Impact of orphaning on field colonies of Southeast Asian Macrotermes gilvus (Hagen) and M. carbonarius (Hagen) (Termitidae, Macrotermitinae)

Field colonies of Macrotermes gilvus (Hagen) and M. carbonarius (Hagen) were experimentally orphaned to examine their potential for producing replacement reproductives. Orphaned colonies were investigated only once for caste composition at selected time intervals at 3, 6, 9 or 12 months after orphaning. Of the 38 orphaned colonies of M. gilvus, 15 colonies (39.5%) re-established. In M. carbonarius, three colonies out of 20 (15%) re-established. Re-established colonies were headed by adultoids which were morphologically indistinguishable from primary reproductives. In naturally orphaned colonies of M. gilvus, we often found multiple adultoids with normal pigmentation but torn wings, i.e. the colonies retained alates as replacement reproductives. The number of reproductives probably declines over time. It may take alates of M. gilvus 6 months to develop into functional adultoids, and up to 12 months for alates of M. carbonarius. Our results also demonstrate that the presence of sexual castes (nymphs or alates) at the time of orphaning does not necessarily guarantee the success of colonies in re-establishing themselves as breeding colonies. We also found a high prevalence of occupation of the mounds by other termite species, after the death of M. gilvus (18.4%) or M. carbonarius (30.0%) colonies, probably using them as foraging sites.     

Interspecific Agonistic Behavior of <Emphasis Type="Italic">Macrotermes gilvus</Emphasis> (Isoptera: Termitidae): Implication on Termite Baiting in the Philippines

The study investigated interspecific agonistic behavior of Macrotermes gilvus Hagen (Isoptera: Termitidae: Macrotermitinae) against three economically important subterranean termites in the Philippines, viz., Coptotermes gestroi Wasmann, Nasutitermes luzonicus Oshima and Microcerotermes losbanosensis Oshima. Termite-termite interactions after a 1:1 pairing experiment showed that M. gilvus workers and soldiers were highly aggressive against C. gestroi, N. luzonicus and M. losbanosensis leading to severe injury or death of the opponent termite species in a short period of time. The levels of agonism were caste and species specific. Worker termites of M. gilvus showed an equally aggressive behavior as soldiers contributing to the high mortality of opponent species used in this study. It is likely that the highly aggressive behavior of M. gilvus limits foraging activity of C. gestroi, N. luzonicus and M. losbanosensis around in-ground bait stations contributing to the low success of termite baits containing chitin synthesis inhibitors in the Philippines.     

Desiccation of intertidal limpets: Effects of shell size,fit to substratum,and shape

For undisturbed, field populations of the northeast Pacific, intertidal limpet Collisella pelta (Rathke), water loss was found to be proportional to apertural circumference rather than mantle cavity surface area. This contrasts with previous laboratory measurements for other limpet species and emphasizes the importance of the fit of shell to substratum for non-homing limpets. This close fit maintained a meniscus of water between the edge of the shell and the substratum, thereby reducing evaporative surface area and increasing the advantage of large size for reducing the rate of water loss. Shell-raising behavior eliminated the meniscus, increasing the rate of desiccation, and water loss appeared to become proportional to mantle cavity surface area. Indirect evidence suggests that C. pelta may utilize shell-raising behavior for evaporative cooling under thermally stressful conditions in the field.Both shell size and shape affect the ratio of water stores (proportional to internal shell volume) to evaporative surface area (proportional to apertural circumference) and per cent water loss is, potentially, a function of this ratio. Shape, however, (when defined as volume/circumference) exhibits an average allometric increase with increasing size (volume) for C. pelta, as well as for three other sympatric limpet species: C. Paradigitalis (Fritchman), C. Digitalis (Rathke), and Notoacmea scutum (Rathke). An independent measure of shape is, therefore, required to separate shape effects on desiccation from size effects; this measure was obtained by rearranging the allometry equation. In contrast to significant size effects, no measurable effect of shape on desiccation was detected. Variation in limpet shell shape may be partially or wholly maintained by factors other than an adaptational response to desiccation.     

