Changes in the selection differential exerted on a marine snail during the ontogeny of a predatory shore crab

Empirical estimates of selection gradients caused by predators are common, yet no one has quantified how these estimates vary with predator ontogeny. We used logistic regression to investigate how selection on gastropod shell thickness changed with predator size. Only small and medium purple shore crabs (Hemigrapsus nudus) exerted a linear selection gradient for increased shell‐thickness within a single population of the intertidal snail (Littorina subrotundata). The shape of the fitness function for shell thickness was confirmed to be linear for small and medium crabs but was humped for large male crabs, suggesting no directional selection. A second experiment using two prey species to amplify shell thickness differences established that the selection differential on adult snails decreased linearly as crab size increased. We observed differences in size distribution and sex ratios among three natural shore crab populations that may cause spatial and temporal variation in predator‐mediated selection on local snail populations.     

The biology of Placostylus ambagiosus (Pulmonata: Bulimulidae) in New Zealand: Part 1. Behaviour,habitat use,abundance, site fidelity,homing and the dimensions of eggs and snails

The large land snail Placostylus ambagiosus (Pulmonata: Bulimulidae) was investigated during a long-term study in northernmost New Zealand. The snails fed at night on fallen leaves from a variety of broadleaf trees and bushes. During the day, most snails in shrubland rested under plants close to their food sources at densities of up to 53 snails per m² and mean live biomasses of 47–72 g/m². Individual snails stayed close to their food plants for up to 12 years, seldom moving away. Resting behaviour and site fidelity in forest was not investigated in detail, but some adult snails in forest returned to their original sites after being moved up to 60 m into a fenced area built to protect them from feral domestic animals. The ovoid eggs (mean 7.0 mm long×5.6 mm wide; ranges 5.5–7.7 mm long; 5.1–6.7 mm wide) were laid in clutches averaging 43 eggs (range 1–113 eggs) in shallow cavities covered with soil. More than one snail may contribute to a clutch. Adult shell height ranged from 43 mm to 97 mm and varied with habitat quality. Where snail abundance was high adults were small. Implications of these results for conservation management are discussed.     

Predation as the primary selective force in recurrent evolution of gigantism in Poecilozonites land snails in Quaternary Bermuda

During the last half million years, pulses of gigantism in the anagenetic lineage of land snails of the subgenus Poecilozonites on Bermuda were correlated with glacial periods when lower sea level resulted in an island nearly an order of magnitude larger than at present. During those periods, the island was colonized by large vertebrate predators that created selection pressure for large size and rapid growth in the snails. Extreme reduction in land area from rising seas, along with changes in ecological conditions at the onset of interglacial episodes, marked extinction events for large predators, after which snails reverted to much smaller size. The giant snails were identical in morphology during the last two glacials when the predators included a large flightless rail Rallus recessus (marine isotope stages (MIS) 4-2) and a crane Grus latipes and a duck Anas pachysceles (MIS 6). In a preceding glacial period (MIS 10), when the fauna also included the tortoise Hesperotestudo bermudae, the snails were not only large, but the shells were much thicker, presumably to prevent crushing by tortoises. Evolution of Poecilozonites provides an outstanding example of dramatic morphological change in response to environmental pressures in the absence of cladogenesis.     

Relevance of body size and shell colouration for thermal absorption and heat loss in white garden snails,Theba pisana (Helicidae), from Northern France

The internal temperature of land snails depends on environmental factors, such as exposure to electromagnetic radiation and airflow as well as biotic factors including shell size, shell colouration and thickness or the resting position of the snail. In controlled field experiments, we quantified heating by thermal absorption of light and airflow-induced heat loss in the white garden snail, Theba pisana, from Normandy, France. Heating experiments revealed a significant positive relation of the internal body temperature with illumination period, shell temperature and air temperature at different times of day. The size of the snails was negatively related with both of the given illumination times: smaller animals heated up stronger than larger ones. The temperature at the surface of the shell significantly depended on the illumination period and the time of day. An AIC-based quality assessment of multiple linear modelling showed that, for explaining both shell surface and internal temperature of the soft body, several factors, i.e., exposure time, daytime, shell size and colouration contributed to the best models, respectively. Similarly, heat loss of the soft body after and during exposure of the snails to sunlight by a constant airflow depended on the initial body temperature, shell size, colouration and ambient air temperature. Our study revealed also the importance of both shell size and colouration for the loss of body temperature under natural conditions: small and banded animals that had heated up to temperatures above 30 °C cooled down faster than large and un-banded ones.     

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.     

