Why are mayflies (Ephemeroptera) lost following small increases in salinity? Three conceptual osmophysiological hypotheses

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    Abstract

    The salinity of many freshwaters is increasing globally as a result of human activities. Associated with this increase in salinity are losses of Ephemeroptera (mayfly) abundance and richness. The salinity concentrations at which Ephemeroptera decline in nature are lower than their internal salinity or haemolymph osmolality. Many species also suffer substantial mortality in single species laboratory toxicity tests at salinities lower than their internal salinity. These findings are problematic as conventional osmoregulation theory suggests that freshwater animals should not experience stress where external osmolality is greater than haemolymph osmolality. Here I explore three hypotheses to explain salt sensitivity in Ephemeroptera. These conceptual hypotheses are based on the observations that as the external sodium ion (Naþ) concentration increases so does the Naþ turnover rate (both uptake and elimination rates increase). Sulphate (SO2 4 ) uptake in mayflies also increases with increasing external SO2 4 although, unlike Naþ, its rate of increase decreases with increasing external SO2 4 . The first hypothesis is premised on ion turnover being energetically costly. The first hypothesis proposes that individuals must devote a greater proportion of their energy to ion homeostasis at the expense of other uses including growth and development. Lethal levels of salinity presumably result from individuals not being able to devote enough energy to maintain ion homeostasis without critical loss of other vital functions. The second hypothesis is premised on the uptake of Naþ exchanged for (an outgoing) Hþ, leading to (localized) loss of pH regulation. The third hypothesis is premised on localized Naþ toxicity or poisoning with increased Na turnover as salinity increases. None of the proposed hypotheses is without potential problems, yet all are testable, and research effort should be focused at attempting to falsify them. This article is part of the theme issue ‘Salt in freshwaters: causes, ecological consequences and future prospects’.

    Original languageEnglish
    Article number20180021
    Pages (from-to)1-9
    Number of pages9
    JournalPhilosophical Transactions of the Royal Society B: Biological Sciences
    Volume374
    Issue number1764
    DOIs
    Publication statusPublished - 21 Jan 2019

    Fingerprint

    Salinity
    Ephemeroptera
    Ions
    salinity
    Toxicity
    osmolality
    Salts
    Fresh Water
    Osmolar Concentration
    ions
    Hemolymph
    uptake mechanisms
    Sulfates
    hemolymph
    homeostasis
    Animals
    Homeostasis
    Sodium
    salts
    Osmoregulation

    Cite this

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    title = "Why are mayflies (Ephemeroptera) lost following small increases in salinity? Three conceptual osmophysiological hypotheses",
    abstract = "The salinity of many freshwaters is increasing globally as a result of human activities. Associated with this increase in salinity are losses of Ephemeroptera (mayfly) abundance and richness. The salinity concentrations at which Ephemeroptera decline in nature are lower than their internal salinity or haemolymph osmolality. Many species also suffer substantial mortality in single species laboratory toxicity tests at salinities lower than their internal salinity. These findings are problematic as conventional osmoregulation theory suggests that freshwater animals should not experience stress where external osmolality is greater than haemolymph osmolality. Here I explore three hypotheses to explain salt sensitivity in Ephemeroptera. These conceptual hypotheses are based on the observations that as the external sodium ion (Na{\th}) concentration increases so does the Na{\th} turnover rate (both uptake and elimination rates increase). Sulphate (SO2 4 ) uptake in mayflies also increases with increasing external SO2 4 although, unlike Na{\th}, its rate of increase decreases with increasing external SO2 4 . The first hypothesis is premised on ion turnover being energetically costly. The first hypothesis proposes that individuals must devote a greater proportion of their energy to ion homeostasis at the expense of other uses including growth and development. Lethal levels of salinity presumably result from individuals not being able to devote enough energy to maintain ion homeostasis without critical loss of other vital functions. The second hypothesis is premised on the uptake of Na{\th} exchanged for (an outgoing) H{\th}, leading to (localized) loss of pH regulation. The third hypothesis is premised on localized Na{\th} toxicity or poisoning with increased Na turnover as salinity increases. None of the proposed hypotheses is without potential problems, yet all are testable, and research effort should be focused at attempting to falsify them. This article is part of the theme issue ‘Salt in freshwaters: causes, ecological consequences and future prospects’.",
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    T1 - Why are mayflies (Ephemeroptera) lost following small increases in salinity? Three conceptual osmophysiological hypotheses

    AU - Kefford, Ben J.