Physiological changes in major soldiers of Macrotermes gilvus (Isoptera: Termitidae) induced by the endoparasitoid Misotermes mindeni (Diptera: Phoridae)

The majority of true parasitoids manipulate their host’s physiology for their own benefit. In this study, we documented the physiological changes that occurred in major soldiers of the subterranean termite Macrotermes gilvus (Hagen) (Isoptera: Termitidae) parasitized by the koinobiont larval endoparasitoid Misotermes mindeni Disney and Neoh (Diptera: Phoridae). We compared the metabolic rate, body water content, body water loss rate, cuticular permeability, and desiccation tolerance between parasitized and unparasitized major soldiers. The metabolic rate of parasitized hosts was significantly higher than that of unparasitized termites. Mean total body water content of parasitized major soldiers (64.73 ± 3.26%) was significantly lower than that of unparasitized termites (71.99 ± 2.23%). Parasitized hosts also had significantly lower total body water loss rates (5.72 ± 0.06%/h) and higher cuticular permeability (49.37 ± 11.26 μg/cm/h/mmHg) than unparasitized major soldiers (6.75 ± 0.16%/h and 60.76 ± 24.98 μg/cm/h/mmHg, respectively). Parasitized major soldiers survived almost twice as long as unparasitized termites (LT₅₀ = 6.66 h and LT₅₀ = 3.40 h, respectively) and they had significantly higher tolerance to water loss compared to unparasitized termites (45.28 ± 6.79% and 32.84 ± 7.69%, respectively). Body lipid content in parasitized hosts (19.84 ± 6.27%) was significantly higher than that of unparasitized termites (6.17 ± 7.87%). Finally, parasitized hosts had a significantly lower percentage of cuticular water content than unparasitized major soldiers (10.97 ± 1.84% and 13.17 ± 2.21%, respectively). Based on these data, we conclude that the parasitism-induced physiological changes in the host are beneficial to the parasitoids as the alterations can clearly increase the parasite’s chances of survival when exposed to extreme environmental conditions and ensure that the parasitoids are able to complete their larval development successfully before the host dies.     

Physical features and chitin content of eggs from the mosquito vectors Aedes aegypti,Anopheles aquasalis and Culex quinquefasciatus: Connection with distinct levels of resistance to desiccation

Mosquito eggs are laid in water but freshly laid eggs are susceptible to dehydration, if their surroundings dry out at the first hours of development. During embryogenesis of different mosquito vectors the serosal cuticle, an extracellular matrix, is produced; it wraps the whole embryo and becomes part of the eggshell. This cuticle is an essential component of the egg resistance to desiccation (ERD). However, ERD is variable among species, sustaining egg viability for different periods of time. While Aedes aegypti eggs can survive for months in a dry environment (high ERD), those of Anopheles aquasalis and Culex quinquefasciatus in the same condition last, respectively, for one day (medium ERD) or a few hours (low ERD). Resistance to desiccation is determined by the rate of water loss, dehydration tolerance and total amount of water of a given organism. The ERD variability observed among mosquitoes probably derives from diverse traits. We quantified several attributes of whole eggs, potentially correlated with the rate of water loss: length, width, area, volume, area/volume ratio and weight. In addition, some eggshell aspects were also evaluated, such as absolute and relative weight, weight/area relationship (herein called surface density) and chitin content. Presence of chitin specifically in the serosal cuticle as well as aspects of endochorion external surface were also investigated. Three features could be related to differences on ERD levels: chitin content, directly related to ERD, the increase in the egg volume during embryogenesis and the eggshell surface density, which were both inversely related to ERD. Although data suggest that the amount of chitin in the eggshell is relevant for egg impermeability, the participation of other yet unidentified eggshell attributes must be considered in order to account for the differences in the ERD levels observed among Ae. aegypti, An. aquasalis and Cx. quinquefasciatus.     