Contrsating anti-predator defenses of sympatric marine gastropods (family Trochidae)

Three of four sympatric gastropods found in southern California waters, Tegula aureotincta Forbes, Norrisia norrisi (Sowerby), and Astraea undosa (Wood), display behavioral flight responses to predatory asteroids and a gastropod. The fourth species, Tegula eiseni Jordan, lacks this behavior but does have morphological features that deter predators. The shell of T. eiseni is thicker and the tissue more difficult to acid hydrolyze than that of its sympatric congener, T. aureotincta. Juvenile seastars fed T. aureotincta grew three times faster than those reared on T. eiseni. The dissimilarity in growth rates is not attributed to differences in nutritive value of the two Tegula species, but is due to differences in the rate at which the seastars can consume the snails. Laboratory choice tests demonstrate that the anti-predator defense of T. eiseni reduces its acceptability as food. In contrast, the behavioral flight defense of the three remaining gastropods decreases the probability of their capture by certain predators.     

Stereotype,ontogeny, and heritability of drill site selection in thaidid gastropods

Laboratory observations on the feeding behavior of four species of thaidid gastropods (Muricacea), when fed on the intertidal barnacle Balanus glandula (Darwin), revealed two interesting patterns. First, large inividuals of Thais (or Nucella) emarginata (Deshayes) (> 15 mm shell length) exhibited remarkably little variation in the locations at which they drilled barnacles, either among individual snails, among populations along a wave exposure gradient (≈ 5 km), or among populations along the Pacific coast of North America (≈ 3000 km). The results of laboratory crosses suggested that the small differences which did exist between populations from southeast Alaska (U.S.A.) and Vancouver Island, British Columbia (Canada) were genetically determined.A second pattern of interest was an ontogenetic shift in the preferred location of drilling in all four species of Thais examined (T. canaliculata (Duclos), T. emarginata T. lamellosa (Gmelin), and T. lima (Gmelin)): larger snails drilled much more frequently in the opercular region and, concomitantly, more frequently at the occludent margins of the scutal plates. The ontogenetic shift in these snails appeared to be primarily genetically predetermined rather than learned, since individuals of T. emarginata grown from one size class to the next on mussels (Mytilus edulis L.) did not differ greatly in their selection of drilling locations on Balanus glandula from those grown similarly on barnacles.     

Testosterone,gonadotropins and androgen receptor during spermatogenesis of Biomphalaria alexandrina snails (Pulmonata: Basommatophora)

Endocrine regulation of reproductive processes of the snail Biomphalaria alexandrina is poorly recognized. Thus, the aims of the study were: (1) to acquire histological images of the ovotestis; (2) to determine the hemolymph concentrations of testosterone (T) and gonadotropic hormones (luteinizing hormone: LH and follicle stimulating hormone: FSH), (3) to demonstrate androgen receptor (AR) immunolocalization in the ovotestis, and (4) to show LH and FSH protein expression in cerebral ganglia of small (diameter shell: 4–6 mm), medium (7–11 mm) and large (12–16 mm) B. alexandrina snails. These three groups represented different reproductive stages of the snail. The AR immunoexpression was found in the periphery and inside the acini of small (immature) snails as well as in spermatocytes, spermatids, Sertoli cells, the interstitial cells and the acinus lining epithelium of medium (mature) snails. Low AR immunoexpression was demonstrated in the interstitial cells of large (aged) snails. The neurons at the periphery of the cerebral ganglia and connective sheath of the ganglia showed a positive FSH and LH immunostaining. T concentration in the hemolymph was higher in medium snails than in small and large snails. In contrast, LH concentration was higher in medium snails than in small and large snails. These data suggests that gonadotropins and T play a role in the gonadal development in B. alexandrina.     

Feeding behavior of black carp Mylopharyngodon piceus (Pisces: Cyprinidae) on fry of other fish species and trematode transmitting snail species

Fish raised in aquaculture ponds may get infected with fishborne zoonotic trematodes (FZT) during the nursing stage. Freshwater snails serve as intermediate hosts for FZT and we wanted to explore the possibility of controlling snails by stocking nursery ponds with a few juvenile specimens of the mollusc-eating fish, black carp (Mylopharyngodon piceus). Obviously, the risk that black carp might also prey on the juvenile fishes in nursery ponds should first be assessed. Laboratory trials showed that all size classes of juvenile black carp consumed fry of common carp (Cyprinus carpio) even when offered snails as food; the odds of survival of fry from tanks with medium sized and large black carp was 5.6% and 39.9%, respectively of that of fry in tanks with small sized black carp. Since the large black carp also consumed fewer snails than medium sized fish, we believe that large specimens were stressed in the experimental aquaria. Under semi-field conditions, presence of the black carp had no effect on survival of fry of Oreochromis niloticus and C. carpio both in the absence and presence of snails as alternative food. The black carp consumed most snails offered with the exception of some of the large snails. We conclude that under field conditions, predation by black carp on fish fry is minimal and field trials in nursery ponds are warranted. Due to the risks that black carp pose to native imperiled snails and other molluscs, trials should be restricted to ponds within the fish’s native or existing range.     

