Distribution of Azadinium spinosum in Chinese coastal waters and its response to climate change

Azaspiracid toxins are the most recently discovered group of major marine shellfish toxins. Azadinium spinosum is a key species producing azaspiracid toxins. The distribution pattern of Azadinium spinosum and its response to climate change remain unclear in China's coastal waters. We obtained distribution records of this species based on environmental DNA survey from 600 stations across China's coastal waters. Then, by combining relevant environmental data with the MaxEnt model, we investigated its distribution characteristics under current conditions and predicted its distribution under three climate scenarios (SSP1-2.6, SSP2-4.5, and SSP5-8.5) for the 2050s and 2100s. The results showed that: 1) Nitrate concentration (contribution rate of 50.6%) and sea surface temperature (contribution rate of 28.0%) were the dominant factors determining the distribution of A. spinosum in China's coastal waters. 2) Its current potential distribution areas mainly concentrate in the South China Sea and the East China Sea. The total area is 1.7136 million km², of which the South China Sea accounted for 58.2% and the East China Sea for 34.7%. 3) Under different climate scenarios, the potential distribution area of A. spinosum would shrink by 9.8%-22.0% by the 2050s, with the greatest reduction occurring under the SSP5-8.5 scenario, while the distribution centroid shifting northeastward from the northern South China Sea to the East China Sea. By the 2100s, the distribution area would further shrink by 17.7%-38.9%, with the smallest area occurring under the SSP5-8.5 scenario (1.0477 million km²). The distribution margins exhibit a pronounced "southern contraction and northern expansion" trend, while the distribution centroid shift further northeastward, potentially reaching the Yellow Sea under the SSP5-8.5 scenario.