Altered behavior of the snail Biomphalaria glabrata as a result of infection with Schistosoma mansoni

In this comparative behavioral study, the effect of infection with Schistosoma mansoni on its snail intermediate host Biomphalaria glabrata was investigated. Three groups of snails were compared for their activity: (1) uninfected, (2) infected with male parasites, and (3) infected with female parasites. In solitary movement trials, uninfected snails traveled greater distances at faster rates, explored more surface area, and had shorter rest periods than snails infected with either male or female schistosomes. In Y-maze experiments designed to determine attraction, the uninfected snails more often and more quickly moved toward other snails than the infected individuals. Snails from all 3 groups were more attracted to infected individuals than to uninfected ones. There was no difference in attraction toward snails infected with male or female parasites. These experiments provide evidence that behavioral alterations as a result of infection may lead to aggregation of infected snails in the field. We propose that such an effect may result in enhanced parasite transmission.     

Effects of aestivation and starvation on the neutral lipid and phospholipid content of Biomphalaria glabrata infected with Schistosoma mansoni

The effects of aestivation or starvation on the neutral lipid and phospholipid content of Biomphalaria glabrata patently infected with Schistosoma mansoni were determined by high-performance thin-layer chromatography-densitometry. Infected-aestivated snails were maintained in a moist chamber at 24 +/- 1 C and a relative humidity of 98 +/- 1%. Infected-starved snails were maintained in artificial spring water (ASW) at 23 +/- 1 C without exogenous food. Infected snails (the controls) were maintained in ASW at 23 +/- 1 C and fed lettuce ad libitum. The 3 groups were maintained in the laboratory for 7 days, and then the lipids from the digestive gland-gonad complex (DGG) were extracted and analyzed by class. Infected-aestivated snails exhibited greater mortality rate and weight loss after 7 days than did the infected-starved snails. The steryl ester concentration in the infected-starved snails was significantly increased (P = 0.010) compared with the controls but not compared with infected-aestivated snails; the concentration of phosphatidylcholine in infected-aestivated snails was significantly decreased (P = 0.007) compared with the controls but not when compared with the infected-starved snails. Aestivation or starvation had a significant effect on the concentration of certain lipid classes in the DGG of B. glabrata infected with S. mansoni.     

Schistosoma mansoni: effect of infection on reproduction and gonadal growth in Biomphalaria glabrata

Sexually mature Biomphalaria glabrata were exposed to 12 miracidia of Schistosoma mansoni, and egg production of snails was monitored over a period of 5 weeks. During the study period, exposed snails grew at approximately the same rate as unexposed controls. Castration, as measured by a reduction in the mean number of eggs laid per snail occurred between 14 and 21 days postexposure (PE). The reduction in fecundity in infected snails coincided with the migration and establishment of daughter sporocysts in the digestive gland and gonad. Enumeration of individual oocytes in longitudinal sections of the ovotestis revealed that uninfected snails contained significantly more oocytes per section than infected snails at 27, 31, and 40 days PE. In addition, the mean area of gonadal sections of control snails increased over the 40-day experimental period, whereas there was no such increase in gonadal area of infected snails. These data suggest that there is an inhibition in gonadal growth in infected snails. When oocyte data were expressed in terms of mean gonadal area, the mean number of oocytes per mm2 of gonad of uninfected and infected snails did not differ significantly over the study period, except at Day 14 PE, when infected snails contained a significantly greater number of oocytes per mm2 of gonad than did uninfected controls. It is hypothesized that daughter sporocysts of S. mansoni are primarily responsible for the inhibition of host reproductive activity, and may be mediating their effects through mechanisms involved in the regulation of gonadal growth.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of isolation on the life-history traits of Pseudosuccinea columella (Pulmonata: Lymnaeidae)

A population of Pseudosuccinea columella was raised under laboratory conditions and its life tables were determined in isolated and paired snails. Isolated snails were significantly larger in shell size than paired snails from five weeks of age onward. Also, statistically significant differences were found for the number of eggs per mass per individual from week 5 to 9, isolated snails exhibiting the highest values. The intrinsic and finite rates of increase were greater in isolated than in paired snails. Either an inhibition of the reproductive output between individuals or the advantage of selfing may be the cause of the differences in this species, acting as a possible mechanism that increase the fitness of isolated snails.     

