Cenocorixa bifida (Hungerford) and Cenocorixa expleta (Hungerford) have different distributions which appear to be correlated with lake water sodium, chloride and osmotic pressure. C. bifida occurs in the lower and C. expleta in the higher salinities. An attempt was made to study the salinity relations of these two species. In low salinities both species maintain blood osmotic pressure, sodium, potassium and chloride well above those of the water but C. expleta tends to maintain lower levels than C. bifida. In high salinities both species are iso-osmotic to the water but C. expleta maintains blood sodium and potassium hypotonic to the water. C. bifida blood sodium and potassium are never hypotonic to the water. C. expleta contains more body water in all salinities. Both species appear to drink regularly in all salinities and mouth closure results in lowered blood sodium and chloride in hypo-osmotic media. The rostral ridges and third leg femoral-tibial intersegmental membranes are the only areas of cuticle which allowed penetration of silver ions. The abolishment of the osmotic gradient by adding sucrose to hypo-osmotic water appears to impair entry of water, sodium and chloride into C. bifida but not into C. expleta. C. bifida does not occur in water to which it is iso-osmotic as does C. expleta. This study suggests physiological adaptation of these species to different salinities. / Science, Faculty of / Zoology, Department of / Graduate

Facets of the biology of the sibling species Cenocorixa bifida and C. expleta (Corixidae) were compared in different habitats, constituting both sympatric and allopatric populations. The study aimed to identify reasons for the differing distribution of the two species. In particular, it sought to (a) explain why and how the species coexisted over part of their range in lakes of moderate salinity, and (b) deduce why C. expleta is absent from the more freshwater lakes. The thesis is divided into six sections. An introduction, reviewing competition theory and summarising the problem, is followed by studies on the environments investigated, in Chapter II. Chapter III discusses distribution, abundance and breeding pattern of the corixid species encountered, to obtain data relevant to certain indirect measures of competition, and Chapters IV and V describe feeding experiments and serological gut analyses of field-collected corixids. The feeding experiments aimed to obtain one measure of the fundamental niche, while the gut enalyses were carried out fer a measure of the realised niche. The final section discusses the findings in terms of potential species interaction. The study area comprised six lakes on Becher's Prairie, in the Chilcotin area of B.C., and another near Kamloops; all lakes were similar in general morphology. Although temperature patterns were similar between lakes, they varied considerably in conductivity. Temperatures and conductivity data corroborate earlier findings, suggesting that the studied years were not atypical. Oxygen levels and phytoplankton primary production were generally higher in the more freshwater lakes, but the contribution of aquatic macrophytes and benthic algae there was not measured. Phytoplankon production was restrained. Plankters were abundant in all lakes, especially inshore, but the levels fluctuated most in the three freshwater lakes, being very low there before mid-June. Diaptomid copepods and fairy shrimps were confined to the higher salinity lakes, while chaoborids and amphipods were restricted to the three freshwater lakes; other invertebrates were widely distributed. Both biomass and diversity of organisms were highest in the freshwater lakes, but they were by no means low in the saline lakes studied. The data suggest that there was an abundance of food in all lakes at all times. C. bifida bred in all lakes investigated, although excluded apparently through increasing salinity from lake LB 2 after spring. C. expleta bred sympatrically with C. bifida in waters of over 6000 umhos cm-1 conductivity, whereas Cymatia americana, Hesperocorixa laevigata, Callicorixa audeni and Sicjara sp. bred only in the three most freshwater lakes. C. expleta produced three generations a year in the highest salinities (LB 2 and Barnes Lakes) and two in other high salinities. C. bifida produced two generations a year in the higher salinities, but like all the other corixids, in the three most freshwater lakes often produced only one generation a year. Corixids showed no definite trends of different abundance with alkalinity, and the two species of Cenocorixa did not show depressed population levels in sympatry. Further, the phenology was essentially contemporary in sympatry. C. expleta when allopatric occurred in both marginal and mid-lake areas, whereas C. bifida when allopatric was more confined to the littoral areas. In both species, habitat preferences of instars varied with season, but in general the largest individuals of each species preferred the most complex environments. Overall, C. expleta in sympatry was relatively commanest over deeper waters and over plain mud or silt, whereas C. bifida preferred rocks, logs and reeds. However, the segregation was not marked. Both C. bifida and C. expleta took a wide variety of preys offered in experiments. C. expleta adults accepted more of the preys offered than did C. bifida. Juveniles showed less marked species