The role of growth,predation, and habitat selection in the population distribution of the crown conch,Melongena corona Gmelin

The intertidal gastropod Melongena corona Gmelin exhibits a size gradient along the shore as a function of habitat. Small, juvenile snails were found on the sand beach and larger adults were found on hard substratum habitats (shell rubble beach and oyster bar). A transplant experiment was performed to test three proximate explanations for this pattern:

• 1.(1) differential growth rate of conchs in different habitats,
• 2.(2) differences in predation intensity between habitats and
• 3.(3) active habitat selection by snails.

Shell scars on the last shell whorl were used as an index of predation pressure. Growth and predation were not significantly different for snails of similar size in different habitats, but snails were found to return to their original habitat when displaced. The ability to home did not differ between sexes or juveniles and adults. Active habitat selection appears to be a significant proximate factor maintaining the population distribution. A number of potential ultimate causes of the size class segregation are suggested.     

Dependence on aerial respiration and its influence on microdistribution in the invasive freshwater snail <Emphasis Type="Italic">Pomacea canaliculata</Emphasis> (Caenogastropoda,Ampullariidae)

The invasive Neotropical snail Pomacea canaliculata is usually regarded as amphibious, although the relative significance of aerial and aquatic respiration is unknown. To investigate the degree of dependence on aerial respiration and its influences on microdistribution, experiments were performed in the laboratory and under seminatural and natural conditions. Restriction of aerial respiration negatively affected survivorship, activity and feeding, its effects worsening with temperature and water fouling; females were more seriously affected than males although the effect depended on reproductive effort. Snails were unevenly distributed relative to the access to air, both in a stream and in an outdoor tank, being concentrated less than 2–4 m from the nearest emergent substratum. Accessibility to air would be an important trait of waterbodies prone to invasions of P. canaliculata, especially in tropical areas. The effectiveness of some control measures could be increased by focusing on areas where snails are concentrated due to their dependence on air.     

Keeping warm in the cold: On the thermal benefits of aggregation behaviour in an intertidal ectotherm

Temperature is a primary determinant for species geographic ranges. In the context of global warming, most attention focuses upon the potential effects of heat stress on the future distribution of ectothermic species. Much less attention has, however, been given to cold thermal stress although it also sets species thermal window limits, hence distribution ranges. This study was conducted in winter on a South-Australian rocky shore in order to investigate the potential thermal benefits of the aggregation behavior observed in the dominant gastropod Nerita atramentosa. Thermal imaging was used to measure the body temperatures of 3681 aggregated individuals and 226 solitary individuals, and surrounding substratum temperature. N. atramentosa aggregates and solitary individuals were significantly warmer than their surrounding substratum. The temperature deviation between aggregates and substratum was, however, ca. 2 °C warmer than the one observed between solitary individuals and substratum. This result is critical since a body temperature increase of only a few degrees might enhance individual performance, hence organismal fitness, and could potentially drive changes in interspecific relationships. Besides, the potential higher thermal inertia of aggregates might increase the snail adaptive ability to abrupt environmental changes. We further investigate the potential thermal heterogeneity within an aggregate in order to identify any thermally advantageous position. Patch centers are significantly warmer than their edges, hence snails experience greater thermal advantages in the aggregate center. Finally, we examined the potential effect of aggregate size on snail temperature and thermal spatial heterogeneity. We identified an aggregate size threshold (216 individuals) beyond which all snails had equal thermal benefits, regardless of their spatial positions within an aggregate. While the determinism of this aggregate size threshold requires further investigations, the present work uniquely identified the thermal benefits of aggregation behavior for intertidal ectotherms under cold weather conditions. The implications of the present finding are discussed in the general framework of the ability of ectothermic populations to face environmental changes.     

Behavioral response of the New England dog whelk,Nassarius trivittatus,to a temperature gradient

Thirty New England dog whelks (Mollusca: Gastropoda: Nassariidae), Nassarius trivittatus, were tested as a group for 3 days in a long linear gradient trough encompassing a thermal gradient ranging from 5°C to 45°C. The modal thermal preferendum of the snails lay between 30°C and 35°C. The lowest temperature interval voluntarily occupied by the snails was 15–20°C, the highest 35–40°C. W e believe this to be the first published study of temperature preference and avoidance behavior of gastropod molluscs.     