Is bigger better? Water balance in the polymorphic desert harvester ant Messor pergandei

Abstract. Foraging activity of ants in xeric areas may be limited by desiccation stresses. To assess the extent of such stresses on a polymorphic ant species [Messor pergandei (Mayr), body mass range 1-12mg], we measured body water reserves, absolute rates of water loss, and cuticular permeability over the species' foraging temperature range (15–45C). Cuticular permeability of M. pergandei was typical of xeric arthropods (17μgcm^-2h^-1mmHg^-1). However, the effect of polymorphism on both absolute and relative rates of water loss was unexpected. Large workers lost water more slowly than small workers, even after adjusting for cuticular surface area. In addition, the body water reserves of large workers were larger, even after adjusting for body mass. Small workers of this ant species are therefore disproportionately prone to dehydration, yet, in spite of this, constitute a large and important component of the foraging force. To assess the practical relevance of desiccation to polymorphic ants, we developed a 'worst-case' model of desiccation-limited foraging parameters (time to complete loss of locomotor coordination in 5% of the foraging force). In this model, average actual foraging duration was substantially less than would be required to incapacitate the ants by dehydration. We discuss direct and indirect evidence suggesting that desiccation may, nevertheless, impose limitations on the foraging activity of ants.     

Does lack of intraspecific aggression or absence of nymphs determine acceptance of foreign reproductives in <Emphasis Type="Italic">Macrotermes</Emphasis>?

The rejection or acceptance of a foreign reproductive by an alien colony may not always be as straightforward as cue recognition between worker termites. This paper aims to determine whether adoption of foreign reproductives is caused simply by lack of intraspecific aggression or is contingent on the reproductive status of the host colony. In the fungus-culturing termites, Macrotermes gilvus (Hagen) and Macrotermes carbonarius (Hagen), major workers showed low intraspecific aggression towards non-nestmates irrespective of geographic distance between source colonies. Our results indicated that workers were hardly aggressive towards non-nestmates. In royal cell-swapping experiments, both species responded in a similar way: (1) in host colonies with nymphs present, the foreign reproductives were rejected; while (2) in host colonies without nymphs the foreign reproductives were either accepted and breeding resumed or the host colonies died eventually. Workers from the host colonies preferentially maintained offspring nymphs from which adultoid replacement reproductives develop rather than accepting foreign reproductives. There is no fitness gain for the queenless workers in accepting foreign reproductives; however, there is overall benefit to the newly born population.     

Body shape in terrestrial isopods: A morphological mechanism to resist desiccation?

Woodlice are fully terrestrial crustaceans and are known to be sensitive to water loss. Their half‐ellipsoidal shapes represent simple models in which to investigate theoretical assumptions about organism morphology and rates of exchange with the environment. We examine the influence of surface area and mass on the desiccation rates in three eco‐morphologically different species of woodlice: Oniscus asellus, Porcellio scaber, and Armadillidium vulgare. Our analysis indicates that the rate of water loss of an individual depends on both the initial weight and the body surface area. Interspecific and intraspecific analyses show that the mass‐specific water loss rate of a species decreases along with the ratio of surface area to volume. In particular, we show that body shape explains the difference in mass‐specific water loss rates between A. vulgare and P. scaber. This observation also explains several known ecological patterns, for example, the distribution and survivorship of individuals. However, in addition to body size and shape, water loss in terrestrial isopods depends also on the coefficient of permeability (i.e., a measure of water loss rate per surface unit), which is high in O. asellus and lower (and at similar levels) in P. scaber and A. vulgare. We discuss morphological, physiological, and behavioral aspects of water loss avoidance in terrestrial isopods. J. Morphol. 276:1283–1289, 2015. © 2015 Wiley Periodicals, Inc.     

