Abstract
Osmoregulation is a key physiological function, critical for homeostasis. The basic physiological mechanisms of osmoregulation are thought to be well established. However, through a 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 modelled 96 h LC10, LC50 and LC99 of 2.4, 4.8 and 10 g l−1 added synthetic marine salt (SMS), respectively. They were strong osmoregulators. At aquarium water osmolality of 256 ± 3.12 mmol kg−1 (±s.e.; equivalent to 10 g l−1 added SMS), the haemolymph osmolality of A. pusillus was a much higher 401 ± 4.18 mmol kg−1 (±s.e.). 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 increased internal osmotic pressure. 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 salinization is an increasing global threat to freshwaters, there is an urgent need for studies into the osmophysiology 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.
Highlights
Freshwater invertebrates have internal salinity concentrations higher than the environment in which they live, and so have to cope with two main osmoregulatory challenges
Given that salinization is an 2 increasing global threat to freshwaters, there is an urgent need for studies into the osmophysiology 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
Austrophlebioides pusillus directly transferred to 4 g l−1 synthetic marine salt (SMS) maintained haemolymph osmotic pressure within the range of those in the control treatment, despite this salinity leading to 52% mortality
Summary
Freshwater invertebrates have internal salinity concentrations higher than the environment in which they live, and so have to cope with two main osmoregulatory challenges. Large volumes of water enter the body. For freshwater insects, this occurs via drinking [1] and/or cuticular permeability [2,3]. From a dilute environment, they need to acquire specific ions at concentrations that support metabolic activity. Osmoregulation is the active control of intra- and extracellular ionic concentrations and volume. It is critical to homeostasis [3,4], and the osmoregulatory capacity of aquatic animals places limits on the salinity range they can inhabit [5]. The basics of osmoregulation are thought to be among the best understood physiological processes (see [6,7])
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