Biology of Salpa thompsoni at the Chatham Rise, New Zealand: demography, growth, and diel vertical migration

Abstract

The abundant pelagic tunicate Salpa thompsoni is a major grazer in the Southern Ocean (SO) with high ingestion rates, fast-sinking faecal pellets, and the potential to rapidly grow and form dense blooms. We investigated the length-frequency distribution, maturity stage composition, growth, development speed, and size-specific diel vertical abundance patterns at one of the northernmost habitats of S. thompsoni (Chatham Rise, east of New Zealand, ~ 44°S 178°E). By observing the in situ growth of distinct size cohorts and ex situ on-board experiments, relative growth was estimated for 6–10 mm blastozooids to be between 8.8–11.7% d−1 at ambient temperatures of 10 °C, with good agreement between our cohort-based field estimates and ex situ growth chambers. Transition times between development stages were estimated to be 3.8 ± 1.5 days, allowing for the succession from newly released aggregate chain to functional male blastozooid within 23 days. Integrative Bongo tows in the upper 200 m showed that S. thompsoni not only have daytime-dependent vertical abundance patterns, but also that these are size-specific, with medium-sized blastozooids and large oozooids contributing most to the elevated values during the night. The present study highlights the need for more in-depth biological studies of S. thompsoni at lower latitudes, which will help in understanding their increasingly structuring role in a gradually warming SO.