Ascaris sp.: water loss during desiccation of embryonating eggs

The ability of embryonating eggs of Ascaris lumbricoides to avoid desiccation by reducing the loss of water through the egg shell was investigated. When exposed to desiccation the eggs lost water at a rate dependent upon the relative humidity and ambient temperature, eventually resulting in the collapse of the eggs and the death of the enclosed embryo. The eggs are small with a large surface to volume ratio. A low permeability to gaseous exchange thus restricts water loss while still ensuring an adequate supply of oxygen for embryonic development. Relative humidity did not appear to affect the rate of development. In eggs exposed to desiccation at various constant temperatures, the rate of water loss increased as an exponential function of increasing temperature. When eggs were exposed to various temperatures before exposure to desiccation at 22 C, the rate of water loss increased as a function of increasing pretreatment temperature. After exposure to 63–65 C, the ability of the egg shell to slow down the loss of water was destroyed. These phenomena suggest that there is not a simple “critical” or “transition” temperature, but a gradual melting of the complex mixture of components forming the lipid layer.     

不同抗旱性花生品种的根系形态发育及其对干旱胁迫的响应

为明确不同抗旱性花生品种的根系形态发育特征,探讨其根系形态发育特征对不同土壤水分状况的响应机制,在防雨棚旱池内进行土柱栽培试验,研究抗旱型品种“花育22号”、“唐科8号”和干旱敏感型品种“花育23号”3个不同抗旱性花生品种根系形态发育特征及其对干旱胁迫的响应.结果表明:抗旱型品种根系较发达,具有较大的根系生物量、总根长、总根系表面积.干旱胁迫使抗旱型品种根系总表面积和体积增加,而干旱敏感型品种则相反.干旱胁迫显著增加抗旱型品种“花育22号”20 cm以下土层内根长密度分布比例及根系表面积和体积,但“唐科8号”相应根系性状仅在20-40 cm土层内增加;干旱胁迫使干旱敏感型品种“花育23号”40 cm以下土层内各根系性状升高,但未达显著水平且其深层土壤内各根系性状增加幅度小于“花育22号”.花生根系总长、总表面积及0-20 cm土层内根系性状与产量间呈显著或极显著正相关.土壤水分亏缺条件下,花生主要通过增加深层土壤内根长、根系表面积和体积等形态特性,优化空间分布构型,以调节植株对水分的利用.     

Blood volume and total water content of the woodlouse, Oniscus asellus, in conditions of hydration and desiccation

Changes in the blood volume and total water content of Oniscus asellus were measured by means of inulin dilution and weight. The relationship between body weight and blood volume, total water content, and weight loss does not change during desiccation. The process of desiccation is similar in 0% r.h. and 62% r.h., although the rates of reduction of blood volume, total water content and weight are different in these two humidities. During desiccation most of the water lost by the animal comes from the blood. There is no evidence to suggest that O. asellus takes up an excess of water in moist conditions.     

Root desiccation and drought stress responses of bareroot Quercus rubra seedlings treated with a hydrophilic polymer root dip

Root hydrogel, a hydrophilic polymer, has been used to improve transplanting success of bareroot conifer seedlings through effects on water holding capacity. We examined mechanisms by which Terra-sorb® Fine Hydrogel reduces damage that occurs when roots of 1-year old, dormant northern red oak (Quercus rubra L.) were subjected to short-term (1, 3, and 5 h) pre-transplanting desiccation and long-term (45 days) drought stress following transplanting in a controlled environment chamber or greenhouse conditions. Hydrogel-treated seedlings had 80% greater root moisture content than non-root dipped control seedlings following the pre-transplanting desiccation period. Hydrogel reduced root membrane leakiness by 31% 5 h after the desiccation exposure. Hydrogel-treated seedlings did not show greater differences in shoot length, plant dry mass, root volume, net photosynthesis, and stomatal conductance compared with control seedlings following the 45-day drought stress exposure. A reduction in mean number of days to bud break in hydrogel-treated seedlings, combined with delayed tissue moisture loss (linked to higher stem water potential), suggests that hydrogel may have provided stress protection to aid survival under short-term desiccation, which may be beneficial toward alleviating initial transplanting stress.     

