Reproductive biology, elemental composition and diel vertical migration of the cosmopolitan warm-temperate pelagic tunicate Soestia zonaria

Pelagic tunicates (appendicularians, doliolids, pyrosomes, salps) are cosmopolitan members of open-ocean food webs that serve as a link to the microbial loop and play a disproportional role in vertical carbon flux. Soestia zonaria is an oceanic salp species studied for more than a century, but little information exists on its ecology. Specimens of Soestia collected between 2008 and 2021 during four research expeditions (three to the Chatham Rise, New Zealand, and one to the Northeast Atlantic) using MOCNESS-1 net and large midwater trawl were analyzed for reproductive biology, stoichiometry and vertical distribution. Populations at the Chatham Rise sampled in early winter were more developed than in spring/early summer and dominated by small and medium-sized sexually reproducing blastozooids. Whole Soestia specimens had high organic content (mean ± SD = 32.8 ± 7.5%) and carbon-to-nitrogen values (6.8 ± 0.9) compared with other salp species, indicating a stronger dependency on carbon to meet its nutritional needs. Depth-stratified sampling showed that Soestia is primarily a low-amplitude diel vertical migrator occurring in the top 150 m of the water column, but also found at depths exceeding 500 m. Soestia is primarily an epipelagic salp species adapted to living in warm-temperate nutrient-depleted and more productive ocean regions.