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    N2 - The salinity of many freshwaters is increasing globally as a result of human activities. Associated with this increase in salinity are losses of Ephemeroptera (mayfly) abundance and richness. The salinity concentrations at which Ephemeroptera decline in nature are lower than their internal salinity or haemolymph osmolality. Many species also suffer substantial mortality in single species laboratory toxicity tests at salinities lower than their internal salinity. These findings are problematic as conventional osmoregulation theory suggests that freshwater animals should not experience stress where external osmolality is greater than haemolymph osmolality. Here I explore three hypotheses to explain salt sensitivity in Ephemeroptera. These conceptual hypotheses are based on the observations that as the external sodium ion (Naþ) concentration increases so does the Naþ turnover rate (both uptake and elimination rates increase). Sulphate (SO2 4 ) uptake in mayflies also increases with increasing external SO2 4 although, unlike Naþ, its rate of increase decreases with increasing external SO2 4 . The first hypothesis is premised on ion turnover being energetically costly. The first hypothesis proposes that individuals must devote a greater proportion of their energy to ion homeostasis at the expense of other uses including growth and development. Lethal levels of salinity presumably result from individuals not being able to devote enough energy to maintain ion homeostasis without critical loss of other vital functions. The second hypothesis is premised on the uptake of Naþ exchanged for (an outgoing) Hþ, leading to (localized) loss of pH regulation. The third hypothesis is premised on localized Naþ toxicity or poisoning with increased Na turnover as salinity increases. None of the proposed hypotheses is without potential problems, yet all are testable, and research effort should be focused at attempting to falsify them. This article is part of the theme issue ‘Salt in freshwaters: causes, ecological consequences and future prospects’.

    AB - The salinity of many freshwaters is increasing globally as a result of human activities. Associated with this increase in salinity are losses of Ephemeroptera (mayfly) abundance and richness. The salinity concentrations at which Ephemeroptera decline in nature are lower than their internal salinity or haemolymph osmolality. Many species also suffer substantial mortality in single species laboratory toxicity tests at salinities lower than their internal salinity. These findings are problematic as conventional osmoregulation theory suggests that freshwater animals should not experience stress where external osmolality is greater than haemolymph osmolality. Here I explore three hypotheses to explain salt sensitivity in Ephemeroptera. These conceptual hypotheses are based on the observations that as the external sodium ion (Naþ) concentration increases so does the Naþ turnover rate (both uptake and elimination rates increase). Sulphate (SO2 4 ) uptake in mayflies also increases with increasing external SO2 4 although, unlike Naþ, its rate of increase decreases with increasing external SO2 4 . The first hypothesis is premised on ion turnover being energetically costly. The first hypothesis proposes that individuals must devote a greater proportion of their energy to ion homeostasis at the expense of other uses including growth and development. Lethal levels of salinity presumably result from individuals not being able to devote enough energy to maintain ion homeostasis without critical loss of other vital functions. The second hypothesis is premised on the uptake of Naþ exchanged for (an outgoing) Hþ, leading to (localized) loss of pH regulation. The third hypothesis is premised on localized Naþ toxicity or poisoning with increased Na turnover as salinity increases. None of the proposed hypotheses is without potential problems, yet all are testable, and research effort should be focused at attempting to falsify them. This article is part of the theme issue ‘Salt in freshwaters: causes, ecological consequences and future prospects’.

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    KW - Major ions

    KW - Mayfly

    KW - Osmoregulation

    KW - Salinity

    KW - Stream invertebrates

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    SN - 0962-8436

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