Efficacy of Metarhizium anisopliae isolate MAX-2 from Shangri-la, China under desiccation stress

Background

Metarhizium anisopliae, a soil-borne entomopathogen found worldwide, is an interesting fungus for biological control. However, its efficacy in the fields is significantly affected by environmental conditions, particularly moisture. To overcome the weakness of Metarhizium and determine its isolates with antistress capacity, the efficacies of four M. anisopliae isolates, which were collected from arid regions of Yunnan Province in China during the dry season, were determined at different moisture levels, and the efficacy of the isolate MAX-2 from Shangri-la under desiccation stress was evaluated at low moisture level.

Results

M. anisopliae isolates MAX-2, MAC-6, MAL-1, and MAQ-28 showed gradient descent efficacies against sterile Tenebrio molitor larvae, and gradient descent capacities against desiccation with the decrease in moisture levels. The efficacy of MAX-2 showed no significant differences at 35% moisture level than those of the other isolates. However, significant differences were found at 8% to 30% moisture levels. The efficacies of all isolates decreased with the decrease in moisture levels. MAX-2 was relatively less affected by desiccation stress. Its efficacy was almost unaffected by the decrease at moisture levels > 25%, but slowly decreased at moisture levels < 25%. By contrast, the efficacies of other isolates rapidly decreased with the decrease in moisture levels. MAX-2 caused different infection characteristics on T. molitor larvae under desiccation stress and in wet microhabitat. Local black patches were found on the cuticles of the insects, and the cadavers dried without fungal growth under desiccation stress. However, dark black internodes and fungal growth were found after death of the insects in the wet microhabitat.

Conclusions

MAX-2 showed significantly higher efficacy and superior antistress capacity than the other isolates under desiccation stress. The infection of sterile T. molitor larvae at low moisture level constituted a valid laboratory bioassay system in evaluating M. anisopliae efficacy under desiccation stress.     

Desiccation resistance in arboreal and terrestrial ants

ABSTRACT Arboreal and terrestrial ants were exposed to 0, 25, 50, 75 and 100 (control)% r.h., at 30^oC. Desiccation resistance increased with body size (as dry weight^0.55), but not as quickly as expected from the consequences of the surface area and volume relationship (as dry weight^0.67). Arboreal ants took 8 times longer to die than terrestrial ants of comparable size. Even after size effects were removed, desiccation resistance differed between various terrestrial species and showed a correlation with foraging patterns.
Arboreal and terrestrial ants whose waterproofing epicuticular lipids were removed by chloroform: methanol extraction had equally high water loss rates at 0% r.h. Unextracted arboreal ants had water loss rates half those of unextracted terrestrial ants, suggesting that differences between them were based on differences in epicuticular lipids. The lower water loss rates of arboreal ants contributed significantly to their longer survival under desiccation. Arboreal ants also had greater total rectal pad area than terrestrial ants, suggesting that they may be able to reclaim faecal water more effectively. There were no differences in the minimum viable water content between the two groups of ants. Both had water loss tolerances comparable with those of arthropods adapted to xeric environments. Initial water loss rates could not account for all of the differences in desiccation resistance between arboreal and terrestrial ants. Other adaptations to desiccation stress by arboreal ants are likely.
Comparisons of water loss rates and desiccation resistance between arboreal and desert ants suggest that the arboreal habitat is at least as stressful as the desert habitat.     

Rapid changes in desiccation resistance in Drosophila melanogaster are facilitated by changes in cuticular permeability

Insects can improve their desiccation resistance by one or more of (1) increasing their water content; (2) decreasing water loss rate; or (3) increasing the amount of water able to be lost before death. Female Drosophila melanogaster have previously been reported to increase their resistance to desiccation after a desiccation pre-treatment and recovery, but the mechanism of this increased desiccation resistance has not been explored. We show that female, but not male adult D. melanogaster increased their resistance to desiccation after 1 h of recovery from a 3 to 4.5 h pre-treatment that depletes them of 10% of their water content. The pre-treatment did not result in an increase in water content after recovery, and there is a slight increase in water content at death in pre-treated females (but no change in males), suggesting that the amount of water loss tolerated is not improved. Metabolic rate, measured on individual flies with flow-through respirometry, did not change with pre-treatment. However, a desiccation pre-treatment did result in a reduction in water loss rate, and further investigation indicated that a change in cuticular water loss rate accounted for this decrease. Thus, the observed increase in desiccation resistance appears to be based on a change in cuticular permeability. However, physiological changes in response to the desiccation pre-treatment were similar in male and female, which therefore does not account for the difference in rapid desiccation hardening between the sexes. We speculate that sex differences in fuel use during desiccation may account for the discrepancy.     