Ecology of the intertidal snail Littorina acutispira Smith

Littorina acutispira Smith, a minute gastropod of < 3 mm shell height, lives at great densities in pools and on rock-surfaces at the highest levels of sea-shores in New South Wales. Populations from pools and dry areas were sampled on two shores for 18 months to investigate seasonal changes in density, size-structure, rates of growth and reproductive biology of the snails. Densities of snails increased between February and May, due to an influx of juveniles, and then declined until the following February, when they increased again. The decrease in density was due to the death of the largest snails at the end of summer, and the mortality of medium-sized snails between June and January. Longevity was estimated as 1–2 yr, but most individuals died by ≈ 16 months from settlement on the shore. Newly-settled snails grew to merge in size with those of the previous year's population by winter. During the summer months, the rate of growth of snails from a sheltered shore was greater than that of snails on a shore exposed to wave-action. Laboratory experiments revealed that this could be attributed to the presence of better quality food, or food in greater abundance on the sheltered shore, compared with the exposed shore.During winter months, but not during the summer, snails from dry areas grew more slowly than those from pools. An experiment demonstrated that some snails from dry areas might be able to compensate for reduced periods of feeding by being able to feed faster when submersed. This could not explain the differences in natural rates of growth.L. acutispira bred from October–November to March–April. Spawning in the laboratory was greatest during late summer (January to March). The percentage of mature oocytes in the gonads was small in winter and increased in early summer. Among the largest-sized snails, females outnumbered males. Two experiments, on unsexed and pre-sexed snails, demonstrated that the biased sex-ratio of the largest snails was due to faster growth by females.There was a greater density of snails on the exposed shore, which was correlated with the presence of barnacles. When barnacles were removed from experimental areas, the density of the snails declined within 24 h. This suggested that barnacles provided a refuge from wave-shock, rather than shelter from desiccation or high temperature. In laboratory experiments, snails were exposed to higher temperatures and less humidity than they would normally encounter on the shore. There was negligible mortality of small or large snails after 24 h of these conditions.This minute species grows quickly, recruits annually and has a short life-span. This type of life-history is discussed in comparison with similar small species from other habitats.     

Natural selection by avian predators on size and colour of a freshwater snail (Pomacea flagellatd)

We identify two avian predators of the Neotropical apple snail, Pomaceaflagellata, and estimate the strength, direction and form of multivariate natural selection by these predators on size and colour of snail shells. Limpkins are tactile predators and act as agents of disruptive selection on snail size, selecting average-sized snails disproportionately more often than small or large snails (y = 0.39, SE = 0.08). In addition, we were able to identify variation in handling behaviours and snail size selection among individual limpkins. Individual limpkins showed preferences for snails of different sizes and punctured the snail shells opposite the aperture mainly when handling large snails. Snail kites are visual predators and seem to be agents of directional selection against lighter coloured snails (β= 0.66, SE = 0.33). The ecological interaction between the apple snail and its predators provides a powerful system to further explore the role of predation in determining evolutionary changes in snail behaviour, morphology and life history.     

