium spp., which produce paralytic shellfish toxins (PSTs), have threatened marine ecosystems for decades. However, their impacts on benthic copepods remain underexplored. Here, we investigated the physiological and biochemical responses of marine benthic copepod Tigriopus japonicus to Alexandrium pacificum. Adult female showed no mortality within 24 h and maintained high survival (>80%) across all A. pacificum concentrations over 96 h, with no significant difference from non-toxic controls. Ingestion rates on A. pacificum and non-toxic controls were similar, when algal prey concentrations were lower than about 2500 µgC/L. However, above this threshold, feeding of T. japonicus was significantly depressed by A. pacificum. Six-day exposure induced oxidative stress, marked by elevated level of reactive oxygen species, malondialdehyde, total antioxidant capacity and superoxide dismutase activity, while catalase activity showed a non-significant increase. Moreover, concentration-dependent increase was observed in acetylcholinesterase activity, and its positive correlation with oxidative biomarkers suggests additional roles in coping with stress. Chronic exposure over three generations did not consistently affect development, except for complete development failure in F2 and F3 at the highest concentration. However, moderate-to-high concentrations of A. pacificum significantly impaired reproduction, reducing egg production by 9.66%–96.07% and clutch numbers by 33.33%–55.56%. Our findings indicate that Alexandrium blooms can adversely impact benthic copepod feeding and population recruitment through neurotoxicity and oxidative stress, highlighting the ecological risks of prolonged algal blooms in benthic habitats.