Mayfly Species Research Articles

Effects of temperature on the size of aquatic ectotherms: Exceptions to the general rule

1. 1. Of 61 studies of aquatic ectotherms, increased rearing temperature (apparently not stressful for growth and development) caused a reduction in organism size at a given developmental stage in 55 cases (90.2%) and an increase in only six (9.8%). 2. 2. The six exceptions to the size-reduction rule included one diatom ( Phaeodactylum tricornutum), one copepod ( Salmincola salmoneus) and four species of mayfly (Ephemeroptera). The extent to which these exceptions could be explained by each of four mechanisms was investigated by comparing their life cycles and niches with those of other closely-related species. 3. 3. No satisfactory explanation could be found for the response of P. tricornutum, but mechanisms consistent with the response of S. salmoneus were: reduced risk of oxygen shortage, low risk of ectotherm predation and seasonal constraints on the life cycle. The latter may also help explain the four mayfly exceptions.

An Update on the Occurrence of Three Species of Mayflies (Ephemeroptera) in Maine

Mayflies, members of the order Ephemeroptera, are a diverse group of aquatic insects. Unlike many kinds of insects that spend most of their lives on land, mayflies spend most of their lives in the water as immature forms called nymphs. Adult mayflies are short lived and reproduce soon after they complete their development to the final winged stage called an imago. Mated adult females deposit their eggs in the waters of streams and ponds, where they hatch. After hatching, nymphs live on or among the sediments and submerged stems of aquatic plants. Nymphs can feed and grow for as much as two years before they complete their development to the first winged stage called a subimago, but often this development is accomplished in one year or less. The subimago is a winged terrestrial form that usually flies away from the water to a place of safety where it soon sheds its skin and becomes the imago. For further details of mayfly biology the reader is directed to publications by Edmunds et al. (1976) and McCafferty (1981). In the late 1980's an extensive survey of Maine's mayfly fauna was conducted (Burian and Gibbs 1990). The results of that work showed Maine to have a diverse mayfly fauna with a majority of its 160 species broadly distributed from the mountains of the Moosehead Plateau to the coast. However, about 41 species (26% of the fauna) were recorded from less than three sites. Species such as these may indeed be rare, endemic, or indicative of special habitats. But, they may have been simply overlooked at other sites or were collected in a life stage that could not be determined to species. In general, mayflies with few records have ranges and habitat requirements that are largely unknown. For these species each new record improves our understanding of its range and habitat. This is especially important for the purpose of habitat management and conservation because critical decisions, usually involving rare species, cannot be made without basic range and habitat information. In this paper new locality records are presented for Siphlonurus securifer McDunnough, Metretopus borealis Eaton, and Hexagenia rigida Eaton, which have few known localities in Maine. Siphlonurus

Experimental acidification of Little Rock Lake, Wisconsin: The first four years of chemical and biological recovery

Little Rock Lake was experimentally acidified in 1984-1990 during which sulfuric acid was added to one basin, decreasing pH from 6.1 to 5.6, 5.1 and 4.7. The lake has been allowed to recover without manipulation since autumn 1990. By the third year of recovery, ∼40% of the change necessary to return to pre-acidification values of pH, acid neutralizing capacity (ANC), sulfate (SO 4 2- ) and calcium (Ca 2+ ) had occurred. During recovery years 1-2, ANC was closely predicted by models based on acidification phase observations, but recovery during years 3-4 was slower than predicted. A possible explanation for the slowed recovery is acidification of the upper 0-5 cm of sediment, which acts as a sink for the ANC generated via SO 4 2- reduction, the primary recovery mechanism. Trends for zooplankton did not follow pH recovery very closely. Species diminished by acidification (e.g. Keratella cochlearis, Daphnia dubia) have not recovered, but species that dominated the community at pH 4.7 (e.g. K. taurocephala, D. catawba) have not maintained high populations. The time required for the zooplankton community to recover to pre-manipulation conditions is uncertain. Delays also have been observed for the mayfly species Caenis, which had disappeared at pH 4.7. In contrast, reproductive success of largemouth bass (Micropterus salmonides) mirrored that observed during acidification ; egg hatch and survival of young-of-the-year to autumn recurred when pH exceeded response levels documented during acidification. Overall, recovery has not closely followed the pattern predicted by acidification responses.

