QUANTITATIVE GENETICS OF SHELL FORM OF AN INTERTIDAL SNAIL: CONSTRAINTS ON SHORT-TERM RESPONSE TO SELECTION

We investigated the genetic and environmental determinants of shell form in an intertidal snail (Prosobranchia: Littorina sp.) to identify constraints on the short-term response to selection. Our quantitative genetic parameters were estimated from a half-sib experimental design using 288 broods of snails. Each brood was divided into two treatments differing in snail population density, and therefore in grazing area per snail. Differences in population density induced marked differences in shell form. Snails in the low density treatment grew faster and had lighter shells with narrower whorls and narrower apertures than their siblings at high density. Despite this environmental plasticity in shell shape we found significant additive genetic variance for components of shell shape. We discuss two mechanisms that may maintain additive genetic variance for shell shape in intertidal snail populations: migration between environments with different selective pressures and migration between environments with different mean growth rates. We also estimated a genetic variance-covariance matrix for shell form traits and used the matrix to identify constraints on the short-term response to selection. We predict the rate of response to selection for predator-resistant morphology such as would occur upon invasion of predatory crabs. The large negative genetic correlation between relative spire height and shell weight would facilitate simultaneous selection for a lower spire and a heavier shell, both of which would increase resistance to predatory crabs.     

Constrating short-term and long-term effects of predation risk on consumer habitat use and resources

I contrasted the short-term and long-term effects of predationrisk on snail habitat use and resource dynamics. Pulmonate snails(Physella gyrina) were placed into experimental pools and exposedto four levels of predation risk while holding their densityconstant. Periphyton resources were made available in two habitats:open and covered. I hypothesized that a behavioral responseby snails to predation risk would influence periphyton standingcrop in the open and covered habitats. Snails responded to increasingpredation risk by moving into safer (covered) habitats, andthe magnitude of their response was sensitive to the actuallevel of risk: intermediate levels of risk resulted in intermediatehabitat use. However, use of the risky (open) habitat by snailswas time dependent Snails initially responded strongly to predationrisk, but they exhibited similar patterns of habitat use atall risk levels by the end of the experiment Periphyton standingcrop was positively related to predation risk. In contrast tosnail habitat use, this response was initially weak and becamestronger as the experiment progressed. Thus, the short-and long-termresponses of snail habitat use and periphyton standing cropcontrasted sharply. I suggest that the changing patterns ofsnail habitat use over time are consistent with the idea thatsnails balance predation risk against foraging gains when selectinghabitats and that the manner in which they balance foraginggains and predation risk determines the pattern of periphytonstanding crops across habitats     

Schistosoma mansoni: influence of Biomphalaria glabrata size on susceptibility to infection and resultant cercarial production

Biomphalaria glabrata snails of the same age, but different sizes, were used to determine size-related susceptibility to Schistosoma mansoni miracidial infection and the influence of snail size on total cercarial production. Snails with shell diameters from less than 5 to greater than 17 mm were individually exposed to one or several miracidia, depending on the experiment. In snails exposed to multiple numbers of miracidia, the percentage of snails which developed patent infections was lower in snails with larger shell sizes. This was also reflected by fewer primary sporocysts per infected snail found in tissues of the larger snails. Upon determining cercarial production in these groups over a 1-month period there were no statistical differences between any groups in the numbers of cercariae produced per snail. However, upon determining the number of successful primary sporocysts found in cohort snails of each size group, cercarial production increased as a function of the number of successful primary sporocysts. This was verified by examining cercarial production in various size snails with known monomiracidial infections. Our data therefore confirm and extend earlier work using snails infected with unknown numbers of miracidia and clearly show that total S. mansoni cercarial development and decreased susceptibility of snails is a direct reflection of snail size and not necessarily age of the snail.     

The effects of exposure and predation on the shell of two British winkles

Changes in shell size and shell shape of the two British winkles Littorina nigrolineata and L. rudis were studied in relation to exposure and to crab-size. In both species, shells from exposed shores are smaller and more globose than those from sheltered shores. Also, in rudis of exposed shores the mouth is relatively wider. In shores of equally sheltered conditions, shells are bigger at those localities where crabs are large than at those localities where they are small. The largest shells are found in those localities where it is extremely sheltered, and the crabs are very large.
It is argued that on exposed shores, small shells are favoured because they have more possibilities than large ones to shelter in crevices and in barnacle interspaces, from the impact of winds and waves. A globose shell could accommodate more foot muscle and thus enable a stronger adherence to the rock; and an increased mouth diameter would increase the area of foot adherence to the rock. On sheltered shores, on the other hand, large, narrow-mouthed shells are favoured because they discourage crab predation, large crabs being abundant mainly on sheltered shores.
The possible significance of shell size and shape in relation to zonation is discussed, in view of the different predatory and physical conditions which prevail in different zones of the shore, and the different shell specializations which these conditions would require.     