Supercooling ability in two populations of the land snail Helix pomatia (Gastropoda: Helicidae) and ice-nucleating activity of gut bacteria

The land snail Helix pomatia (Gastropoda: Helicidae) is widely distributed in Northern and Central Europe where it may experience subzero temperatures during winter months. Its supercooling ability was studied in two populations of H. pomatia. One population originated from Southern Sweden (Gotaland) and the other from Central France (Auvergne). In the experimental design, they were acclimated, over 2 weeks, to artificial winter conditions (hibernation, T=5 degrees C). The Swedish snails showed a rather limited supercooling ability (temperature of crystallization, T(c)=-6.4+/-0.8 degrees C), significantly greater, however, than the supercooling capacity of the population from France (T(c)=-4.6+/-1.4 degrees C). In artificial spring conditions (3 months of hibernation followed by a progressive acclimation, over 2 weeks, to activity at T=20 degrees C), both populations exhibited a similar high T(c) (-2.0+/-1.0 degrees C). The lower T(c) of hibernating Swedish snails could be due to a greater loss of body water, accompanied by a higher concentration of solutes in the hemolymph. In both populations, the variation in hemolymph osmolality measured between hibernating (250-270 mOsm kg(-1)) and active (165-215 mOsm kg(-1)) snails may be explained by the variation in body water mass and did not suggest the production of colligative cryoprotectants. Moreover, the three bacterial strains, Buttiauxella sp., Kluyvera sp., and Tatumella sp. (Enterobacteriaceae) which were isolated from fed snails, but absent in starved snails, did not show any ice-nucleating activity at temperatures higher than -9 degrees C. Only the strain Kluyvera sp. initiated nucleation at -9 degrees C. This strain, therefore, is a weak, also termed a Type III or Class C ice-nucleating active bacterium, but with no influence on the supercooling ability of individual snails. In summary, fluctuations in body water mass of hibernating snail populations, triggering changes in osmolyte concentration, rather than the presence of endogenous ice-nucleating-active bacteria, accounts for fluctuations in their T(c).     

Echinostoma paraensei: hemocytes of Biomphalaria glabrata as targets of echinostome mediated interference with host snail resistance to Schistosoma mansoni

Earlier in vivo work by Lie et al. (1977) indicated that the innate resistance of the 10R2 strain of Biomphalaria glabrata to PR1 Schistosoma mansoni could be interfered with if the snails were infected previously with another trematode, Echinostoma paraensei. We have studied this interference phenomenon using in vitro methods in an attempt to understand its mechanistic basis. Hemolymph, derived from 10R2 snails infected with E. paraensei for 14-28 days, killed 25% of S. mansoni sporocysts in vitro, significantly less (P less than 0.001) than the 90% killing rate observed with hemolymph from uninfected, control 10R2 snails. Hemolymph from the infected 10R2 snails and from schistosome susceptible M line snails did not differ significantly (P greater than 0.1) in their relative inability to kill S. mansoni sporocysts in vitro. The defect in sporocyst killing exhibited by echinostome infected 10R2 snails was traced to the cellular, rather than the humoral, component of the hemolymph. Preparations containing uninfected 10R2 snail hemolymph and echinostome daughter rediae exhibited significantly less (P less than 0.001) killing of S. mansoni sporocysts than did controls containing only 10R2 hemolymph and S. mansoni sporocysts. Our results suggest that echinostome larvae release factors that interfere with the ability of B. glabrata hemocytes to kill S. mansoni sporocysts.     

Altered feeding response as a cause for the altered heartbeat rate and locomotor activity of Schistosoma mansoni-infected biomphalaria glabrata

Biomphalaria glabrata infected with Schistosoma mansoni for 33 days fed more often than uninfected snails. Whereas uninfected snails had nocturnal increases in feeding, snails with a 33-day-old infection of S. mansoni fed as often during the day as in the night. Using direct observation and film analysis, we found that feeding increased the heartbeat rate and locomotor activity of B. glabrata. When snails were allowed to feed ad lib., infected snails had higher heartbeat rates than uninfected snails both during the day (P less than 0.01) and the night (P less than 0.001). However, when the snails were deprived of food for 24 hr, infected snails had slightly higher heartbeat rates than uninfected snails only during the day (P less than 0.05). There was no difference between the heartbeat rates of feeding, infected snails and the heartbeat rates of uninfected snails that were starved for 8 hr, and then allowed to feed. Uninfected snails had nocturnal increases in heartbeat rate regardless of feeding schedule, but infected snails had greater nighttime heartbeat rate than daytime heartbeat rate only when they were not allowed to feed. Infected snails had less nocturnal locomotor activity than uninfected snails when feeding, but there was no difference between the locomotor activity of infected and uninfected snails when the snails were deprived of food for 24 hr. Absence of food also resulted in an increased nighttime to daytime ratio of locomotor activity of infected snails. These results suggest that the increased heartbeat rate and altered rhythms of heartbeat rate and locomotor activity in B. glabrata infected with S. mansoni for 33 days were caused by the altered feeding response of these snails.     