Life Histories of Two Ameletus Mayflies (Ephemeroptera) in Two Mountain Streams: The Influence of Temperature, Body Size, and Parasitism

The life histories of Ameletus celer and A. similior were compared at Ford Creek and Elbow Creek, two sites in the same drainage system that differed in their annual temperature regimes. We used electrophoresis to identify very small larvae and to determine parasitism by a mermithid nematode. Ameletus similior had an overwintering egg diapause and a univoltine summer cycle at both sites. Ameletus celer eggs appeared to take about 1 yr to hatch and larvae took a further year to develop at the warmer site but 2 yr at the colder site. Both species were parasitized by a mermithid nematode, the prevalence of parasitism reaching ∼10% in the A. celer population at the colder site. Parasitism had the effect of prolonging larval life by 2 mo, and parasitized mayflies laid no eggs, even if they survived to adulthood. Temperature and larval size had significant effects on growth rates in the laboratory but larvae of both species of mayfly grew at the same rate at each temperature between 0.6°C and 18°C. The number of day-degrees at the colder site was sufficient to allow the smaller A. similior to mature in one year, but not the larger A. celer. Extension of physiological time to complete the life cycle at the colder site could not be satisfactorily explained by delayed development, a thermal minimum above 0°C, a nonlinear relationship between temperature and growth rate, placement of recruitment within the thermal cycle, food supply, or parasitism. However, some extension of the life cycle in A. celer may be achieved by increase in final size.

Diel Patterns of Drift by Five Species of Mayfly at Different Levels of Fish Predation

Stream insects that drift downstream in the water column are at risk from fishes that feed visually in mid-water, and the restriction of drifting to nighttime is hypothesized to have evolved as an avoidance response to this type of predator. I tested the hypothesis that low daytime drifting is a proximate response to the presence of drift-feeding fishes by manipulating fish density (brook trout, Salvelinus fontinalis) in two field experiments. Daytime drift activity of five prey taxa (all mayflies: Ephemeroptera) was low in both experiments and did not change following fish removal. By night, however, nighttime drift activity of two mayflies, Baetis and Paraleptophlehia, was increased in areas containing fish. The other three mayflies, Ephemerella, Stenonema, and Eurylophella, which showed less drift activity overall, displayed no consistent responses to fish presence. The original hypothesis predicts that risk from drift-feeding fishes should result in reduced daytime drifting. The increases in nighttime drifting away from patches dense with trout were not predicted by the original hypothesis, but may be an additional avoidance response to reduce long-term predation risk in streams where trout are distributed patchily in space.

Actions and interactions of temperature, ph and photoperiod on mercury bioaccumulation by nymphs of the burrowing mayfly Hexagenia rigida, from the sediment contamination source

AbstractBased on a three‐compartment system — water, natural sediment, Hexagenia rigida nymphs — an experimental study was set up, using a complete factorial design, to quantify the actions and interactions of three abiotic factors (temperature: 10, 18, and 26°C; photoperiod: 6, 12, and 18 h per day; pH: 5.0 and 7.5) on inorganic mercury (HgCl2) and methylmercury (CH3HgCl) bioaccumulation by Hexagema rigida (whole organism and gills). The two chemical forms of the metal were initially introduced into the sediment; the exposure duration was 15 d. Total Hg burdens measured at the whole‐organism level revealed a very high bioaccumulation capacity of this burrowing mayfly species and important differences between the two contamination conditions of the sediment source, a factor close to 20 observed in favor of methylmercury, for similar exposure conditions. Among the three abiotic factors taken into account, temperature and water‐column pH played an important role on Hg bioaccumulated by the nymphs, when considered in isolation and in interaction. An increase in temperature from 10 to 26°C gave rise to an increase in Hg bioaccumulation, with the higher differences close to a factor of 1.7. On the other hand, acidification of the water column from 7.5 to 5.0 led to a decrease in the amounts of the metal accumulated by Hexagenia rigida. These effects were similar for the two Hg compounds, but they were more pronounced when the experimental units were contaminated by methylmercury. This comparative analysis of the amounts of metal bioaccumulated by whole organism and by the gills, estimates of nymph activity within the sediment, and results from earlier lab studies have generated several hypotheses on the involved mechanisms. We propose that ingested sediment is the predominant route of exposure and that the gut acts as a selective barrier that favors organic Hg absorption.