Effects of habitat on growth and shape of contrasting phenotypes of Bembicium vittatum Philippi in the Houtman Abrolhos Islands,Western Australia

Translocation experiments were used to test the effect of habitat on growth and shape of three contrasting phenotypes of Bembicium vittatum: dwarf, highly domed snails from an usually dry tidal pond; large, moderately domed snails from a sheltered, regularly inundated pond; and relatively flat snails from a vertical, exposed shore. Snails from both ponds grew nearly twice as fast in the wet pond as in the dry pond, indicating a high degree of plasticity of growth. Associated with these changes in growth rates was convergence of shape. Under conditions of rapid growth, the dwarf snails became relatively flatter, and hence more similar to the native snails at that site. These results indicate that the dwarf phenotype is largely a plastic stunting in response to conditions of little submersion time. The snails from the exposed shore also grew faster in the sheltered, wet pond than at their native site. However, they not only retained their flat shape, but actually became flatter (and hence divergent from the pond snails) when grown in the pond. Thus, variation in shell shape was due to interactions between source population and a common plastic association of flatter growth profile with more rapid growth. Previous experiments had demonstrated high heritability of the flat phenotype, while the present results show that the expression of the genetically different types is affected substantially by the conditions of growth, and that phenotypic differences among populations may either overestimate or underestimate the underlying genetic differences. This unpredictability of the relationship between variation in shell form and its underlying genetic basis complicates interpretations of geographical variation or palaeontological sequences based on shell form.     

螺类与着生藻类的相互作用及其对沉水植物的影响

浅水湖泊的富营养化常导致水生植被的退化与浮游藻类的爆发［１０,１８,２９］。可利用光通常是决定沉水植物分布、生物量和生产力的最重要因子,因此,伴随高营养负荷的浮游藻类繁殖,极大地削弱了沉水植物的光合能力［２０］。然而,Ｐｈｉｌｉｐｓ等人［２８］认为,...     

Zebra mussels decrease burrowing ability and growth of a native snail, <Emphasis Type="Italic">Campeloma decisum</Emphasis>

Invasive species can drive native organisms to extinction by limiting movement and accessibility to essential resources. The purpose of this study was to determine if zebra mussels (Dreissena polymorpha) affect the burrowing ability and growth rate of a native snail, Campeloma decisum. Snails with and without zebra mussels were collected from Douglas Lake, MI, and burrowing depths were studied in both the laboratory and Douglas Lake. Growth rates were calculated as the amount of shell growth from 2004 to 2005. Both the tendency of snails to burrow and the depth to which they burrowed into the substrate were significantly decreased by the presence of zebra mussels on snail shells in both laboratory and lake experiments. There was no difference in the percentage of snails that exhibited growth as a function of zebra mussel presence. However, for those snails that grew, there was a 50% higher growth rate for snails without zebra mussels compared to snails with zebra mussels. These negative effects of zebra mussels on growth and burrowing ability will likely lead to decreases in snail population densities in the future. Handling editor: S. Wellekens     

Influence of shell size of Lymnaea columella on infectivity and development of Fasciola hepatica

Experimental infections of Lymnaea columella with Fasciola hepatica were carried out to determine the influence of shell size on the infection rate and on the outcome of rediae and cercariae. Snails were divided into seven groups according to shell size: 2-4 mm, 5-6 mm, 7-8 mm, 9-10 mm, 11-12 mm, 13-14 mm and 15 mm or more. One hundred snails in each group were infected by using four miracidia for each snail. Snails with larger shell size showed a lower infection rate, the groups presenting the highest (79%) and lowest (2%) proportions of positives being those of 5-6 mm and 15 mm or more, respectively. Cercariae were present in 21% of them at 31 days post-infection, and cercarial shedding was observed 61 days post-infection. It was concluded that there is a non-linear negative association between shell size and infection rate.     

