Exposure to Alexandrium spp. impairs the development of Green-lipped mussel (Perna canaliculus) embryos and larvae.

The green-lipped mussel (GLM) Perna canaliculus is an economically, ecologically, and culturally important species in Aotearoa New Zealand. Since 2011, harmful algal blooms (HABs) of Alexandrium spp. have occurred annually in the Marlborough Sounds, the largest GLM aquaculture region in New Zealand. Across a similar timeframe, there has been a severe reduction in wild spat (juvenile mussel) catch. This research investigated the effects of Alexandrium pacificum (which produces paralytic shellfish toxins; PSTs) and A. minutum (a non-producer of PSTs) on the development of four GLM larval life stages (gametes, embryos, D-stage and settlement). Early life stages of GLM were exposed to environmentally relevant concentrations of Alexandrium spp. as whole cell, lysate and filtrate treatments. A 48-h exposure of embryos to whole A. pacificum cells at 500 cells mL caused lysis of embryos, severe abnormalities, and reduced development through to veliger (D-stage) larvae by 85%. GLM growth was impaired at cell concentrations as low as 250 cells mL during a 4-day exposure of D-stage larvae to both Alexandrium spp. Exposure of GLM to both whole and lysed treatments of Alexandrium spp. at 500 cells mL resulted in halved larval growth rates (2.00 µm day vs 4.48 µm day in the control) and growth remained impeded during a 4-day recovery period. Both A. pacificum and A. minutum were found to negatively impact D-larvae. Both whole-cell and lysed-cell treatments of A. pacificum had similar negative effects, suggesting that Alexandrium spp. toxicity to D-larvae is independent of PSTs. Additionally, cell membrane-free treatments of A. pacificum had no negative effects on embryo development, indicating that cell surface-associated bioactive compounds may be responsible for the observed negative effects during this early life stage. Conversely, non-PST-producing A. minutum was toxic to D-stage larvae but not to embryos; larval growth was reduced following a brief 1 h exposure of sperm to cell membrane-free treatments of A. pacificum. No effects were recorded in GLM larvae exposed during settlement, highlighting the potential for differences in susceptibility of early life stages to Alexandrium spp. exposure and the influence of exposure durations. In the wild, blooms of Alexandrium spp. can persist for several months, reaching cell densities higher than those investigated in the present study, and as such may be detrimental to the vulnerable early life stages of GLM.