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⁺) concentration increases so does the Na⁺ turnover rate (both uptake and elimination rates increase). Sulphate ([Formula: see text]) uptake in mayflies also increases with increasing external [Formula: see text] although, unlike Na⁺, its rate of increase decreases with increasing external [Formula: see text] 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⁺ exchanged for (an outgoing) H⁺, leading to (localized) loss of pH regulation. The third hypothesis is premised on localized Na⁺ 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'.

Keywords: Ephemeroptera; major ions; mayfly; osmoregulation; salinity; stream invertebrates.

Figure 1.

The conventional model of (a) osmoregulation and its (b) energetics in freshwater animals. (Online version in colour.)

Figure 2.

The conventional model of osmoregulation in freshwater animals. The individual animal is represented by a blue rectangle in either water with low salinity (a) or moderate salinity but less than the osmolality of its haemolymph (b). The level of shading in the low salinity, moderate salinity and the animal itself represent the osmolality of the internal/external fluid. The green elbow arrows represent the production of energy via respiration, red arrows the uses of this energy (with the red elbow arrow the use of energy for ion turnover), and yellow arrows the movement of substances, i.e. inorganic ions and water. The size and direction of the arrows represent the amount and direction of the substances/energy flow. The sizes of the green and red rectangles represent the size of these functions/stores. Repr, reproduction. (Online version in colour.)

Figure 3.

A conceptual model of hypothesis 1 in salt-sensitive Ephemeroptera (mayflies). The individual animal is represented by a blue rectangle in either water with low salinity (a) or moderate salinity but less than the osmolality of its haemolymph (b). The levels of shading in the low salinity, moderate salinity and the animal itself represent the osmolality of the internal/external fluid. The green arrows represent the production of energy via respiration, red arrows the uses of this energy and yellow arrows the movement of substances, i.e. inorganic ions and water. The size and direction of the arrows represent the amount and direction of the substances/energy flow. The sizes of the green and red rectangles represent the size of these functions/stores. Repr, reproduction. (Online version in colour.)