Experimental analysis of diet specialization in the snail kite: the role of behavioral conservatism

We examined factors maintaining extreme diet specialization in the snail kite (Rostrhamus sociabilis), a medium-sized hawk which feed almost exclusively on Pomacea snails, by determining why during some months kites eat crabs (Dilocarcinus dentatus) in the Ilanos of Venezuela. We offered snails and crabs of different sizes to wild free-flying birds to develop estimates for a prey choice model. Handling times of Pomacea doliodes snails averaged 90±39 s and were positively correlated with snail size. Handling times for crabs (x=353±130 s) were significantly longer and exhibited greater variation than for snails, and were not correlated with crab size. Edible crab tissues had greater dry weights and contained more energy (25.37 kJ/g) than tissues of snails (16.91 kJ/g). Total energy of crabs was much greater than that of snails, and total energy of both foods was highly related to body length. We constructed an allometric equation for profitability of snails and crabs. Snails were more profitable than all but the largest crabs, but estimates of variance in profitability were greater for crabs. Predictions from the model were tested by offering crabs that represented equal, greater and much greater profitability than snails, to determine whether kites chose prey according to profitability. Only 15.6% of 289 food items chosen were crabs. Half of the 18 kites tested did not eat crabs and only 3 birds switched from snails to more profitable crabs. Four fledglings showed no preference for snails. The role of neophobia in food choice was investigated by offering unfamiliar snails (Pomacea urceus) to kites. Kites exhibited neophobic behaviors, and 5 of 12 birds chose not to capture P. urceus. Two-thirds of the 12 snails chosen were rejected immediately, but the others were handled efficiently (x=133±89 s). Although morphological adaptations allow kites to specialize on snails, the costs of specialization were overcome for kites when the profitability of alternative food increased sufficiently. Our results suggest a role for behavioral conservatism, in the form of risk-averse foraging and neophobia, in maintaining severe diet specialization in the snail kite.     

GROWTH, MORTALITY AND FECUNDITY IN SUCCESSIVE GENERATIONS OF HEIX ASERSA MULLER CULTURED INDOORS AND CROWDING EFFECTS ON FAST-, MEDIUM- AND SLOW-GROWING SNAILS OF THE SAME CLUTCH

This paper examines the optimum conditions for edible snailsHelix aspersa to be cultured indoors successfully in successivegenerations (originating from the crossing of snails comingfrom different clutches of a previous generation), and the effectof crowding on growth and reproduction in fast-, medium-, andslow-growing snails coming from the same clutches. The timeneeded for the snails to reach marketable size (25–32mm)varied from 2.5 to 5 months up to the 7th generation. The timeneeded for the snails to mature and reproduce from 4 to 7 monthsuntil the fifth generation. After the F5 x F5 generation, thefinal size of the snails decreased. The number of eggs did notdiffer statistically among the different generations but thereproductive success (how many snails reproduced/cage) increasedfrom Fl = Fl generation onwards to F5 x F5. In F6 x F6 onlythree (out of 26) snails reproduced and in F7 x F7 none, althoughthe snails remained under controlled conditions for 15 moremonths. Mortality in the different generations varied from 0–10%up to F5 x F5 but from F6 x F6 onwards increased and reached25%. Concerning the origin of snails, it was found that largersnails (originating from Kyparissia, Peloponnesos) lay statisticallymore eggs (138.40 ± 29.60, N =5) than smaller ones (77.38± 40.42, N=4) (originating from Hania, island of Crete).Hatching success was greater, too. (Received 10 September 1996; accepted 24 March 1997)     