differences than did adults, and accepted prey more often than them. Both species accepted both live and dead organisms. Juveniles of both species preferred dead prey to live; however, only for C. expleta was this trend significant overall. When offered live or dead mixed plankton, C. bifida took chiefly ceriodaphniids, whereas C. expleta took both diaptomids and ceriodaphniids in the proportions offered. In other choice situations, both species fed more often on chironomids than daphniids, taking zygopterans least. However, unsuccessful attacks were far more frequent on daphniids than on the benthic and littoral organisms. In summary, the experimental feeding results suggest that the fundamental niches of the two species of Cenocorixa are similar, but not identical. Guts of field-collected corixids were analysed by serological techniques using 10 active antisera. Mcst reactions indicated a clearly carnivorous habit, less than 1% being positive for algae. Results suggest that foods eaten varied dynamically between instars, lakes and seasons. In C. bifida daphniids and zygopterans were more frequently recorded in guts from the sympatric lakes than from the freshwater lakes. Ephemeropterans and amphipods were identified chiefly from summer-collected corixids, and were chiefly restricted to the freshwater lakes, paralleling their distribution. However, some reactions were also seen in guts from these lakes with diaptomid antiserum, perhaps a reaction to other copepod species. In C. expleta most positive results were seen for daphniids in spring and fall corixid samples. More male than female C. exjaleta took ephemeropterans, and more male than female C. bifida took zygopterans. Specific differences between saline and freshwater lakes seemed to relate to prey distribution, but in Lake LB 2 C. exgleta took significantly more diaptomids in fall than in summer. In sympatry, most juveniles of both species took an increasingly diverse diet with increasing instar size, although the preferred order chironomids, then zygopterans, ephemeropterans and diaptomids, remained relatively constant. Second instar C. expleta reached a plateau level of feeding intensity only reached by third instar C. bifida ; this seems related to instar size. In larger instars, C. bifida showed more serological reactions with most foodstuffs than e. expleta. Overall, C. expleta showed more positive reactions than C. bifida with chironomids and daphniids, but less with diaptomids and zygopterans. In sympatry, C. bifida fed more on daphniids and ephemeropterans than in allopatry, which tends to contradict ideas of competition for food. The serological data suggest that the realised niche with respect to feeding is not markedly more restricted than the fundamental niche. Other corixid species had similarly carnivorous diets, C. audeni showing more reactions for amphipods and chironomids than H. laevigata , but less for chaoborids, suggesting more benthic orientation for the former. In field-collected corixids, red guts usually contained diaptomids, while brown guts reacted with a variety of antisera. In several brown and olive guts there was no serological reaction, indicating that the antisera did not cover the total dietary range. Occasional guts were bright blue-green, suggesting cyanophyte material. Despite, a wide geographical overlap between the species, C. bifida and C. expleta possess somewhat different osmotic and ionic regulatory capacities, and show slightly different habitat and food preferences. Therefore they cannot be considered as true ecological homologues, and hence competition need not necessarily result in exclusion of one or other. With regard to habitat, some of the differences observed may be related to variation in the environments between lakes studied, but other differences show the species not to be identical. C. expleta seems to have a rather wider fundamental niche in terms of habitat than C. bifida, but neither show marked restriction in sympatry. The serological feeding results suggest that in sympatry, each species took different proportions of the same foodstuffs. The realised niches appear somewhat restricted from the fundamental niche data obtained from feeding experiments. C. expleta seemed somewhat more specialised in feeding than C. bifida, thus fulfilling the requirements cof an included niche species; again a point usually considered to permit coexistence. However, diet and habitat preferences of species and instars varied with season and lake, suggesting that pooling data obscures much of the real variation. In general, the evidence shows that the two species of Cenocorixa are not exact ecological homologues, and competition between them in sympatry was not evident. The reason for the absence of C. expleta from the more freshwater lakes, to which it appears physiologically suited, still remains obscure. / Science, Faculty of / Zoology, Department of / Graduate