Parasite-induced changes in nitrogen isotope signatures of host tissues

To estimate isotopic changes caused by trematode parasites within a host, we investigated changes in the carbon and nitrogen isotope ratios of the freshwater snail Lymnaea stagnalis infected by trematode larvae. We measured carbon and nitrogen stable isotopes within the foot, gonad, and hepatopancreas of both infected and uninfected snails. There was no significant difference in the delta13C and delta15N values of foot and gonad between infected and uninfected snails; thus, trematode parasite infections may not cause changes in snail diets. However, in the hepatopancreas, delta15N values were significantly higher in infected than in uninfected snails. The 15N enrichment in the hepatopancreas of infected snails is caused by the higher 15N ratio in parasite tissues. Using an isotope-mixing model, we roughly estimated that the parasites in the hepatopancreas represented from 0.8 to 3.4% of the total snail biomass, including the shell.     

Crayfish induce a defensive shell shape in a freshwater snail

Abstract. This study was undertaken to determine whether intraspecific variation in shell morphology of the freshwater snail Elimia (= Goniobasis ) livescens is caused by predator-induced morphological changes. Juvenile snails from 3 populations were grown in the presence of effluent from predatory crayfish feeding on conspecific snails or in effluent from conspecific snails only. Snails from one population, Clear Creek, exhibited a predator-induced morphology; they grew a narrow body whorl when exposed to the effluent from crayfish. Experimental feeding trials with crayfish and snails from Clear Creek were conducted to determine whether a narrow body whorl reduced predation in the presence of the crayfish Orconectes propinquus . In the feeding trials, snails with a narrow body whorl were eaten less frequently than snails with wider shells. However, there was no difference in overall size (length) between snails that were eaten and those that were not. Thus, juvenile snails from Clear Creek exposed to the presence of crayfish were induced to develop relatively narrow shells, which reduce the risk of successful attack by these crayfish. Hence the induced shell morphology is probably a defense against predation by crayfish.     

The evolution of extreme shell shape variation in the land snail Ainohelix editha: a phylogeny and hybrid zone analysis

Ainohelix editha from Hokkaido, Japan, exhibit great geographical variation in their shell morphology. In particular, A. editha in two quite separate locations, Shimamaki and Samani, are striking because they are extremely flat and have a sharp keel, whereas at adjacent sites the shells are globular or depressed-globular. We used mitochondrial 16S rRNA and nuclear ITS-2 sequences to infer a phylogeny among 47 snails from 29 locations. Snails from the two keeled-flat populations clustered separately in the phylogeny, suggesting that this unusual shell form could have evolved independently. A morphological analysis of shells collected along a transect between keeled-flat and globular snail sites showed a cline for shell shape and the angle of the keel. Two different mtDNA lineages were found across the transect, with a cline for an ITS-2 single nucleotide polymorphism. Together, the results may suggest a lack of reproductive isolation between keeled-flat and globular snails, with possible introgression by hybridization.     

The importance of crevices to the intertidal snail Littoraria articulata (Philippi) in a tropical mangrove forest

The intertidal gastropod, Littoraria articulata (Philippi), is common on the trunks of mangroves at Coorooman Creek, Central Queensland. Individuals of shell length less than 3 mm were only found in empty tests of the barnacle Hexaminius popeiana (Foster). Larger snails were exposed on the trunk or occasionally in crevices such as knot holes. The relationship between shell length and shell strength differed between barnacle dwelling and larger L. articulata: in both cases it was linear, but the rate of increase of strength with increasing length was significantly greater for barnacle dwelling snails than for exposed ones. In contrast, there was an exponential relationship between shell strength and length for the weaker shelled Littoraria filosa (Sowerby), which occurs higher on the tree and does not inhabit barnacles. The survival of L. articulata was affected by the presence of crevices: significantly fewer disappeared from posts with artificial crivices than from those which lacked them, and some snails testhered to posts appeared to have been eaten by fish. The importance of crevices is discussed in relation to predation pressure, growth and habitat selection by larval L. articulata. This revised version was published online in July 2006 with corrections to the Cover Date.     