Anti-Predator Responses of Mayfly Larvae to Conspecific and Predator Stimuli

Hydrodynamic and water-borne chemical stimuli are considered important cues that initiate anti-predator behaviours of mayfly larvae. We tested the hypotheses that chemical stimuli from conspecifics and the predacious fish Rhinichthys cataractae and hydrodynamic stimuli from a Rhinichthys model would initiate anti-predator responses by mayfly larvae of Ephemerella aurivillii, Paraleptophlebia heteronea, and Baetis tricaudatus. A second set of experiments tested the hypotheses that chemical stimuli from conspecifics and the predacious stonefly Claassenia sabulosa and hydrodynamic stimuli from a Claassenia model would initiate responses by Baetis. These hypotheses were tested in laboratory streams where mayfly larvae received either chemical stimuli, hydrodynamic stimuli, or the combination of hydrodynamic and chemical stimuli. Responses by larvae to these stimulus types, in terms of use of substratum surfaces and drift rates, were compared with those by larvae in control streams without conspecific and predator stimulus types. Responses to chemical stimuli were variable depending upon mayfly species and the chemical stimulus type. For instance, while Ephemerella and Paraleptophlebia responded to Rhinichthys odours, Baetis did not respond to either Rhinichthys or Claassenia odours. Paraleptophlebia responded to conspecific odours whereas Ephemerella and Baetis did not. In contrast, all three mayfly species responded to hydrodynamic stimuli either alone or when combined with chemical stimuli. Responses by Baetis were more complex when chemical stimuli were combined with hydrodynamic stimuli. For instance, although Baetis did not respond to conspecific odours alone, drift was significantly higher in streams receiving the three stimuli of conspecific odours, predator odours from Rhinichthys, and hydrodynamic stimuli from a Rhinichthys model when compared with streams that received only the conspecific odours combined with the fish model. Similarly, an enhanced response was observed when Baetis odours were combined with Claassenia odours and the Claassenia model. Thus, the lack of a response to a chemical stimulus did not preclude an enhanced response when combined with other stimulus types. These observations suggest that although some mayflies are capable of detecting the presence of some chemical stimuli, a response is not elicited unless the stimulus is accompanied by additional stimuli.

Response of macroinvertebrates to blooms of iron-depositing bacteria

In field measurements and laboratory experiments we assessed the influence of high levels of iron, manganese, and concurrent blooms of iron-depositing bacteria, Leptothrix ochracea, on macroinvertebrates. Macroinvertebrate communities in five of six streams were depauerate inside blooms. Reasons for the decreased abundance vary among taxa, with our experiments demonstrating the importance, for one or more species, of (1) direct toxic effects, and,/or smothering, (2) behavioral avoidance of bacterial-coated substrates, and (3) an inability to use bacteria as food. Three mayfly species showed increased mortality when caged inside the blooms, but five trichopterans and one plecopteran did not. Five invertebrates avoided Leptothrix-coated substrate in choice trials, while three did not. Stenonema fuscum could not ingest Leptothrix, and Neophylax nacatus had reduced growth feeding on it, but Heptagenia umbratica grew equally well on diets of Leptothrix or diatoms. This study demonstrates the important role epilithic organisms play in modifying substrates, and how these changes may act to influence benthic abundance and distribution in streams.

Diel and density-related changes in food consumption and prey selection by brook charr in a New Hampshire stream

We report the results of a field study testing influences of both density and changes over the diel cycle on food consumption and prey selection by brook charr,Salvelinus fontinalis. Charr density in replicate 35 m long sections of a New Hampshire stream was adjusted to either medium or high levels (relative to natural densities). Diets of charr and the availability of drifting prey were then sampled every four hours for 24 hours. There were no significant diel changes in the weight of prey consumed by charr per four hours, though there was some indication of reduced feeding at night. Chary fed selectively on different prey taxa, showing most preference for cased caddis larvae. Several species of mayflies and stoneflies were selected more strongly during the day than at night. Charr fed selectively on larger prey during the daytime but showed no size-selection at night. The density of charr had no significant effects on either their rate of food consumption or on selection for prey of different taxa or sizes.

