The giant Antarctic amphipod Paraceradocus miersi will not fare well if freshening continues in the Southern Ocean

Sea water provides a stable environment for marine animals. Most oceanic invertebrates have body fluids similar in ionic composition to sea water. However, Antarctic coastal ecosystems are increasingly exposed to freshening—a reduction in salinity driven by glacial melt, increased precipitation, and extreme weather events. Such changes can cause mortality in Antarctic invertebrates, which generally are intolerant to reduced salinity. This study investigated osmotic and ionic regulation of the Antarctic amphipod Paraceradocus miersi exposed to simulated freshening. When salinity was reduced (from S = 34.6 to 28.0) for 12–48 h, haemolymph osmolality and sodium concentration declined, while calcium remained stable; magnesium increased markedly, suggesting poor regulation. At S = 22.1, irreversible, catastrophic gill damage was clearly visible. We conclude that P. miersi is an osmoconformer capable of limited osmo- and variable iono-regulation, at least on the timescale investigated, and so is vulnerable to hyposaline damage, via compromised gill integrity Compared with Arctic amphipods, which possess stronger osmoregulatory capacity, P. miersi reflects adaptation to the historically stable Antarctic salinity regime. Haemolymph ion composition was broadly similar to sea water for Na + but differed markedly for K+ and Mg2+, with values more closely-aligned with Antarctic isopods. Freshening events in Ryder Bay have been rare but not absent; a 2017 event (S = 28.4) may have impaired osmotic and ion balance or even caused mortality in shallow-dwelling individuals. Continued ‘summer’ coastal freshening may therefore pose a significant physiological threat to this Antarctic amphipod and other marine invertebrates.