Foraging strategy switch in detour behavior of the land snail Anguispira alternata (Say)

Abstract. The ability of an olfactory-dominant organism, the land snail Anguispira alternata , to negotiate an odor-permeable barrier to obtain food in still air was tested. Thirty-nine snails were challenged with a "V"-shaped wire mesh fence that allowed them to get within 1 cm of their food but required them to turn away from the food to move around the fence. Fifteen snails were tested under the same conditions but without a food odor source present. Of the 39 snails tested with food, 22 moved up the odor gradient directly toward the food. Twentyone of these snails altered their foraging behavior by moving away from the odor source after they encountered the barrier. Twelve of these 21 moved along the fence to the food. The other 9 snails moved away from the fence and wandered about within the arena. Of the 39 snails tested, 17 did not move toward the food but by-passed the fence by either following the arena edge or moving toward the gap between the fence and the arena wall. Since some of these snails obtained the food without encountering the fence, a second experiment challenged 19 snails with a "J"-shaped fence in which one of the possible detour paths was blocked. There was no difference in the number of snails taking the blocked path vs. snails taking the open path. Therefore it is unlikely that these snails were able to detect the fence and plot a path around it before actually moving. In the 2 experiments, 91% of the snails that followed the odor gradient directly toward the food subsequently switched their foraging behavior. The ability of these snails to switch their foraging behavior was unexpected in an olfactory-dominant animal and suggests that their behavior is based on a significant amount of sensory processing.     

Spatial microhabitat selection by Biomphalaria pfeifferi in a small perennial river in Tanzania

A study was carried out in the Mlali river in south-central Tanzania with two aims. First, to determine microhabitat availability in two sites (A and B) with respect to water depth, water velocity and dominant substratum type. Second, to assess microhabitat use by Biomphalaria pfeifferi, the intermediate host snail of intestinal schistosomiasis and to investigate whether these snails show preferences for certain microhabitats. The two sites differed significantly with respect to width, water depth, water velocity and substratum composition. It is suggested that the absence of B. pfeifferi from site B is mainly associated with the high water velocities at that site, where 62% of the measurements exceeded30 cm s^-1. In site A, the microhabitat use by 327 B. pfeifferi snails was assessed by means of direct observation. No significant relationships were found between snail size and the habitat variables investigated, indicating that snail size appeared to be of no importance in spatial microhabitat selection. B. pfeifferi snails showed statistically significant preferences for shallow water (depth: 2–7 cm) and the preferred water velocities ranged between 12 and 21 cm s^-1 with an estimated optimum at13.3 cm s^-1. No statistically significant preferences for substratum type were found. This revised version was published online in August 2006 with corrections to the Cover Date.     

Symptoms of behavioural anapyrexia--reverse fever as a defence response of snails to fluke invasion

The subject of the research was the thermal preferences of Planorbarius corneus individuals infected by larvae of digenetic trematodes. Snails were obtained over two consecutive years, 2009 and 2010, from 10 water bodies located in central Poland. The relationship between the seasons and the occurrence of patent invasions in hosts found in the shore-zone of lakes was observed. Behavioural experiments conducted on P. corneus individuals placed in a thermal gradient demonstrated that parasite infection had an impact on the thermal preferences of the snails. Individuals that shed cercariae of Bilharziella polonica, Cotylurus sp., Notocotylus ephemera, Rubenstrema exasperatum/Neoglyphe locellus, Rubenstrema opisthovitellinum, or Tylodelphys excavata displayed symptoms of behavioural anapyrexia, similarly to experimentally injured snails. This response increased the survival of infected individuals while simultaneously prolonging the period of shedding of dispersive forms of parasites. This point of view was upheld by the observation that infected snails bred at 19°C lived longer than at 26°C and the shedding rate of cercariae at a lower temperature was lower than at a higher one.     