A Model for Seasonal Synchrony in Stream Mayflies

Many mayfly species have synchronous univoltine life histories over broad geographic ranges, but the life-history adaptations underlying their seasonality remain unidentified. We investigated whether simple adaptations in the response of development rate to temperature might account for the observed phenology of adult emergence and the 1- and 2-year life histories of eight mayfly species studied in Piedmont streams of eastern North America between 34°N and 50°N latitude. We present a model consisting of two sequential life-history stages. Development rate in each stage is a linear function of streamwater temperature above a lower threshold but, in the first stage, a developmental quiescence occurs whenever a maximal temperature is exceeded. The five model parameters are assumed not to vary with latitude. Using daily field temperatures, the model simulated several successive generations beginning from an arbitrary day of the year. With parameters fitted from the field data, the model could reproduce both t...

Consequences and Plasticity of the Specialized Predatory Behavior of Stream‐Dwelling Stonefly Larvae

The purpose of this study was to determine experimentally the potential mechanism(s) favoring specialized foraging behavior of stream—dwelling predatory stonefly larvae (Megarcys signata), and whether this specialized behavior was fixed or flexible. We measured stonefly growth rates after 10 d of conditioning on Baetis bicaudatus (Ephemeroptera) or one of two alternative mayfly prey species in replicated circular plexiglass flow—through chambers powered by water from a stream in western Colorado. We then compared attacks per encounter (as an index of predator choice) of conditioned stoneflies to those of unconditioned controls given equal densities of all three prey types. We conducted additional behavioral observations to determine whether predators differentially encountered the alternative prey types on natural substrates, and to determine whether predator—prey microhabitat overlap was mechanism explaining differential prey encounter rates among prey species. Megarcys exhibited a stereotyped attack bias toward swimming mayflies, such as Baetis, following the simple rule, "attack any prey that swim," which results in Megarcys' selection of prey types that swim in response to stonefly encounters. This behavior remained unmodified after extended experience with alternate but palatable nonswimming mayfly species. A potential fitness advantage (measured as relative growth rate) of this behavioral specialization occurred for female Megarcys, but not for males. Female Megarcys grew faster on Baetis diets because they ate more Baetis, not because Baetis was a more intrinsically profitable prey item (in terms of promoting predator growth). Further, high feeding rates on Baetis were a result of disproportionately high encounter rates and attacks per encounter with Baetis compared to the other mayfly species. Our data are consistent with the hypothesis that this predatory behavior arose and is maintained in female stoneflies due to a fitness advantage to individuals specializing on a relatively abundant, easily recognized prey resource. We speculate that this specialized behavior has not been lost in male stoneflies, for which we did not measure a fitness advantage, because there is no strong selection pressure against it. Effective pre—and—post—contact prey defenses may exert enough selection pressure to prevent stoneflies from evolving on alternative mayfly types.

Habitat Partitioning among Three Species of Ephemerelloidea

ABSTRACT Niche volumes according to several physical and water quality parameters are shown for Drunella grandis, Drunella doddsi and Tricorythodes minutus. Generalist strategies have been adopted by D. grandis for physical habitat characteristics and by T. minutus for water quality characteristics. Drunella doddsi, a specialist, has the smallest physical habitat and water quality niche dimensions of the three mayfly species. Even though each has its own unique niche, there is some overlap between them for most niche dimensions studied. These overlaps are probably present because density independent seasonal limiting factors in western United States streams are as strong or stronger selectors than many density dependent factors. Also, each species has its own unique niche “hypervolume” that changes from station to station and from time to time.

A new monotypic fungal genus, Allantomyces, and a new species of Legeriomyces (Trichomycetes, Harpellales) in the hindgut of a Western Australian mayfly nymph (Tasmanocoenis sp.)

Allantomyces caenidarum sp.nov., a trichomycete fungus that lives in the hindgut of Tasmanocoenis sp. (Caenidae) mayfly nymphs in southwestern Western Australian streams, is described. In addition to normal trichospores and zygospores that serve to reinfest other nymphs, A. caenidarum also produces what may be modified trichospores that serve to increase gut infestation endogenously. The same species of mayfly also harbored a new species, Legeriomyces raus, which belongs to a genus previously known only from North America and Europe. Key words: Caenidae, Allantomyces, Australia, Tasmanocoenis, Trichomycetes, Legeriomyces.