Most terrestrial, and many aquatic insects are made waterproof by a layer of lipid in or on the epicuticle. At a specific temperature, which is determined by their composition, these lipids undergo a phase transition which markedly increases the permeability of the integument. The major purpose of this study was to assess the possibility that epicuticular wax transition could differentially affect the distribution of four species of water boatmen: Cenocorixa bifida hungerfordi Lansbury, Cenocorixa expleta (Uhler), Cenocorixa blaisdelli. (Hunger-ford , and Callicorixa vulnerata (Uhler). The rates of water loss and cuticle temperatures of adult corixids were measured in a stream of dry carbon dioxide in steps of increasing temperature. The temperatures at which transition occurred in these species were all approximately the same. Although they ranged from 30.3 to 32.6 C there were no major differences between the two genera, among congeners with different distributions, or between two coexisting congeners. This was true, however, only for corixids which had been acclimated to the same temperature: a positive correlation between transition temperature and acclimation temperature was demonstrated in C. befida. Both the short-term and long-term effects of transition on these insects were examined. Immersion of live C. bifida adults in water warmer than their transition temperature did not appear to cause any irreversible changes in cuticular permeability. Survival tests at various temperatures showed that the survival time of C. bifida adults decreased with increased temperature. However, insects placed in warm water did not show any outward signs of osmoregulatory failure or loss of surface wax as a result of transition. In addition, C. bifida placed in water as warm as or several degrees warmer than their transition temperature survived much longer than the length of time that these insects would .be exposed to these temperatures in the field. The present study on transition effects suggests that the transition of epicuticular lipids does not affect the distribution of these corixids in the field. However, it appears that salinity has a pronounced effect on the permeability of C. bifida adults. Individuals from fresh water habitats and highly saline ponds exhibited roughly equal cuticular permeability, but those from lakes of intermediate salinities were up to twice as permeable. This phenomenon was shown to be one of physiological acclimation, since individuals placed into distilled water showed a significant decrease in permeability after five days. It is possible that this influence of salinity on cuticular permeability may affect the relative dispersal success of the corixids which inhabit inland saline lakes. / Science, Faculty of / Zoology, Department of / Graduate

The problems of, osmoregulation and oxygen consumption were Investigated In two species of Corixidae (Hemiptera), Cenocorixa bifida and C. expleta. Field collections support a distribution correlated with salinity. C. bifida and C. expleta acclimated to a range of different lake waters for three days showed a gain and loss of about five per cent body water in hypo and hyper-osmotic media when compared with their natural environment. Determinations of freezing point depression of blood showed that both species are able to maintain the osmotic pressure of their body fluids higher than that of the surrounding medium except in highly saline Gr₂ lake water. Both species became approximately iso-osmotic In Long lake water (concentration 435 mOs/1). The results of this study Indicate that C. bifida and C. expleta can tolerate lake waters having as low as 5 and as high as 435 mOs/1 concentration under laboratory conditions and show no visible adverse effects. On transfer to media of lower concentration than the normal environment, C. expleta became swollen and sluggish. It appears that this species is unable to tolerate very low salinities. The results also Indicate that in Gr₂ water (concentration 753 mOs/1) the blood of both species becomes hypo-osmotic probably due to the breakdown of the hyperosmoregulatory mechanism, thus resulting in death. The presence of filtered solution of amaranth and phenol red In different lake waters showed that these species drink water In hypo and hyper-osmotic media to obtain salts and water lost, in excretory products. Sections of midgut showed swelling and vacuolation of epithelial cells in G. expleta when transferred to very low salinities like Spring House water. In highly concentrated Gr₂ lake water the midgut cells In both species were shrunken and detached from the basement membrane and in time they appeared to be completely broken down. To maintain a difference of osmotic pressure between the body fluids and the surrounding medium both species do metabolic work. The measurement of respiratory rate by Warburg method and the periods of locomotory activity in nitrogen saturated media showed that both species consume more oxygen on transfer to lower and higher salinities than in their normal medium. These results suggest that both species are under osmotic stress and are doing increased metabolic work with the increasing osmotic gradient, but because of the difficulty in distinguishing this from activity, the latter may also be causing some increase in the respiratory rate. The rate of oxygen consumption in the females of both species was found to be significantly higher than the male in all salinities. It is possible that this difference could be due to a greater expenditure of energy on egg maturation. / Science, Faculty of / Zoology, Department of / Graduate

Krum waste stabilization lagoon is a hypereutrophic system with relatively high thermal regimes and primary production. Corixids, particularly Corisella edulis and Corisella inscripta utilize the vast quantities of algae produced.

Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, Discipline of Ecological and Evolutionary Biology, 2008. / "December 2007" Bibliography: leaves 117-123. Also available in print form.

Sodium balance and mortality rates were examined in five species of adult waterboatmen (Hemiptera: Corixidae) exposed to neutral and low pH waters. The five species were chosen to reflect a wide range of pH conditions in waters where they naturally occur. Cenocorixa bifida and C. expleta normally inhabit high pH waters, whereas C. blaisdelli and Hesperocorixa atopodonta can be found most often at neutral pH. Sigara omani occur in acidic waters. Haemolymph [Na], whole-body [Na], and sodium influx rates were recorded during exposures of 6-9 days at pH 7.0, 4.5, and 3.0. C. blaisdelli and H. atopodonta were studied throughout the year (spring, summer, and fall) . C. bifida and C. expleta were examined in the fall, while S. omani were tested in the spring. Overall, these corixids appeared to be tolerant of short-term exposure to low pH. Mortality for all species remained below 50% in both neutral and acidic pH levels throughout the year. Additionally, differences in haemolymph [Na], whole-body [Na], and sodium influx rates from pH 7.0 to either pH 4.5 or 3.0 were rarely significant. However, both inter- and intraspecific variation in sodium balance over the range of pH levels tested were apparent. Most notably, C. blaisdelli and H. atopodonta exhibited their highest concentrations of haemolymph and whole-body Na in the summer, under both natural conditions and in the laboratory at all pH levels. For C. blaisdelli, summer was also the time of highest mortality, with mortality increasing as pH was lowered. The observed correlation between high haemolymph/whole-body [Na] and high mortality in the summer appears to result from a relatively large decrease in haemolymph and whole-body [Na] when bugs were exposed to pH 3.0, at a time when these values were initially high. Fall was the season of lowest haemolymph/whole-body [Na], and also of lowest mortality, for both C. blaisdelli and H. atopodonta. Interspecific variation in the ability to tolerate low pH did not appear to reflect variation in the pH of water these bugs normally inhabit. Of the five species tested, H. atopodonta appeared best able to maintain internal homeostasis under acidic conditions, despite being common to neutral waters. Haemolymph [Na], whole-body [Na], and sodium influx rates did not change significantly from pH 7.0 to pH 3.0 in any season. Furthermore, C. blaisdelli, which was collected from the same pond as H. atopodonta, seemed to be least able to regulate internal milieu in low pH waters. In C. blaisdelli, exposure to pH 3.0 usually resulted in decreased haemolymph and whole-body [Na], relative to values recorded in those individuals exposed to pH 7.0. The difference in the size of these two species (H. atopodonta is approximately twice that of C. blaisdelli) may account for the observed variations in their respective sodium balance at low pH. / Science, Faculty of / Zoology, Department of / Graduate

Compared with the free atmosphere, the aquatic environment is oxygen poor. As a result many secondarily aquatic organisms have adaptations that allow them to continue to use the atmosphere, directly or indirectly, to supply their oxygen requirements. This thesis examines how diving insects use bubbles of air collected at the surface of the water as oxygen reserves, gills and flotation devices, and how an aquatic angiosperm channels convective flows of air from its emergent leaves to its submerged organs. 