Mechanisms and consequences of shell fouling in the kelp snail,Norrisia norrisi (Sowerby) (Trochidae): Indirect effects of octopus drilling

The kelp snail, Norrisia norrisi (Sowerby), dwells on the large kelps Macrocystis pyrifera (L.) and Eisenia arborea Areschoug, and is rare on benthic substrata. Approximately 4% of the snail population is dislodged from plants each day. Per capita mortality of snails on the benthos is an order of magnitude greater than individuals on kelp. Even though snails displaced to the bottom actively seek a plant to climb, we found that between 20–40% of marked snails released on the bottom were killed over a 24-day period. While most N. norrisi are free of epibionts on their shells, ≈30% of the population are encrusted by the barnacle Megabalanus californicus (Pilsbry). The majority of barnacle fouling was the result of failure by Octopusbimaculatus (Verrill) to kill Norrisia norrisi; barnacle cyprids settled in octopus drill holes in the shells of live kelp snails. N. norrisi encrusted by barnacles were less effective in exploiting kelp compared with unfouled snails. Moderately to heavily fouled snails were dislodged from kelp twice as frequently and remained on the benthos much longer than snails without epibionts. Laboratory tests revealed that escape velocities of N. norrisi from asteroid predators were much reduced by shell fouling, and that snails carrying barnacles were most likely to be captured and consumed. Hence octopus increase mortality of kelp snails in two ways. First, octopus can kill N. norrisi encountered on the bottom. Second, kelp snails drilled by octopus, but not killed outright, will become fouled by barnacles. This results in a subsequent risk of mortality from all benthic predators that is at least 8-fold greater than for unfouled kelp snails.     

Trichobilharzia ocellata: physiological characterization of giant growth, glycogen depletion, and absence of reproductive activity in the intermediate snail host, Lymnaea stagnalis

Giant growth, depletion of energy stores, and inhibition of reproductive activity are striking effects of many trematode parasites on their intermediate snail hosts. Two hypotheses have been put forward to explain these phenomena: (1) host and parasite compete for energy rich and other essential nutrients, with the parasite as the winner, and (2) the parasite intervenes in the endocrine control of reproduction of the snail. These hypotheses were tested in the present study with the Trichobilharzia ocellata/Lymnaea stagnalis association. The snails were infected at a juvenile stage, and release of cercariae started on Day 55 after exposure. It was shown that enhanced growth of infected snails is not paralleled by a greater increase in dry weight, but hemolymph volume does increase, being 35% greater than in the noninfected controls. Control snails, on the other hand, showed an increase in the percentage body dry weight during sexual maturation. The conclusion is that infected snails retain an essentially juvenile body structure. In control snails, glycogen was depleted from the mantle store at the start of egg laying but the onset of cercariae production marked a severe glycogen depletion from the headfoot and the mantle in infected snails, being nearly complete on Day 68 after exposure. The hemolymph glucose concentration was only slightly lower in infected than in control snails and it did not change (in both groups) during glycogen mobilization. This suggests that glycogen mobilization does not result from the snail and the parasite competing directly for metabolites within the hemolymph. Infection inhibited the maturation of the accessory sex organs: there was no increase in the relative wet weights nor in the amounts of DNA and secretion products in the albumin and prostate glands. Infected snails did not lay eggs. It is presumed that the parasite produces one or more agents which intervene in the action of the gonadotrophic hormones. The release of these agents commences at an early stage of infection.     

