Chaetognaths of the Leeuwin Current system: oceanographic conditions drive epi-pelagic zoogeography in the south-east Indian Ocean

Chaetognaths are ubiquitous marine zooplankton strongly influenced by oceanographic conditions. Epi-pelagic plankton samples taken across the Leeuwin Current system, the poleward flowing eastern boundary current of the Indian Ocean, were examined for chaetognath diversity and distribution. Sampling occurred at shelf, shelf break, and oceanic environments at each degree of latitude from 22 to 34°S. Nineteen species were identified, of which six are new records for the region with one an undescribed species. Low-moderate abundances (19.0 ± 2.4 individuals m⁻³) were observed compared with other eastern boundary currents and can be attributed to the unique oligotrophic nature of the system. The anomalous oceanography of the Leeuwin Current resulted in two distinct zoogeographic regions with distributions of tropical and temperate species separated around 28°S. Here, tropical waters transitioned to cooler, saltier waters and multivariate analyses identified temperature and salinity as the best indicators of community change. Community variation across the continental shelf was largely driven by one species, Flaccisagittaenflata, which was numerically dominant. Despite spatial variations, widely distributed species caused high overall community similarity. As the first system-wide investigation of Leeuwin Current Chaetognatha, this study provides benchmark biological information and highlights an unusual oceanography that produces a distinct planktonic zoogeography.