New insides into ecology of the Antarctic tunicate Salpa thompsoni: ingestion, growth and development dynamics

Salps were collected during austral summer 2018 in the vicinity of the Antarctic Peninsula onboard the German RV Polarstern. At 106 stations, using a 0-170 m double oblique 2.5 m² Isaacs-Kidd Midwater Trawl equipped with the 0.5 mm mesh, salps were quantified and analyzed for basic biology (length, sex, stage). The grazing, including fecal pellet production, studies were undertaken during two 24-h process studies near the Deception and Elephant Islands. Salpa thompsoni was the most abundant and frequently caught pelagic tunicate with densities reaching 3.5 ind.m^-3. Average salp densities in the Bransfield Straight, north of the South Shetlands Islands and in the vicinity of the Elephant Island were 0.04, 1.02 and 0.98 ind.m^-3, respectively. Waters influenced by the Weddell Sea near the Antarctic Peninsula were nearly devoid of salps. In the vicinity of Deception and Elephant Islands, the salp density in the top 170 m water layer during the daytime and nighttime was significantly different pointing to the active vertical migrations of S. thompsoni. Depth stratified sampling showed that salps generally migrated within the top 350 m water layer concentrating in the top 100 m during the nighttime and at 200-300 m during the daytime. Salp population structure and development were similar in three regions and overall dominated by salps smaller than 20 mm represented by recently released aggregate chains. Larger cohorts were not numerous but often distinct. Functional males comprised generally < 3% of total aggregate abundance. In all three regions, 110-130 mm solitary cohort was the most prominent in length frequency distributions and their development stage composition implied active reproduction at the time of our observations. Ingestion and egestion budget revealed low assimilation rates. Noticeable differences in developmental dynamics were documented for the first time shedding light on the possible mechanism of the salp southward expansion.