Water Stress and Protein Synthesis: V. Protein Synthesis, Protein Stability, and Membrane Permeability in a Drought-sensitive and a Drought-tolerant Moss

The effects have been studied of water stress and desiccation on protein synthesis in the drought-tolerant moss Tortula ruralis and the drought-sensitive moss Hygrohypnum luridum. At any particular level of steady state water stress, the inhibition of protein synthesis was greater in H. luridum than in T. ruralis. Water stress-induced changes in the pattern of protein synthesis, as determined by the double label ratio technique, were minor in T. ruralis, but major in H. luridum. Proteins of both mosses were found to be stable during desiccation and subsequent rehydration. Changes in membrane permeability, as indicated by the leakage of amino acid, were observed during rehydration of desiccated moss and were dependent on the rate of desiccation. The leakage was small and reversible in T. ruralis but large and irreversible in H. luridum. Although H. luridum failed to recover from complete desiccation (80% loss in fresh weight), it was able to recover fully from steady state stress under conditions where a maximum loss of 55% in fresh weight was recorded.     

Response of the Succulent Leaves of Peperomia magnoliaefolia to Dehydration: Water Relations and Solute Movement in Chlorenchyma and Hydrenchyma

Relative water content, solute concentrations, and osmolality were determined in the water storage tissue (hydrenchyma) and the assimilatory tissue (chlorenchyma) of the succulent leaves of Peperomia magnoliaefolia (Jac) (Piperaceae) during slow desiccation. Relative water loss was significantly greater for the hydrenchyma than for the chlorenchyma. When whole leaves had lost 50% of their initial water content, the concomitant decrease of the relative water content of the hydrenchyma was 75 to 85%, but of the chlorenchyma only 15 to 25%. In spite of this differential water loss, the osmolality in both tissues increased to the same extent, indicating solute flow from the hydrenchyma to the chlorenchyma during desiccation. Solute translocation appeared to be unspecific, probably reflecting symplastic mass flow from one tissue to the other. The observed volume preservation of the chlorenchyma stabilized photosynthesis of Peperomia magnoliaefolia (Jac) leaves, which was less inhibited by a given decrease of the relative water content of the whole leaves than in nonsucculent leaves.     

Spatial variation in water loss predicts terrestrial salamander distribution and population dynamics

Many patterns observed in ecology, such as species richness, life history variation, habitat use, and distribution, have physiological underpinnings. For many ectothermic organisms, temperature relationships shape these patterns, but for terrestrial amphibians, water balance may supersede temperature as the most critical physiologically limiting factor. Many amphibian species have little resistance to water loss, which restricts them to moist microhabitats, and may significantly affect foraging, dispersal, and courtship. Using plaster models as surrogates for terrestrial plethodontid salamanders (Plethodon albagula), we measured water loss under ecologically relevant field conditions to estimate the duration of surface activity time across the landscape. Surface activity time was significantly affected by topography, solar exposure, canopy cover, maximum air temperature, and time since rain. Spatially, surface activity times were highest in ravine habitats and lowest on ridges. Surface activity time was a significant predictor of salamander abundance, as well as a predictor of successful recruitment; the probability of a juvenile salamander occupying an area with high surface activity time was two times greater than an area with limited predicted surface activity. Our results suggest that survival, recruitment, or both are demographic processes that are affected by water loss and the ability of salamanders to be surface-active. Results from our study extend our understanding of plethodontid salamander ecology, emphasize the limitations imposed by their unique physiology, and highlight the importance of water loss to spatial population dynamics. These findings are timely for understanding the effects that fluctuating temperature and moisture conditions predicted for future climates will have on plethodontid salamanders.     