Migration and trail affinity of snails, Littoraria scabra, on mangrove trees of Nananu-i-ra, Fiji Islands

The distribution of mobile species such as grazing snails may be influenced by migration patterns, which often are excluded from ecological studies. To highlight this point, the migration patterns of Littoraria scabra on mangrove trees were investigated during incoming and outgoing tides at Nanaru-i-ra, Fiji Islands. Marked snails were used to track the position of snails, relative to the ground, during advancing and receding tides. Snails were found to move quickly upward during incoming tides, potentially to avoid immersion. During the outgoing tide, snails migrated downward, but at a slower pace than upward movement, presumably as a consequence of active feeding on the newly replenished micro-organisms on the mangrove structures (i.e., stems and roots). Aggregation behavior during migration was also recorded by measuring the distance of marked snails to two nearest neighbors. Affinity to neighboring snails was found to be maintained throughout tidal cycles, although greater affinity was observed during incoming tides compared to outgoing tides. These migratory and aggregation behaviors may be a result of increased feeding and reproductive efficiency.     

Plastic and heritable components of phenotypic variation in Nucella lapillus: an assessment using reciprocal transplant and common garden experiments

Assessment of plastic and heritable components of phenotypic variation is crucial for understanding the evolution of adaptive character traits in heterogeneous environments. We assessed the above in relation to adaptive shell morphology of the rocky intertidal snail Nucella lapillus by reciprocal transplantation of snails between two shores differing in wave action and rearing snails of the same provenance in a common garden. Results were compared with those reported for similar experiments conducted elsewhere. Microsatellite variation indicated limited gene flow between the populations. Intrinsic growth rate was greater in exposed-site than sheltered-site snails, but the reverse was true of absolute growth rate, suggesting heritable compensation for reduced foraging opportunity at the exposed site. Shell morphology of reciprocal transplants partially converged through plasticity toward that of native snails. Shell morphology of F(2)s in the common garden partially retained characteristics of the P-generation, suggesting genetic control. A maternal effect was revealed by greater resemblance of F(1)s than F(2)s to the P-generation. The observed synergistic effects of plastic, maternal and genetic control of shell-shape may be expected to maximise fitness when environmental characteristics become unpredictable through dispersal.     

Schistosoma mansoni: alterations in total protein and hemoglobin in the hemolymph of infected Biomphalaria glabrata

The total protein concentrations in the hemolymph of noninfected Biomphalaria glabrata and those parasitized by Schistosoma mansoni have been compared. That of the former, averaging 1.42 g/100 ml, is greater than that of the latter which averages 0.96 g/100 ml. The hemoglobin concentrations in the two categories of snails have also been compared and that in noninfected specimens, averaging 0.956 g/100 ml, is higher than that in infected snails which average 0.650 g/100 ml. It has also been ascertained that both the total protein and hemoglobin concentrations in snails of different sizes vary; specimens measuring between 10 and 16 mm in diameter tend to include more hemoglobin than larger ones. On the other hand, larger snails tend to include greater total protein concentrations.     

利用植物他感作用灭螺效果的研究

通过调查有螺滩地林下植被的演变和乌柏（Ｓａｐｉｕｍｓｅｂｉｐｅｒｕｍ）、枫杨（Ｐｔｅｒｏｃａｒｙａｓｔｅｎｏｐｔｅｒａ）等滩地造林树种的灭螺效果发现,鸡矢藤（Ｐａｅｄｅｒｉａｓｃａｎｄｅｎｓ）、水芹（Ｏｅｎａｎｔｈｅｊａｖａｎｉｃａ）的个体数量与钉螺的数量呈正相关关系,Ｘ２分别为１２．８和１１．２（ａ＜０．０２）,而益母草（Ｌｅｏｎｕｒｕｓａｒｔｅｍｉｓｉａ）、问荆（Ｅｑｕｉｓｅｔｕｍａｒｖｅｎｓｅ）、酸模叶蓼（Ｒｕｍｅｘａｃｅｔｏｓａ）、打碗花（Ｃａｌｙｓｔｅｇｉａｈｅｄｅｒａｃｅａ）、紫云英（Ａｓｔｒａｇａｌｕｓｓｉｎｉｃｕｓ）等与钉螺的数量呈负相关关系（ａ＜０．０２）经过一个汛期,枫杨、乌柏叶片使钉螺死亡率从草滩对照的２％升至４３．３％和４４．１％     

