Data from: The mayfly nymph Austrophlebioides pusillus Harker defies common osmoregulatory assumptions

Osmoregulation is a key physiological function, critical for homeostasis. The basic physiological mechanisms of osmoregulation are thought to be well established. However through series of experiments exposing the freshwater mayfly nymph Austrophlebioides pusillus (Ephemeroptera) to increasing salinities, we present research that challenges the extent of current understanding of the relationship between osmoregulation and mortality. A. pusillus had modeled 96 hour LC10, LC50 and LC99 of 2.4, 4.8 and 10 g/L, respectively. They were strong osmoregulators. At aquarium water osmolality of 256 ± 3.12 mmol/kg (±SE), (equivalent to 10 g/L added synthetic marine salt), the haemolymph osmolality of A. pusillus was a much higher 401± 4.18mmol/kg (±SE). The osmoregulatory capacity of A. pusillus did not break down, even at the salinity corresponding to their LC99, thus their mortality at this concentration is due to factors other than directly to external salinity. No freshwater invertebrate has been previously reported as suffering mortality from rises in salinity that are well below the iso-osmotic point. Recently studies have reported reduced abundance/richness of Ephemeroptera with slightly elevated salinity. Given that salinisation is an increasing global threat to freshwaters, there is an urgent need for studies into the osmo-physiology of the Ephemeroptera to determine if their loss at locations with slightly elevated salinity is a direct result of external salinity, or other, possibly physiological, causes.