Use of man-made impoundment in mitigating acid mine drainage in the North Branch Potomac River

The US Department of the Army, Baltimore District Corps of Engineers, oversees a long-term monitoring study to assess and evaluate effects of the Jennings-Randolph reservoir on biota in the North Branch Potomac River. The reservoir was intended, in part, to mitigate effects of acid mine drainage originating in upstream and headwater areas. The present study assessed recovery of benthos and fish in this system, six years after completion of the reservoir. Higher pH and lower iron and sulfate concentrations were observed upstream of the reservoir compared to preimpoundment conditions, suggesting better overall water quality in the upper North Branch. Water quality improved slightly directly downstream of the reservoir. However, the reservoir itself was poorly colonized by macrophytes and benthic organisms, and plankton composition suggested either metal toxicity and/or nutrient limitation. One large tributary to the North Branch and the reservoir (Stony River) was shown to have high (and possibly toxic) levels of manganese, iron, zinc, and aluminum due to subsurface coal mine drainage. Macroinvertebrate diversity and number of taxa were higher in sites downstream of the reservoir in the present study. Compared with previous years, the present study suggested relatively rapid recovery in the lower North Branch due to colonization from two major unimpacted tributaries in this system: Savage River and South Branch Potomac. Abundance of certain mayfly species across sites provided the most clear evidence of longitudinal gradients in water quality parameters and geomorphology. Fish data were consistent with macroinvertebrate results, but site-to-site variation in species composition was greater. Data collected between 1982 and 1987 suggested that certain fish species have unsuccessfully attempted to colonize sites directly downstream of the reservoir despite the more neutral pH water there. Our results show that recovery of biota in the North Branch Potomac was attributed to decreased acid inputs from mining operations and dilution from the Savage River, which contributed better water quality. Continued improvement of North Branch Potomac biota may not be expected unless additional mitigation attempts, either within the reservoir or upstream, are undertaken.

Mayflies (ephemeroptera), the most "primitive" winged insects, have telotrophic meroistic ovaries.

Germ line cell cluster formation in ovarioles of three different stages, each from a different mayfly species, was studied using ultra-thin serial sectioning. In the analysed ovariole of Cloeön sp., only one linear, zigzag germ line cell cluster was found, consisting of sibling cells connected by intercellular bridges which represent remnants of preceding synchronized mitotic cycles followed by incomplete cytokinesis. A polyfusome stretched through all sibling cells. At the tip of the ovariole, cytokinesis occurred without preceding division of nuclei; thus, intercellular bridges were lined up but the remaining cytoplasm between the bridges had no nuclei. The analysed Siphlonurus armatus vitellarium contained five oocytes at different stages of development. Each oocyte in the vitellarium was connected via a nutritive cord to the linear cluster of its sibling cells in the terminal trophic chamber. Each cluster had the same architecture as was found in Cloëon. The 3-dimensional arrangement and distribution of closed intercellular bridges strongly suggest that all five clusters are derived from a single primary clone. The position of oocytes within each cluster is random. However, each oocyte is embraced by follicular or prefollicular cells whilst all other sibling cells are enclosed by somatic inner sheath cells, clearly distinguishable from prefollicular cells. In the analysed ovariole of Ephemerella ignita, two small linear clusters were found in the tropharium beside two single cells, two isolated cytoplasmic bags with intercellular bridges but no nuclei, and some degenerating aggregates. One cluster was still connected to a growing oocyte via a nutritive cord. In all species the nurse cells remained small and no indications of polyploidization were found. We suggest that this ancient and previously unknown telotrophic meroistic ovary has evolved directly from panoistic ancestors.

Wundacaenis, a new genus of Caenidae (Insecta : Ephemeroptera) from Australia

A new genus, Wundacaenis, is erected for three new species of Australian caenid mayflies. The genus is diagnosed by possession of distinctive lobes on the anterolateral margins of the mesonotum. The distribution of Wundacaenis extends from the Kimberleys in Western Australia, through the Alligator Rivers Region in the Northern Territory, and down the eastern coast to the Shoalhaven River in New South Wales.

