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

Abstract

Despite the widespread distribution of chaetognaths in the Southwestern Atlantic Ocean and their ecological importance in marine ecosystems, most studies have focused on biogeographic aspects, with limited information on species-specific population dynamics. This study analyzes the late winter distribution, species richness, population structure and sizes of chaetognaths across a coastal - oceanic gradient from the San Jorge Gulf to the slope (45°S - 47°S) in the Patagonian Continental Shelf Waters. Results revealed the presence of chaetognaths at around 70% of the sampled stations, with Parasagitta friderici being the dominant species widely distributed in coastal and shelf waters. In contrast, Pseudosagitta gazellae, Serratosagitta tasmanica and Eukrohnia hamata were restricted to oceanic stations, characterised by colder and more saline waters. Biomass showed spatial heterogeneity, peaking within the coastal zone and at deeper eastern stations with higher species richness. Chaetognath population dynamics varied by species and maturity stage. Pa. friderici exhibited all life cycle stages (0–IV) and showed significant body length differences across them (4-17mm BL). Intraspecific variations in abundance population structure were observed along a coastal-oceanic gradient, as well as along a north-south gradient within the San Jorge Gulf. The ovary index, which is a relation of the ovary length and body length, showed higher values in colder waters, suggesting the influence of temperature in the development rate. The population structure of the four species revealed partial reproductive synchrony, with immature stage I being the most abundant across all species. A low-intensity reproductive peak was also identified at the westernmost stations, probably modulated by the influence of the Malvinas Current. These findings provide new insights into chaetognath diversity and species-specific reproductive features as well as their role as indicators of water mass characteristics in the SW Atlantic Ocean.