Sand lance species play a key ecological role in most temperate to polar shelf ecosystems of the northern hemisphere, but they have remained unstudied with respect to their sensitivity to predicted future CO₂ levels in the ocean. For the past three years (2016 – 2018), we have sampled and spawned with northern sand lance (Ammodytes dubius) from Stellwagen Bank National Marine Sanctuary and subsequently reared their embryos under factorial CO₂ x temperature conditions to hatch and early larval stages. Our results were striking, in all years, high CO₂ conditions severely reduced embryo survival up to 20-fold over controls, with strong synergistic reductions under combined high CO₂ and temperature conditions. High CO₂ also delayed hatching, reduced remaining endogenous energy reserves at hatch, and in combination with higher temperatures, reduced embryonic growth.

Indeed, given the observed effect sizes, northern sand lance might be the most CO₂ sensitive fish species tested to date.