Swarm behaviour and mate competition in mayflies (Ephemeroptera)

Although mayfly swarms are frequently cited as an example of lekking by insects, little is known about the behaviour of individuals within a swarm, or how mate‐selection takes place. A study of five species of mayfly over a period of 10 consecutive years has revealed species‐specific differences in the flight pattern of swarming males and in the ability of males to recognize swarms of their own species. Males of four of the five species jostle other males in the swarm at all times except when mating: mating pairs are not jostled. The pattern of jostling varies with the species. Measurements of the sperm content of the vesicula seminalis and of the wing length of members of individual swarms show that larger wing size is positively correlated with the presence of less sperm. The vesicula seminalis is always filled with sperm at the beginning of the imaginal stage and the testes regress before the beginning of the imaginal stage. If the volume of sperm in the vesicula seminalis is a valid index of mating success then males with larger wings have the highest success. Large wings may bestow an advantage during jostling. The males of Ephemera danica, which do not jostle, glide with outspread wings; these outspread wings may attract females, the largest wings being the most attractive. Females of all five species enter the swarm a few at a time, although many females may be resting beneath the swarm. This phased entry may decrease the attraction of the swarm for predators. The number of females in a swarm is not correlated with swarm size, and the factors which enable females to regulate their entry into a swarm remain obscure.

Use of the mayflyStenonema Modestum(Heptageniidae) in subacute toxicity assessments

This study presents the results of subacute toxicity testing with the mayfly (Ephemeroptera), species Stenonema modestum (Heptageniidae). This species is ubiquitous to many streams in the eastern United States and Canada and is associated with unpolluted sediments and water. The results show that counts of molts or exuviae over a 7–14-d exposure period can be a sensitive indicator of chronic growth effects. Molt production was a more sensitive indicator of pollutant effects than length of organism or width of head capsule measurements. Tests using coal mine effluents indicated that the 7-d molt production end point was often more sensitive than the 14-d molt or survival end point. This was due to either a short-term stimulatory effect on molting or effluent variability over time. Molt stimulation was not observed in tests using NaCl or silver nitrate. Tests with these compounds and coal mine effluents demonstrated that the methods and end points provide repeatable and relatively sensitive data. Test precision was comparable to U.S. Environmental Protection Agency 7-d short-term chronic tests. Molt production of Stenonema modestum in laboratory tests proved to be a reliable predictor of instream effects due to a metal-finishing effluent. Furthermore, simultaneous tests of sediment elutriates and ambient samples from the stream pinpointed probable mechanisms behind the observed instream impact on Stenonema modestum. This method should be amenable to other mayfly species, making it a potentially useful tool in site-specific water quality and sediment quality studies.

Distributions of Ephemeroptera, Plecoptera, and Trichoptera of three maritime catchments differing in pH

SUMMARY1. Two broad‐scale environmental influences affecting species of Ephemeroptera, Trichoptera and Plecoptera inhabiting riffle habitat of three study catchments in New Brunswick and Nova Scotia were stream size‐temperature‐related variables and acidity‐related variables.2. Species richness was most affected by acidity in tributaries of intermediate size <0.001<0.1m3s−1).3. Of the three insect orders investigated, mayfly species richness was affected most by acidity, and stonefly species richness affected least by this factor.4. Zoogeographical isolation of the Nova Scotian catchments resulted in absence of some species of low general abundance, but these species had little influence on the site classification by TWINSPAN analysis.5. Predaceous stoneflies were collected in greater numbers from circumneutral sites and predaceous caddisflies in greater numbers from the Nova Scotian sites.6. Although the Nova Scotian catchments have probably been historically acidic, due to high concentrations of organic acids, pH currently limits the distributions of many indigenous invertebrate species. These species may be vulnerable to the more recent, additional, anthropogenic acidification of these catchments.

Are Chloride Cells Involved in Low pH Tolerance and Sensitivity? The Mayfly Possibility

Sensitivity to low pH in freshwater fish appears to be correlated with density and replacement rates of gill chloride cells, but these cells are difficult to examine. Mayfly nymphs possess cells of similar appearance and probable function and are easily observed in dense accumulations on gill surfaces. We examined chloride cell density on the gills of nymphs of seven species of mayflies (two Heptageniidae, two Leptophlebidae, one Siphlonuridae, and two Baetidae). Although tolerance of low pH exposure varied considerably among the seven species, it was not correlated with gill area or chloride cell density. Gills of nymphs from soft water also had no greater chloride cell density than gills of nymphs of the same species from hard water. Finally, nymphs of the two species able to withstand 2 wk exposure to low pH did not show an increase in chloride cell density. It remains possible that the prolonged ionoregulatory stress of low pH results in cell damage rather than cell proliferation in sensitive species.