Unlocking the past: pteropod biogeographical distributions from the 1929 Discovery expedition in the Scotia Sea, Southern Ocean

Abstract

This study establishes a critical baseline for the population dynamics and biogeographical distribution of pteropods across the Polar Front in the Scotia Sea, between South Georgia and the Falkland Islands. Samples collected during the Discovery investigations aboard RRS William Scoresby in February–March 1929, before significant anthropogenic impacts, provide a historical benchmark for assessing long-term environmental change. An 1100 km transect comprising eight stations was sampled over 6 days using depth-discrete N70v closing plankton nets deployed between the surface and 1000 m (0–50, 50–100, 100–250, 250–500, 500–750 and 750–1000 m), enabling high vertical and regional resolution of pteropod distributions across the Polar Front. We examined pteropod species abundance, size, vertical distribution and diurnal migration. Our findings reveal that the northernmost extent of Limacina rangii is restricted by the Polar Front, positioning the northern Antarctic Zone as the trailing edge of its biogeographical range, while the southern limit of Limacina retroversa australis is similarly defined by the Polar Front. In contrast, gymnosomes occurred throughout the transect, their distribution linked to Limacina abundance rather than temperature, reflecting their broad thermal niche and predator-prey relationship with thecosomes. Vertical migration patterns indicate weak diurnal movement by L. rangii and L. retroversa australis , probably due to predator avoidance, supported by vertical segregation between gymnosomes and their prey. Size-dependent depth patterns were also evident, with veligers concentrated in the upper 100 m and juveniles distributed to 1000 m. These historical data offer a unique reference point for future assessments of climate-driven change in pteropod populations.