Food preference,food quality and diets of three herbivorous gastropods (Trochidae: Tegula) in a temperate kelp forest habitat

Summary The relationship between food preference and food quality (i.e. a food's contribution to growth and reproductive development) was examined in the laboratory for 3 species of herbivorous kelp forest gastropods (Tegula). The preference hierarchies of the 3 Tegula for 6 common algal species are the same: giant kelp (Macrocystis) is the most preferred food and brown algae are consumed at higher rates than red algae. Despite their strong preference for Macrocystis, the 3 species had significantly greater growth and/or reproductive development on a mixed-algae diet than on either brown or red algae alone. Laboratory preferences of the snails did not correspond closely with caloric content, estimated availability or quality of the algal species used in this study. However, the 3 Tegula are subject to strong benthic predation and Macrocystis provides an important spatial refuge in nature. The potential role of non-nutritional factors such as predator avoidance on the formation of food preferences is discussed.     

Effects of trematode parasitism on the shell morphology of snails from flow and nonflow environments

The primary function of the gastropod shell is protection. However, shells that function well in one environment may be maladaptive in another. Upon infection, the snail shell protects internal parasites and it is to the parasite's advantage to optimize, or not interfere with, shell functionality. However, parasites, particularly trematodes, are often pathogenic and it is not clear if parasitism will induce environment‐dependent or ‐independent changes to gastropod shells. We conducted a field study and a complementary laboratory experiment to examine the effects of trematode parasitism on shell characteristics (shape, size, and crush resistance) of Physa acuta snails in flow and nonflow environments using geometric morphometrics and crush assays. Field results indicate wetland (nonflow) snails had large, crush resistant shells with narrow apertures and tall spires. In contrast, stream (flow) snails had small, weak shells with wide apertures and short spires. Parasitism had no apparent effect on the crush resistance of wetland snails but significantly reduced the crush resistance of stream snails. Parasitism had no significant effect on overall shell shape in stream or wetland snails. Similar to the results of our field study, nonflow tank snails had significantly more crush resistant shells than flow tank snails. Additionally, the shapes of flow and nonflow tank snails significantly differed where nonflow tank snails resembled wetland snails and flow tank snails resembled stream snails. For laboratory snails, parasitism reduced crush resistance regardless of flow/nonflow treatment. Our results demonstrate that habitat and/or flow treatment was the primary factor affecting P. acuta shell morphology and that trematode parasitism played a secondary role. J. Morphol. 277:316–325, 2016. © 2015 Wiley Periodicals, Inc.     

EFFECTS OF HERBIVORE SIZE AND HUNGER LEVEL ON PERIPHYTON COMMUNITIES1

The effects of herbivore size and hunger level were tested on lotic periphyton community structure and ash-free dry mass (AFDM). My hypotheses were 1) that small herbivores would remove more periphyton per unit biomass than large herbivores of the same species because of energetic demands and 2) that within the same size class, starved herbivores would remove more periphyton than nonstarved herbivores. The herbivore used was the prosobranch snail Elimia clavaeformis Lea. Seven treatments were employed: 1) no snails (control); 2) small “starved” snails; 3) large “starved” snails; 4) small and large “starved” snails; 5) small fed snails; 6) large fed snails; and 7) small and large fed snails. Although snails removed significant amounts of periphyton AFDM relative to controls, neither snail size nor degree of starvation had a significant effect on loss of total AFDM. Small snails removed significantly more erect forms of Stigeoclonium tenue (C. A. Ag.) Kütz. than large snails, but snail size had no other significant effect. Starved snails removed significantly more Cocconeis placentula Ehr. than fed snails, suggesting that after the more susceptible growth forms (e.g. erect Stigeoclonium) were removed, Cocconeis cells became more vulnerable to grazing by hungry snails. When small and large snails were combined in chambers, large snails gained weight over time, whereas small snails lost weight. These results suggest that intraspecific competition may occur within populations of Elimia.     