1. Backswimmers (Anisops spp.) begin a dive positively buoyant, but rapidly enter a protracted period of near neutral buoyancy. A bubble of air held on the insect’s abdomen shrinks as respiration consumes its oxygen, while at the same time highly soluble carbon dioxide dissolves into the surrounding water. The reduced air volume confers neutral buoyancy. In response to low oxygen partial pressure (PO2) in the bubble, oxygen is released from large haemoglobin cells in the abdomen. The haemoglobin’s sensitivity to falling PO2 maintains the oxygen tension between 5.1 and 2.0 kPa. This stabilises the volume and buoyancy of the bubble. During a dive the haemoglobin and air-store supply 0.25 and 0.26 μL of oxygen, respectively. 2. The oxygen affinity of backswimmer haemoglobin determines the stability of the neutrally buoyant phase as well as its ability to satisfy the insect’s respiration. An oxygen equilibrium curve (OEC) determined in vivo has a highly sigmoid shape and an oxygen affinity of 3.9 kPa. In comparison with OEC made in vitro, the in vivo measurements show increased cooperativity and oxygen affinity, consistent with the presence of cationic effectors. Models strongly support the accuracy of the in vivo OEC method. 3. It has long been assumed that a bubble of air held over the spiracles of an insect enables the uptake of oxygen from the surrounding water and thus acts as a ‘gas gill’. Oxygen diffuses into a bubble of air when its PO2 is lower than the surrounding water, but a coincident higher nitrogen partial pressure causes it to dissolve. Several models have been produced to describe the gas exchange process, but all are based on untested assumptions of gill parameters. Measurements of gas gill volume and PO2 made on water bugs (Agraptocorixa eurynome) demonstrate that both drop quickly at the beginning of a dive, but PO2 reaches a stable level while the gas gill continues to dissolve. The importance of ventilation in maintaining an acceptable rate of oxygen consumption is also shown. 4. Interconnected gas spaces within the leaves, stems and rhizomes are a common feature of many emergent aquatic plants. Pressurised air from the leaves and culms of these plants ventilate these lacunae, flowing back to the atmosphere through efflux points. Unlike most aquatic plants, which have simple interconnected pith spaces, sacred lotus, Nelumbo nucifera, possess discrete gas canals which only interconnect where a leaf grows from the rhizome. Silicone casts and pneumatic tests of the gas canals reveal a complex repeating pattern of interconnections which channel air from specific regions of the leaf blade to the rhizome and out through efflux points on adjacent leaves. 5. Lotus, Nelumbo nucifera, possess in the centre of their leaves a specialised efflux organ which connects the gas canals in the leaves and stems with the atmosphere through the apertures of large stomata. Measurements made on excised lotus leaves and in situ reveal that the large stomata act as exhaust valves, opening and closing in a diurnal pattern to regulate the flow of pressurised gas from the leaf lamina and gas canals. This behaviour is shown to regulate gas flow rate and direction. The aquatic environment offers similar respiratory challenges to both plants and insects. While the oxygen uptake and transport mechanisms evolved by these groups are markedly different, they all function according to the same physical laws. Diving insects are separated from the atmosphere while underwater, forcing them to rely on oxygen either carried with them from the surface or extracted from the surrounding water. Emergent aquatic plants have permanent access to atmospheric oxygen, but must transport it long distances from their aerial leaves and stems to their roots and rhizomes. This thesis examines the uptake and storage of oxygen by diving insects and the gas transport system of the sacred lotus. / Thesis(Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2008