Impact of hyposalinity on the barrens-forming sea urchin Centrostephanus rodgersii in context with flooding events.

The sea urchin Centrostephanus rodgersii is an ecologically important species in southeastern Australia, where its grazing contributes to the formation of macroalgal-free barrens habitat. While climate warming has facilitated this species southward range extension, climate change is also causing an increase in the frequency of storm-driven hyposalinity events. These events can expose marine organisms to acute or gradual decrease salinity stress, depending on the rate and duration of freshwater influx. The tolerance of C. rodgersii to below ambient salinity was investigated using a range of treatments emulating coastal salinity conditions (22-34‰). The response to decreased salinity was quantified in weight change, mortality, and righting time (a measure of muscular coordination) under abrupt salinity decrease and gradual salinity reduction conditions to reflect flash floods and less intense rain events, respectively. Muscular coordination began to decline at 28‰, with prolonged righting times. Acute exposure to 22‰ salinity caused 100% mortality within 72 h. Urchins in the gradual salinity reduction to 22‰ died following a gradual return to ambient conditions. These findings show that C. rodgersii is highly susceptible to climate-driven hyposaline events, which challenge its survival and thereby disrupt local marine community dynamics.