Elevated CO2 affects shell dissolution rate but not calcification rate in a marine snail

As CO₂ levels increase in the atmosphere, so too do they in the sea. Although direct effects of moderately elevated CO₂ in sea water may be of little consequence, indirect effects may be profound. For example, lowered pH and calcium carbonate saturation states may influence both deposition and dissolution rates of mineralized skeletons in many marine organisms. The relative impact of elevated CO₂ on deposition and dissolution rates are not known for many large-bodied organisms. We therefore tested the effects of increased CO₂ levels—those forecast to occur in roughly 100 and 200 years—on both shell deposition rate and shell dissolution rate in a rocky intertidal snail, Nucella lamellosa. Shell weight gain per day in live snails decreased linearly with increasing CO₂ levels. However, this trend was paralleled by shell weight loss per day in empty shells, suggesting that these declines in shell weight gain observed in live snails were due to increased dissolution of existing shell material, rather than reduced production of new shell material. Ocean acidification may therefore have a greater effect on shell dissolution than on shell deposition, at least in temperate marine molluscs.     

Studies on the prolonged storage of Metarhizium anisopliae conidia: effect of temperature and relative humidity on conidial viability and virulence against mosquitoes

The shell lengths, dried shell weights, soft tissue wet weights, and soft tissue dry weights were ascertained for noninfected Ilyanassa obsoleta and for the same snail species naturally infected with the following digeneans: Himasthla quissetensis, Zoogonus lasius, Cercaria dipterocerca, Lepocreadium setiferoides, Microphalloides nassicola, Stephanostomum tenue, and Microbilharzia variglandis. Analyses of the data obtained indicate that infection with all of the trematodes listed above causes neither enhanced growth of soft tissues nor accelerated lengthening of the shell. However, snails infected with sporocysts of Z. lasius have significantly heavier shells than do noninfected snails. These data suggest that infection with Z. lasius may have caused enhanced growth in the form of increased calcium deposition in the shell of I. obsoleta, possibly as a result of parasitic castration. In addition, the mean dry weight of the soft tissues of snails infected with M. variglandis, when normalized for shell length, is significantly lower than that of noninfected snails, possibly as a result of parasite-induced pathology. Finally, comparisons among infected snails indicate a trend toward increased soft tissue dry weight in snails infected with L. setiferoides, although the mean dry weight of these snails does not differ significantly from the mean dry weight of noninfected snails.     

Life in the Supralittoral Fringe: Microhabitat Choice, Mobility and Growth in the Tropical Perwinkle Cenchritis (=Tectarius) muricatus (Linneaus, 1758)

The littorinid snail, Cenchritis muricatus, inhabits supralittoral vertical rocky walls along Caribbean shores, at times surpassing 14 meters above mean sea level. As the sole macrofaunal representative of this habitat, this marine gastropod presumably experiences extraordinary conditions of thermal load and desiccation. In order to understand the effect of behavioral choices on periwinkle survivorship and growth, snail distribution, microhabitat utilization, and crawling speed were documented in St John (US Virgin Islands). In general, snails rarely inhabited open surfaces; instead, periwinkles were commonly observed in microhabitats that may reduce water and heat stresses (e.g., > 75% in crevices and depressions). Snails found on occasional buttonwood trees (Conocarpus erectus) were larger than elsewhere. Although typically found in repose, C. muricatus crawling speed on moist, shaded, and smooth substrata averaged more than 3 cm^.min^- 1, but did not vary with slope. Repeated mark-recapture of tagged periwinkles exhibited high recovery rates (ca. 35% after 4 yr), absence of mortality, and a projected cessation of growth at 16.5 mm (shell height). Nearly 10% of marked individuals were recaptured every year. Dead, tagged snails were never noted; indeed, seven individuals were only recovered once, a full 4 yrs after release. Site-specific growth rates were absent. Projections using von Bertalanffy growth functions (VBGF) suggest that periwinkles will require 15+ years to achieve the maximum shell height. These VBGF models cannot address extraordinary individuals reaching 22 mm. C. muricatus's remarkable supralittoral distribution may be explained by physiological tolerance, selection of microhabitats, lack of predators and long lifespan.     