Effect of light and substratum structural complexity on microhabitat selection by the snail Helix aspersa muller

Terrestrial snails choose their microhabitat according to anumber of environmental factors. We evaluated the effect oflight intensity and substratum complexity on microhabitat preferenceof the terrestrial snail Helix aspersa using a multi-factorialdesign. The snails were offered two levels of light intensityand two types of structural complexity, hence 16 treatmentsin total were used: 12 in which choice was offered and 4 inwhich no choice was offered. The snails preferred ambient lightover dim light, regardless of substratum complexity, and complexover smooth substrata, regardless of light intensity. The levelof one factor did not affect the response to the other. Thus,the results revealed a preference for microhabitats with greaterlight intensity and that were structurally complex, and a rejectionof dimly lit microhabitats with smooth substrates. (Received 17 March 2006; accepted 21 September 2006)     

Importance of behavior and morphological traits for controlling body temperature in littorinid snails

For organisms living in the intertidal zone, temperature is an important selective agent that can shape species distributions and drive phenotypic variation among populations. Littorinid snails, which occupy the upper limits of rocky shores and estuaries worldwide, often experience extreme high temperatures and prolonged aerial emersion during low tides, yet their robust physiology--coupled with morphological and behavioral traits--permits these gastropods to persist and exert strong grazing control over algal communities. We use a mechanistic heat-budget model to compare the effects of behavioral and morphological traits on the body temperatures of five species of littorinid snails under natural weather conditions. Model predictions and field experiments indicate that, for all five species, the relative contribution of shell color or sculpturing to temperature regulation is small, on the order of 0.2-2 °C, while behavioral choices such as removing the foot from the substratum or reorienting the shell can lower body temperatures by 2-4 °C on average. Temperatures in central California rarely exceeded the thermal tolerance limits of the local littorinid species during the study period, but at sites where snails are regularly exposed to extreme high temperatures, the functional significance of the tested traits may be important. The mechanistic approach used here provides the ability to gauge the importance of behavioral and morphological traits for controlling body temperature as species approach their physiological thresholds.     

The influence of chemical cues and conspecific density on the temperature selection of a freshwater snail (Melanoides tuberculata)

1. The effects of predation risk (via chemical cues) and conspecific density on temperature selection of the freshwater snail Melanoides tuberculata were assessed within a circular thermal gradient.2. Chemical cues from crushed conspecifics elicited the strongest avoidance response of snails in isothermal conditions.3. Following the addition of chemical cues to the thermal gradient, snails became more active and did not exhibit significant preference for any area within the gradient. Thus, average selected temperatures were higher when exposed to chemical cues.4. Although average selected temperatures increased with density of snails within the gradient, differences among densities were non-significant.5. This species seems to modify its thermoregulatory behavior when exposed to chemical cues (i.e. predation risk) suggesting that snails favor predator avoidance over thermal selection when presented with both demands.     

Relationships between height on the shore and size distributions of Thais SPP. (Gastropoda: Muricidae)

Average body sizes in a variety of marine animals have been reported to be correlated with height in the intertidal zone. This paper reports field observations on four species of Thais showing that the relationship between size distribution and intertidal height or subtidal depth is highly variable. Intertidal size gradients in Thais have been explained by a size dependent set of responses to light and gravity, but experiments reported here do not indicate that snails of different sizes behave differently with respect to the intertidal gradient per se and it is argued that, when there is a size gradient, it is produced by the direct responses of snails to factors such as shelter, temperature, desiccation, and food. It is suggested that a response to token stimuli is unlikely to evolve when the token stimuli are poorly correlated with the relevant environmental factors, especially if the latter can be monitored directly by the animal.     

Does parasitic infection compromise host survival under extreme environmental conditions? The case for Cerithidea californica (Gastropoda: Prosobranchia)

Summary This laboratory study examined the influence of parasitic infection by larval trematodes on the survival of extreme environmental conditions by the salt marsh snail, Cerithidea californica. Experimental treatments simulated the durations, combinations, and levels of potentially lethal environmental extremes to which the snail is exposed in its natural habitat, as determined from long-term field measurements. No significant difference was found in the rates of mortality suffered by infected and uninfected snails when exposed to simulated natural extremes of water temperature, water salinity, or exposure in air. Exposure to low levels of dissolved oxygen was the only treatment that caused differential mortality: infected snails died at higher rates than uninfected. This differential mortality was accentuated by high water temperature, and varied with the species of infecting parasite. The potential impact of this interaction between parasitism and anoxia on snail survival and population dynamics is discussed.     

