Geographical origin determines responses to salinity of Mediterranean caddisflies

Regional and fine-scale local adaptation in salinity tolerance in Daphnia inhabiting contrasting clusters of inland saline waters

Understanding the spatial scales at which organisms can adapt to strong natural and human-induced environmental gradients is important. Salinization is a key threat to biodiversity, ecosystem functioning and the provision of ecosystem services of freshwater systems. Clusters of naturally saline habitats represent ideal test cases to study the extent and scale of local adaptation to salinization. We studied local adaptation of the water flea Daphnia magna, a key component of pond food webs, to salinity in two contrasting landscapes-a dense cluster of sodic bomb crater ponds and a larger-scale cluster of soda pans. We show regional differentiation in salinity tolerance reflecting the higher salinity levels of soda pans versus bomb crater ponds. We found local adaptation to differences in salinity levels at the scale of tens of metres among bomb crater pond populations but not among geographically more distant soda pan populations. More saline bomb crater ponds showed an upward shift of the minimum salt tolerance observed across clones and a consequent gradual loss of less tolerant clones in a nested pattern. Our results show evolutionary adaptation to salinity gradients at different spatial scales, including fine-tuned local adaptation in neighbouring habitat patches in a natural landscape.

The acclimatory response of the mayfly Neocloeon triangulifer to dilute conditions is linked to the plasticity of sodium transport

Relative to a growing body of knowledge about the negative consequences of freshwater salinization, little is known about how aquatic insects respond to progressively ion-poor conditions. Here, we examined life-history and physiological acclimation in Neocloeon triangulifer by rearing nymphs from 1-day post-egg hatch to adulthood across a gradient of decreasing Na concentrations (15, 8, 4, 2 and 1 mg l^-1 Na). We found no significant changes in survival, growth, development time and whole-body Na content across these treatments. Radiotracer data revealed that nymphs acclimated to their dilute exposures by increasing their rates of Na uptake and were able to maintain a relatively narrow range of uptake rates (±s.e.m.) of 38.5 ± 4.2 µg Na g^-1 h^-1 across all treatments. By contrast, the Na uptake rates observed in naive nymphs were much more concentration dependent. This acclimatory response is partially explained by differences in ionocyte counts on the gills of nymphs reared under different salinities. Acclimated nymphs were surprisingly less retentive of their sodium composition when subjected to deionized water challenge. By contrasting our findings with a previous N. triangulifer salinity acclimation study, we show a physiological affinity for dilute conditions in this emerging mayfly model.

Tolerance of glacial-melt stoneflies (Plecoptera) and morphological responses of chloride cells to stream salinity

Aquatic insects within glacial-melt streams are adapted to low dissolved inorganic ion concentrations. Increases in ion concentrations in glacial-melt streams are predicted with increasing air temperatures, which may impact future aquatic insect survival in these streams. We hypothesized that stonefly (Plecoptera) naiads from glacial-melt streams acclimated to different conductivity would differ in survival, median lethal concentrations, and chloride cell responses to elevated conductivity above that expected in our study streams. We conducted field bioassays in remote glacial-melt streams in southwestern China in 2015 and exposed representative stonefly naiads (Chloroperlidae, Nemouridae, Taeniopterygidae) from stream sites differing in conductivity to experimental conductivity ranging from 11 to 20,486 μS/cm for up to 216 h. We examined survivorship, calculated 96-h median lethal concentrations, and measured chloride cell responses with scanning electron microscopy. Chloroperlidae survival after 120 and 216 h did not differ (P > 0.05) among conductivity treatments. The combined Nemouridae/Taeniopterygidae survival after 120 and 216 h was the least (P < 0.05) in conductivity treatments >16,349 μS/cm. Taeniopterygidae survival after 120 h was also the least (P < 0.05) in conductivity treatments >16,349 μS/cm. The 96-h median lethal concentrations did not differ (P > 0.05) between the combined Nemouridae/Taeniopterygidae group (2306 μS/cm) and Taeniopterigydae (2002 μS/cm) and were lower (P < 0.05) than the 96-h median lethal concentration for Chloroperlidae (8167 μS/cm). Chloroperlidae caviform cell number, density, and area decreased (P < 0.05) with increasing conductivity. Taeniopterygidae caviform cell count decreased (P < 0.05) with increasing conductivity, but cell density and area did not. Chloroperlidae and Taeniopterygidae coniform cell characteristics and Nemouridae bulbiform cell characteristics were not affected by conductivity. Our results suggest that Chloroperlidae, Nemouridae, and Taeniopterygidae from glacial-melt streams in China may be able to tolerate moderate increases in conductivity (i.e., 100 to 200 μS/cm).

Physiological responses of freshwater insects to salinity: molecular-, cellular- and organ-level studies

Salinization of freshwater is occurring throughout the world, affecting freshwater biota that inhabit rivers, streams, ponds, marshes and lakes. There are many freshwater insects, and these animals are important for ecosystem health. These insects have evolved physiological mechanisms to maintain their internal salt and water balance based on a freshwater environment that has comparatively little salt. In these habitats, insects must counter the loss of salts and dilution of their internal body fluids by sequestering salts and excreting water. Most of these insects can tolerate salinization of their habitats to a certain level; however, when exposed to salinization they often exhibit markers of stress and impaired development. An understanding of the physiological mechanisms for controlling salt and water balance in freshwater insects, and how these are affected by salinization, is needed to predict the consequences of salinization for freshwater ecosystems. Recent research in this area has addressed the whole-organism response, but the purpose of this Review is to summarize the effects of salinization on the osmoregulatory physiology of freshwater insects at the molecular to organ level. Research of this type is limited, and pursuing such lines of inquiry will improve our understanding of the effects of salinization on freshwater insects and the ecosystems they inhabit.