Shell shape and land-snail habitat in a Mediterranean and desert fauna

The bimodal distribution of shell shape (height: diameter), that is found in various geographically widely separate and taxonomically distinct land snail faunas of many different regions of the world, occurs also in a Mediterranean fauna and in a desert fauna that is derived from it. The desert fauna is, however, closer to the bisector than the Mediterranean one. High-spired snails are mainly rock-dwellers, and equidimensional to low-spired snails are bush-dwellers or soil-diggers, with a few rock-dwellers; litter-dwellers are small-sized species that may have either high- or low-spired shells. These results are discussed in adaptive terms. Litter is probably the more primitive of these micro-environments. Many of the small, litter-dwelling snails are ovo-viviparous rather than oviparous, perhaps so as to avoid attacks on the eggs by saprophytic fungi. The shift away from the litter environment is accompanied by a trend to abandon the ovo-viviparous strategy, in favour of oviparity, the snail using its foot to dig into the soil and lay eggs. The conchometric differences between bush-, ground- and rock-dwelling snails may perhaps reflect selective pressure to increase the size of the foot; and constraints of a habitat that consists of narrow interspaces between rocky boulders. Snails that habitually dig into the ground during periods of inactivity, and roam over the ground when active, requires a very large foot and, consequently, a very large-mouthed shell to accommodate it; the result is an equidimensional shell, globose or turbiniform in shape. Snails that climb up vertical vegetation would also require a large foot, and consequently a large-mouthed shell to contain it. A fully globose shell would however be disadvantageous, since it might cause undesired torque. Hence, bush-dwellers tend to be flatter than soil-diggers. Snails that habitually live in rock crevices, and on hard substrata, would not require a very large foot; they would need a narrow shell, both to enable easy manoeuvring through crevices and to reduce torque, the result being a small-mouthed, usually high-spired shell. The classification of land snails into bush-, soil- or rock-dwellers closely follows the taxonomic classification. In those species that depart from the habitat that is typical of their taxonomic group towards another habitat, the shell alters its shape accordingly.     

Prey-induced changes to a predator's behaviour and morphology: Implications for shell-claw covariance in the northwest Atlantic

Whereas many plasticity studies demonstrate the importance of inducible defences among prey, far fewer investigate the potential role of inducible offences among predators. Here we ask if natural differences in a snail's shell hardness can induce developmental changes to a predatory crab's claw size. To do this, we fed Littorina obtusata snails from either thick- or thin-shelled populations to captive European green crabs Carcinus maenas. The crabs' shell-breaking behaviour dominated among those fed thin-shelled snails, whereas crabs fed thick-shelled snails mostly winkled flesh through the shell opening without damaging the shell itself (a.k.a. aperture-probing behaviour). Significantly, the size of crab crusher claws grew in proportion to the frequency of shell-crushing behaviour and, for a same shell-crushing frequency, crabs fed thick-shelled snails grew larger crusher claws than those fed thin-shelled snails after two experimental moults. Diet and behaviour had no effect on the growth of the smaller cutter claws of same individuals, providing good evidence that allometric changes to crusher claws were indeed a result of differential use while feeding. Findings indicate that both predation habits and claw sizes are affected by green crabs' diet, supporting the hypothesis that prey-induced phenotypic plasticity contributes to earlier accounts of shell-claw covariance between this predator and its Littorina prey in the wild.     

Low‐pH seawater alters indirect interactions in rocky‐shore tidepools

Ocean acidification is expected to degrade marine ecosystems, yet most studies focus on organismal‐level impacts rather than ecological perturbations. Field studies are especially sparse, particularly ones examining shifts in direct and indirect consumer interactions. Here we address such connections within tidepool communities of rocky shores, focusing on a three‐level food web involving the keystone sea star predator, Pisaster ochraceus, a common herbivorous snail, Tegula funebralis, and a macroalgal basal resource, Macrocystis pyrifera. We demonstrate that during nighttime low tides, experimentally manipulated declines in seawater pH suppress the anti‐predator behavior of snails, bolstering their grazing, and diminishing the top‐down influence of predators on basal resources. This attenuation of top‐down control is absent in pools maintained experimentally at higher pH. These findings suggest that as ocean acidification proceeds, shifts of behaviorally mediated links in food webs could change how cascading effects of predators manifest within marine communities.     

Vertical migration and spawning inNodilittorina exigua (Gastropoda: Littorinidae)

The relation between the vertical migration in summer and spawning of a littoral-fringe periwinkle,Nodilittorina exigua, was studied by field observations and laboratory experiments. The snails moved down soon after the first appearance of their planktonic eggs in the sea and stayed at low shore-levels until early August. It was shown that the migration increased the chance of immersion of the snails, and therefore of releasing eggs by the females. Females immersed every day in the laboratory released more eggs than those which experienced some days of exposure to air, during the same period. The results obtained suggest the reproductive advantage of immersion at low levels. Nevertheless, the snails moved upward before the end of the spawning season. Typhoon swells increasing in frequency through the summer seemed to be responsible for the upward migration.