Size-dependent zoogeographical distribution of gelatinous thaliaceans associated with current velocity and...

Thaliaceans are globally distributed and play an important role in the world's biological carbon pump and marine ecosystems by forming dense swarms with high feeding rates and producing large amounts of fecal pellets and carcasses. The contribution of thaliacean swarms to the downward transport of carbon depends not only on their abundance but also on their body size. However, the key factors influencing the distribution of different-sized thaliaceans remain unstudied. To discriminate thaliacean assemblages and examine the key factors determining the zoogeographical distribution and abundance of different-sized thaliaceans during different monsoon periods, we conducted three cruises in the South China Sea from before the southwest monsoon to the peak of the northeast monsoon. Our results revealed that high thaliacean abundance corresponded to high chlorophyll a concentration, which were associated with hydrodynamic processes, such as upwelling and eddies. Hierarchical partitioning and niche difference analyses demonstrated that current velocity and temperature are key factors that shaped the zoogeographical distribution of different-sized thaliaceans. The global dataset indicated that small-sized thaliacean species tend to occur in coastal areas where the current velocity is generally high, while large-sized species tend to occur in open ocean areas where the current velocity is generally low. The results revealed that global warming-induced changes in surface current velocity and temperature may alter the zoogeographical distribution and abundance of thaliaceans with different sizes, thereby affecting the biological carbon pump and surrounding marine ecosystem. Overall, this study sheds light on the potential responses of pelagic tunicates to global climate change through changes in their hydrodynamic conditions.