Traits underpinning desiccation resistance explain distribution patterns of terrestrial isopods

Predicted changes in soil water availability regimes with climate and land-use change will impact the community of functionally important soil organisms, such as macro-detritivores. Identifying and quantifying the functional traits that underlie interspecific differences in desiccation resistance will enhance our ability to predict both macro-detritivore community responses to changing water regimes and the consequences of the associated species shifts for organic matter turnover. Using path analysis, we tested (1) how interspecific differences in desiccation resistance among 22 northwestern European terrestrial isopod species could be explained by three underlying traits measured under standard laboratory conditions, namely, body ventral surface area, water loss rate and fatal water loss; (2) whether these relationships were robust to contrasting experimental conditions and to the phylogenetic relatedness effects being excluded; (3) whether desiccation resistance and hypothesized underlying traits could explain species distribution patterns in relation to site water availability. Water loss rate and (secondarily) fatal water loss together explained 90 % of the interspecific variation in desiccation resistance. Our path model indicated that body surface area affects desiccation resistance only indirectly via changes in water loss rate. Our results also show that soil moisture determines isopod species distributions by filtering them according to traits underpinning desiccation resistance. These findings reveal that it is possible to use functional traits measured under standard conditions to predict soil biota responses to water availability in the field over broad spatial scales. Taken together, our results demonstrate an increasing need to generate mechanistic models to predict the effect of global changes on functionally important organisms.     

Thermal tolerance,evaporative water loss,air-water oxygen consumption and zonation of intertidal prosobranchs: a new synthesis

Duration of emergence increases with tidal height on rocky shores therefore, emergence adaptations in intertidal species such as littorine and other prosobranch gastropods have been considered correlated with zonation patterns; temperature tolerance, desiccation resistance and aerial respiration rate all commonly assumed to increase progressively with increasing zonation level. Such direct correlations are rarely observed in nature. Maximal aerial gas exchange occurs in mid-shore, not high shore species. Temperature tolerance and desiccation resistance do not increase directly with shore height. Thus, hypotheses regarding physiological correlates of zonation require revaluation. A new hypothesis is presented that the high tide mark presents a single major physiological barrier on rocky shores. Above it, snails experience prolonged emergence and extensive desiccation; below it, predictable submergence and rehydration with each tidal cycle. Thus, desiccation stress is minimal below the high tide mark and maximal above it. Therefore, species restricted below high tide (the eulittoral zone) should display markedly different adaptive strategies to emergence than those above it (the eulittoral fringe). A review of the literature indicated that adaptations in eulittoral species are dominated by those allowing maintenance of activity and foraging in air including: evaporative cooling; low thermal tolerance; elevated aerial O₂ uptake rates; and high capacity for radiant heat absorption. Such adaptations exacerbate evaporative water loss. In contrast, species restricted to the eulittoral fringe display adaptive strategies that minimize desiccation and prolong survival of emergence including: foot withdrawal, preventing heat conduction from the substratum; aestivation in air; elevated thermal tolerance reducing necessity for evaporative cooling; position maintenance by cementation to the substratum and increased capacity for heat dissipation. In order to test of this hypothesis the upper thermal limits, tissue and substratum temperatures on emergence in direct sunlight and evaporative water loss and tissue temperatures on emergence in 40 °C were evaluated for specimens of six species of eulittoral and eulittoral fringe gastropods from a granite shore on Princess Royal Harbour near Albany, Western Australia. The results were consistant with adaptation to the proposed desiccation barrier at high tide. The eulittoral species, Austrocochlea constricta, Austrocochlea concamerata, Nerita atramentosa and Lepsiella vinosa, displayed adaptations dominated by maintenance of activity and foraging during emergence while the eulittoral fringe littorine species, Bembicium vittatum and Nodilittorina unifasciata displayed adaptations dominated by minization of activity and evaporative water loss during emergence. The evolution of adaptations allowing tolerance of prolonged desiccation have allowed littorine species to dominate high intertidal rocky shore gastropod faunas throughout the world's oceans.