Does behavioural fever occur in snails parasitised with trematode larvae?

The objective of this study was to determine whether individuals of Lymnaea stagnalis naturally parasitised with trematode larvae had different thermal preferences from non-parasitised ones, and what was the influence of constant temperature on biology of the host and the parasite.

The snails under study were placed in and oblong thermal gradient and their preferences were recorded during 24 h. Non-parasitised individuals chose a rather high temperature (25 °C). In the group of parasitised snails, only individuals which released cercariae showed statistically significant differences in thermal preference than the non-larvae releasing ones. Parasitised snails kept in constant temperatures lived shorter than non-parasitised ones. At high temperatures, they released more cercariae per day than at low temperatures. Temperature had a great influence on the longevity and viability of cercariae.

The thermal behaviour of snails parasitised with trematode larvae is not a symptom of behavioural fever but probably can play a therapeutic role in the host.     

Substratum stability associated with the riverine macrophyte Justicia americana

1. Patches of stable substratum in streams may be important refugia for benthic organisms during scouring floods. Streambed stone stability, packing and embeddedness were assessed within and adjacent to beds of the macrophyte Justicia americana in five Alabama streams. 2. The force needed to dislodge stones and embeddedness was about two times lower outside Justicia beds than within them. Significant positive correlations between stone stability and (i) degree of embeddedness, and (ii) the abundance of binding rhizomes and the presence of attached roots indicate that Justicia may physically modify the local streambed, indirectly enhancing substratum stability and reducing flow, thereby increasing sand deposition. 3. Despite higher stability (i.e. physical refugia during bed‐moving spates) within Justicia beds, the abundance of epilithic plants (moss and Podostemum ceratophyllum) and pleurocerid snails (Elimia spp.) was similar both inside and outside the macrophyte beds. Several physical characteristics within macrophyte beds, such as low light, reduced current and increased sand intrusion, may create suboptimal conditions for benthic organisms in these habitats. 4. Additional work is needed to determine if Justicia biogenically enhances substratum stability or if its presence merely reflects patches of stable substratum within the streambed. Regardless of the mechanism, there is an association between Justicia beds and streambed characteristics.     

Dissecting latitudinal diversity gradients: functional groups and clades of marine bivalves

The latitudinal diversity gradient, with maximum taxonomic richness in the tropics, is widely accepted as being pervasive on land, but the existence of this pattern in the sea has been surprisingly controversial. This is partly due to Thorson's influential claim that the normal latitudinal diversity gradient occurs in marine epifauna (taxa living on the surface of the substratum) but not in infauna (burrowing or boring into the substratum), a contrast he attributed to the greater spatial and temporal environmental homogeneity of infaunal habitats. In an analysis of 930 species of north-eastern Pacific marine shelf bivalves, we found that bivalves as a whole, and both infauna and epifauna separately, show a strong latitudinal diversity gradient (measured as number of species per degree latitude) that is closely related to mean sea surface temperature (SST), even in analyses of residuals and first differences. This agrees with results for marine gastropods, but contradicts Thorson's environmental homogeneity hypothesis. The relationship between SST and diversity is consistent with a species-energy hypothesis, but the linkages from SST to diversity remain unclear. Most bivalve clades within broad functional groups conform to the general latitudinal trend, except for the deposit-feeding protobranchs. This group's non-directional pattern may be related to its mode of development, because a similar effect is seen in several other groups locked into this low-fecundity, non-feeding larval mode.     

Siliceous scales in the centrohelid heliozoan Raphidocystis contractilis facilitate settlement to the substratum

The centrohelid heliozoan Raphidocystis contractilis has hundreds of small scales on the surface of the cell body. To understand the biological functions of the scales, comparative examinations were conducted between wild-type and scale-deficient strains that has naturally lost scales after long-term cultivation. The scale-deficient strain exhibited decreased adhesion to the substratum and had a lower sedimentation rate in water than the wild-type strain, suggesting that the scale may have the ability to attach quickly and strongly to the substratum. Percoll density gradient centrifugation showed that the scale-deficient strain had a lower density than that of the wild-type strain. In the wild-type strain, more scaled cells were observed in the higher specific gravity fractions. During the long-term culture of cells, only the cells suspended in the upper area of the flask were transferred to fresh medium. By repeating this procedure, we may have selected only cells that did not possess normal scales. In the natural environment, centrohelid heliozoans are easily flushed away if they cannot adhere strongly to the bottom. These results suggest that they use scales to ensure effective adhesion to the substratum.     