Oviposition Behavior and Offspring Emergence Patterns in Succinea thaanumi, an Endemic Hawaiian Land Snail

In this field study, newly laid egg clutches of Succinea thaanumi, an endemic Hawaiian land snail, were labeled and observed weekly over a 12‐mo period. Snails usually laid their clutches under leaf tips and, moreover, this placement affected snail emergence by mitigating the effects of dehydration through the leaf‐drip phenomenon. During droughts, the gel surrounding egg clutches dehydrated, pasting the embryos to the backs of leaves. The hygroscopic gel rehydrated once rains resumed. The majority of newly laid clutches contained shell‐less embryos which required more time for emergence than embryos laid with visible shells. More shelled embryos were laid subsequent to a 3‐wk drought, indicating that snails retained their eggs during adverse environmental conditions and laid them when conditions improved. The snails’ abilities to retain eggs in adverse conditions, but to lay more unshelled embryos over their lifetimes, contributed to their overall reproductive success.     

Shell shape and mating behaviour in pulmonate gastropods (Mollusca)

Previous work suggests that low-spired hermaphroditic snails mate face-to-face and have reciprocal sperm exchange, whereas high-spired snails mate by shell mounting and have unilateral sperm exchange. This dichotomy lead others to speculate on the evolution of enigmatic mating behaviours and whole-body enantiomorphy. In the present study, we review the current literature on mating behaviour in pulmonate snails and show that: (1) several pulmonate species show considerable intraspecific variation in mating behaviour; (2) mating position does not predict reciprocity of penis intromission and sperm exchange; (3) dart-shooting may be correlated with reciprocity of sperm exchange but other factors must explain the gain or loss of darts; (4) it is unlikely that the degree of reciprocity is the most important factor in explaining the relationship of whole-body enantiomorphy and shell shape; and (5) the reciprocal intromission of penises does not necessarily involve the reciprocal transfer of sperm. Hence, our survey shows that current ideas on the evolutionary relationship between shell shape and reciprocity with sexual selection (including dart-use) and whole-body enantiomorphy in hermaphroditic snails should be refined. The results obtained demonstrate that our current knowledge on gastropod mating behaviour is too limited to detect general evolutionary trajectories in gastropod mating behaviour and genital anatomy.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 306–321.     

Shell Temperature, Activity and Resistance to Desiccation in the Polymorphic Land Snail Cepaea Vindobonensis

Shell temperature during the morning hours, and the 24-houractivity pattern have been studied in the two distinct morphsof the polymorphic land snail Cepaea vindobonensis. Shell temperaturesbetween the two morphs differed significantly when snails wereplaced in the sun and the temperature difference was about 1°C. Activity observations in the field and in the laboratory revealedthat adult dark-banded snails were in general less active thanboth morphs of juveniles and adult faint-banded snails during sunnymornings or under severe heat load. In the field and duringcloudy mornings no statistically significant differences betweenmorphs were seen. Resistance to desiccation was studied among the two morphs ofthe same population in which activity was observed and anotherpopulation coming from a more northern and more humid habitat.A clear intraspecific difference was found in resistance todesiccation. Snails from the southern population living in amore arid habitat were more resistant than snails from the northernpopulation. Within the same population percentage water lossdid not differ significantly between the two morphs but thefaint-banded snails seemed better adapted to aridity since onlyin these snails was no difference detected in relative bodywater after desiccation. (Received 28 July 1997; accepted 20 June 1998)     

Patterns of shell penetration by Chorus giganteus juveniles (Gastropoda: Muricidae) on the mussel Semimytilus algosus

Patterns are described for shell penetration by the sublittoral muricid snail Chorus giganteus during predatory attacks on the mussel Semimytilus algosus. Location, form and size of shell penetrations were observed in relation to the size of the predator. The results suggested that positions of the perforations on the mussel shells were related to size of the attacking snail. Smaller snails perforated areas near the shell ligament and in the central zone of the shell, while larger snails more frequently attacked shell borders, principally on the ventral side. These observations may be related to: (a) changes in the process of manipulation of the prey during development of the foot and the shell tooth of the predator, (b) changes in internal structure of the snails related to the shell perforation mechanism, or (c) learned behavior acquired experientially by the snails during early growth. Although in other studies of muricid penetration patterns larger boreholes made in shells of the prey were positively correlated with increasing predator size, this relation did not appear to hold with C. giganteus, as larger specimens often made relatively small shell perforations. Areas of boreholes made in the mussel shells by this snail varied from 0.01 to 1.1 mm(2), and were unusually variable in size and shape, especially when compared with literature results on bores characteristic of other muricid species.     