Feeding clusters and olfaction in the mangrove snail Terebralia palustris (Linnaeus) (Potamididae: Gastropoda)

Large numbers of the snail Terebralia palustris (Linnaeus) (Potamididae; Gastropoda) are frequently observed feeding in a cluster on a single fallen mangrove leaf, yet none are present on leaves nearby. Consequently, we investigated the food-finding ability of T. palustris in a Kenyan mangrove forest using field experiments. We estimated the attractive effect of different cues and analysed the paths (video-recorded) of snails when approaching a food-related odour. This intertidal snail can potentially use both air-borne and water-borne odours to locate food. T. palustris is attracted to conspecifics feeding on leaves, while intact leaves as well as non-feeding snails are not attractive. Moreover, the guiding stimulus appears to be compounds released when the leaves are damaged.T. palustris also seems able to discriminate between different food items; it is more strongly attracted to green mangrove leaves than senescent or fallen ones or mangrove propagules, probably because green leaves release a greater amount of attractive cues.Feeding snails thus recruit more snails to feed on the same leaf. The ecological implications of this behaviour are discussed: a large number of snails on the same leaf counteracts the ability of crabs to remove the leaf being eaten by the snails.     

Resolving within- and between-population variation in feeding ecology with a biomechanical model

Studies of phenotypic plasticity have emphasized the effect of the environment on the phenotype, but plasticity can also be used as a tool to study the functional significance of key traits. By inducing variation in phenotypes and testing quantitative models that predict performance based on biological mechanisms, we can develop functionally general models of performance. Pumpkinseed sunfish from lakes with high snail availability have large levator posterior muscles (which are used to crush snail shells), whereas fish from lakes with few snails have relatively small muscles. Here we: (1) quantify differences in the feeding ability of an ontogenetic series of pumpkinseed from two populations; and (2) evaluate whether a biomechanical model can resolve the observed ontogenetic and between-population variation in feeding ecology. Mass, but not length, of the levator posterior muscle in fish from Three Lakes (a lake rich in snails) was greater than for comparably sized fish from Wintergreen Lake (a lake with few snails). Handling times were shorter, crushing strengths were 71% greater, and foraging rate (snail tissue mass consumed per time) and the fraction of thick-shelled snails in the diet were approximately 100% greater for fish from Three Lakes compared to comparably sized fish from Wintergreen. These between-lake differences were not significant after adjusting for variation in pharyngeal morphology, suggesting that the biomechanical model of snail crushing resolved observed ontogenetic and population-level variation in the feeding ecology of pumpkinseed.     

The influence of habitat on the distribution and abundance of metacercariae of Macravestibulum obtusicaudum (Pronocephalidae) in a small Indiana stream

Snails (Elimia livescens) from a central Indiana stream were examined for encysted metacercariae of Macravestibulum obtusicaudum (Pronocephalide) to determine the distribution and abundance of this parasite on its second intermediate host. Five samples of snails were collected, with 2 samples being restricted to high-flow (riffle) or low-flow (pool) areas of the stream. Snails (n = 386) were measured for shell length; the shell and the inner and outer surfaces of the operculum (in most samples) were examined for metacercariae. Seventy-five percent of snails (overall) had encysted metacercariae (range, 52-97%), primarily on the opercula. A significantly lower proportion of snails from riffles were infected, and these snails had significantly fewer cysts as well. Snails collected from pools showed up to 10 times as many metacercariae than those from riffles, although there was no significant difference in the proportion of snails with intramolluscan infections of M. obtusicaudum. The inner opercular surface appeared to be the preferred site of encystment in both flow regimes. Differences in microhabitats, in terms of both snails in the stream and metacercariae on the snails, clearly must be considered when evaluating the infection patterns of this trematode.