Preference versus performance: body temperature of the intertidal snail Chlorostoma funebralis

Evolutionary theory predicts that, in variable environments, it is advantageous for ectothermic organisms to prefer a body temperature slightly below the physiological optimum. This theory works well for many terrestrial organisms but has not been tested for animals inhabiting the hypervariable physical environment of intertidal shores. In laboratory experiments, we allowed the intertidal snail Chlorostoma funebralis to position itself on a temperature gradient, then measured its thermal preference and determined an index of how its performance varied with temperature. Snails performed a biased random walk along the temperature gradient, which, contrary to expectations, caused them to aggregate where body temperature was 15 to 17 °C below their temperature of optimum performance and near the species' lower thermal limit. This "cold-biased" behavioral response may guide snails to refuges in shaded cracks and crevices, but potentially precludes C. funebralis from taking full advantage of its physiological capabilities.     

Thermally mediated body temperature, water content and aggregation behaviour in the intertidal gastropod Nerita atramentosa

Intertidal organisms are vulnerable to global warming as they already live at, or near to, the upper limit of their thermal tolerance window. The behaviour of ectotherms could, however, dampen their limited physiological abilities to respond to climate change (e.g. drier and warmer environmental conditions) which could substantially increase their survival rates. The behaviour of ectotherms is still mostly overlooked in climate change studies. Here, we investigate the potential of aggregation behaviour to compensate for climate change in an intertidal gastropod species (Nerita atramentosa) in South Australia. We used thermal imaging to investigate (1) the heterogeneity in individual snail water content and body temperature and surrounding substratum temperature on two topographically different habitats (i.e. rock platform and boulders) separated by 250 m at both day- and night-times, (2) the potential relationship between environment temperature (air and substratum) and snail water content and body temperature, and (3) the potential buffering effect of aggregation behaviour on snail water content and body temperature. Both substratum and snail temperature were more heterogeneous at small spatial scales (a few centimetres to a few metres) than between habitats. This reinforces the evidence that mobile intertidal ectotherms could survive locally under warmer conditions if they can locate and move behaviourally in local thermal refuges. N. atramentosa behaviour, water content and body temperature during emersion seem to be related to the thermal stability and local conditions of the habitat occupied. Aggregation behaviour reduces both desiccation and heat stresses but only on the boulder field. Further investigations are required to identify the different behavioural strategies used by ectothermic species to adapt to heat and dehydrating conditions at the habitat level. Ultimately, this information constitutes a fundamental prerequisite to implement conservation management plans for ectothermic species identified as vulnerable in the warming climate.     

Life on the edge: survival and behavioural responses of freshwater gill‐breathing snails to declining water level and substratum drying

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Spatially heterogeneous refugia and predation risk in intertidal salt marshes

The effect of habitat structure on interactions between predators and prey may vary spatially. In estuarine salt marshes, heterogeneity in refuge quality derives from spatial variation in vegetation structure and in tidal inundation. We investigated whether predation by blue crabs on periwinkle snails was influenced by distance from the seaward edge of the salt marsh and by characteristics of the primary habitat structure, smooth cordgrass ( Spartina alterniflora ). Spartina may provide refuge for snails and interfere with foraging by crabs. Furthermore, predation risk should decline with distance from the seaward edge because landward regions require more travel time for crabs during tidal inundation. We investigated these processes using a comparative survey of snails and habitat traits, an experiment that assessed the crab population and measured predation risk, and a size-structured model that estimated encounter rates. Taken together, these approaches indicated that predation risk for snails was lower where Spartina was present and was lower in a landward direction. Furthermore, Spartina architecture and distance from the seaward edge interacted. The strength of the predation gradient between seaward and landward regions of the marsh was greater where Spartina was tall or dense. These predation gradients emerge because vegetation and distance inland decrease encounter rates between crabs and snails. This study suggests that habitat modification, a process not uncommon in salt marshes, may have consequences for interactions among intertidal fauna.