不同壳色福寿螺形态性状与体质量的关系

【背景】福寿螺因其食性杂、抗逆性和繁殖力强以及自然天敌少等不断扩散,侵害农作物,被列为我国首批外来入侵物种。国内外学者一直致力于研究对其的防治与监控。自然界中福寿螺存在2种壳色——黄色和黑色,壳色受遗传因素和环境因素的双重影响。广东省福寿螺多以黑色为主,福寿螺倾向于与不同壳色的螺交配。壳色在一定程度上影响其交配的选择性,但2种壳色的福寿螺繁殖力指标差异不显著。而关于这2种壳色的螺在形态学上的差异鲜有报道。【方法】利用生物统计软件和分析方法进行相关性分析、通径分析及多元回归分析,计算相关系数、通径系数和决定系数,研究2种壳色福寿螺形态性状与体质量的关系。【结果】2种壳色福寿螺的体质量、层高的变异系数较大,且黄色比黑色变异系数大。对黄色福寿螺体质量影响较大的依次为壳高、口宽;对黑色福寿螺体质量影响较大的依次为口宽、层高。【结论与意义】2种壳色福寿螺在形态性状方面差异显著,可以将壳色作为特征标记,为福寿螺的监测与灾害评估提供参考。     

Effect of immersion at low tide on distribution and movement of the mud snail, Ilyanassa obsoleta (Say), in the upper Bay of Fundy, eastern Canada

Habitat heterogeneity often affects movement behaviours of animals, and consequently their spatial distribution. We evaluated the effect of immersion at low tide on the distribution, fine-scale movement patterns and daily movement patterns of the mud snail Ilyanassa obsoleta on a mudflat in the upper Bay of Fundy, Canada. Mud snails migrate onto intertidal mudflats in the summer, and our field survey showed that their density was higher inside tide pools relative to adjacent areas that are exposed at low tide. Using time-lapse videography, we evaluated the effect of snail size, snail density, and immersion at low tide on fine-scale movement patterns of I. obsoleta. Time until snails stopped moving and burrowed was unaffected by snail size, but snails at low and high densities burrowed somewhat faster than those at intermediate densities. Snail size and snail density had no detectable effect on displacement speed or linearity of displacement. Immersion affected snail movement: snails within tide pools delayed burrowing and traveled in more convoluted paths compared to those on exposed mud. Snails increased their turning angles within tide pools, which is probably the mechanism by which aggregations are formed. We also performed a mark-recapture experiment to compare daily movement patterns of snails released inside and outside tide pools. Snails released in tide pools moved shorter distances, but did not orient themselves differently than snails released outside tide pools. Both groups exhibited significant directionality, moving against the mean water current direction over 24 h. In sum, immersion at low tide affected the behaviour and spatial distribution of snails, resulting in snail aggregations within tide pools. These snail aggregations, in turn, may be a major factor influencing spatial dynamics on mudflats, including causing changes in distribution patterns of the burrowing amphipod Corophium volutator, a dominant inhabitant and key species in the food web of mudflats.     

Shell size and shape in Madeiran land snails: do niches remain unfilled?

The distribution of shell heights and diameters in the mainly endemic Madeiran land snail fauna shows the bimodal pattern of high- and low-spired shells found in many other faunas. Field and laboratory studies show that shell shape is associated with the angle of substrate on which the snails crawl; as elsewhere, tall spired species use vertical surfaces or burrow in soft material. Flattened species predominate on horizontal surfaces, while globular species are less specific in their preferences. Detailed comparisons with the fauna of N.W. Europe show that the proportion of high-spired species in the Madeiran fauna is low, and large high-spired species associated with vertical surfaces are very few in number despite an apparent abundance of suitable habitats. Amongst low-spired species, one family, the Helicidae, dominates the Madeiran fauna. While the overall distribution of size in these species is much as in Europe, Madeiran helicids extend into smaller size classes than do those in Europe, and they appear to fill a gap in the scatter created by the absence of other families.
Non-endemic species, other than those strictly associated with man-made environments, are generally small in size. In the upper scatter, their size distribution parallels that of endemics, but in the lower scatter they constitute the whole of the smallest size classes.
The role of interspecific competition in determining these distributions is discussed. The range of helicid sizes is compatible with a relaxation of competition or predator pressure relative to other areas, but in the upper scatter there appear to be gaps in the range of size and shape expected despite a long period in which the fauna could evolve. This could indicate the existence of adaptive troughs blocking, or delaying, radiation over the full spectrum of size and shape.     

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.