Chaetognath cross-shelf patterns in the Patagonian Shelf, SW Atlantic Ocean

ABSTRACT Cross-shelf gradients in hydrographic conditions strongly influence zooplankton dynamics in the Patagonian Shelf, yet the responses of chaetognaths remain poorly described. This study tests the hypothesis that chaetognath assemblages exhibit structured variation in species composition, abundance and developmental stage distribution across a coastal–oceanic gradient (45°S–47°S). We analyzed 30 late-winter zooplankton samples from the San Jorge Gulf to the slope, integrating morphological and reproductive data with multivariate analyses. Parasagitta friderici dominated coastal and shelf waters and was the only species present across all developmental stages (0–IV), suggesting multiple reproductive pulses. In contrast, Pseudosagitta gazellae, Serratosagitta tasmanica and Eukrohnia hamata were restricted to oceanic stations, where most specimens were immature. The ovary index increased in colder, deeper waters, indicating temperature-dependent gonadal development. A sharp compositional shift occurred near 62°W, matching hydrographic transitions. Overall, our results demonstrate that chaetognath assemblages respond predictably to cross-shelf hydrographic gradients, confirming our hypothesis. The observed patterns in species distribution and developmental stages reflect the surface coastal–oceanic transition, offering a biological lens through which cross-shelf structure in the Patagonian Shelf can be interpreted. These findings offer an ecological baseline for understanding how physical forcing shapes zooplankton communities in this region of the SW Atlantic.