Juvenile Snails Research Articles

Soil factors influencing the distribution of Oncomelania quadrasi, the intermediate host of Schistosoma japonicum, on Bohol Island, Philippines.

Soil conditions essential to the survival of Oncomelania quadrasi on Bohol Island in the Philippines were examined to clarify the factors limiting distribution of the snail and to develop a method for breeding large numbers of the snail in the laboratory. Soil samples in and around snail habitats were analysed and used for breeding experiments in the laboratory. Experiments using paddy soil derived from different parent materials revealed that the numbers of juvenile snails hatched varied widely between several soil samples. The best soils for reproduction generally had a pH of 5.6-7.9 and > 200 mg of available CaO/100 g. These soil factors, in addition to shade and moisture, determine the optimum conditions for the breeding of O. quadrasi in the field as well as in the laboratory. The determination of the optimum conditions for laboratory breeding of O. quadrasi and other intermediate snail hosts should facilitate detailed study of the hosts and the development of better methods to control or eradicate schistosomiasis and other snail-transmitted diseases.

Host choice by larval parasites: a study of Biomphalaria glabrata snails and Schistosoma mansoni miracidia related to host size.

Within snail/trematode associations the age/size of the host at infection has consequences with regard to miracidial infection success, further intramolluscan parasite development and reproduction, and the host response, mainly in terms of growth and reproductive effort. Taking into account these differences, we were interested in determining whether miracidia could discriminate and make a choice between snails of different sizes. Using the Schistosoma mansoni/Biomphalaria glabrata system, we compared data on the snail infection rate and the mother sporocyst abundance among three size classes of snails (juvenile, subadult, and adult) exposed separately or together to the parasite larvae. When exposed individually, juvenile snails (3-5 mm) had significantly higher prevalence and abundance values than did subadult snails, followed by adult snails. In contrast, when snails of the three size classes were exposed together in heterogeneous size groups the prevalence and abundance values were always significantly higher for subadult snails of the 7- to 9-mm class than for juvenile and adult snails. A host choice experiment confirmed that significantly more miracidia were attracted by subadult snails, suggesting that the parasite has been selected for specific locating and recognition mechanisms increasing the infection rate of subadult snails when the latter have been exposed in a heterogeneous size group. Selective forces that may be responsible for such a preferential infectivity of the parasite vis-à-vis particular host age/size class are discussed in relation to host resources and host responses.

Predation on Juvenile Lagoonal Mud Snails (Hydrobia Neglecta)

Lagoonal mud snails have been shown to suffer approximately 50% mortality between hatching and adulthood. Experiments were conducted in the laboratory to assess the role of predators on the survival of juvenile Hydrobia neglecta (Mollusca: Gastropoda). Adult snails were collected from a lagoon in the Reedland Marshes system at Dunwich, Suffolk, and their eggs hatched in the laboratory, providing a population of juvenile snails which could be presented as a food source. The vessels, kept at environmental regimes close to that of the natural environment, were seeded with juvenile snails at field density. Predator preference was established by exposing the juvenile snails to combinations of potential predators found in the lagoon. One experiment also gave predators a choice of various juvenile snail sizes and other prey species. Juvenile snails remaining alive after the four week exposure to predators were removed, counted and measured. In the absence of other prey, the opisthobranch snail Retusa obtusa preferentially consumed 50% of the available juvenile H. neglecta over a four week period and the nemertean Lineus ruber up to 90%, making them the major contributors to juvenile H. neglecta mortality. The infaunal anemone Nematostella vectensis, did not have a significant impact on snail survival in the experiments but was observed (incidentally) in a separate non-experimental situation engulfing juvenile H. neglecta. The stickleback Gasterosteus aculeatus and the lagoonal prawn Palaemonetes varians, did not contribute directly to the observed mortality of juvenile mud snails, but were found to consume other predators, producing a system of prey preferences in the food chain. Understanding the role of predation contributes to the understanding of ecological processes occurring in rare lagoonal systems.

NADPH-diaphorase activity in the nervous system of the embryonic and juvenile pond snail, Lymnaea stagnalis.

Nicotinamide-adenine-dinucleotide-phosphate-diaphorase (NADPH-d) histochemistry has been applied in the present study to determine the distribution of putative nitric oxide (nitric oxide synthase)-producing cells during embryonic and early postembryonic development in the pond snail, Lymnaea stagnalis L., with special reference to the nervous system. The first NADPH-d-positive structures appear as early as 18% of development (E18, trochophore stage) and correspond to the pair of protonephridia. These structures later show disintegration, although after metamorphosis (E26=75%) staining of their individually spreading cells can be observed until hatching. Peripheral sensory neurons in the foot, mantle edge and lips, and their afferents projecting to the central nervous system reveal NADPH-d activity in the postmetamorphosis period (E25-E27=E60%-E80%) of embryogenesis. After hatching (P1-P3), a number of stained sensory cells appear in the pharynx and esophagus. Some NADPH-d positive neuronal perikarya occur in the pedal and pleural ganglia, and a few weakly stained cells in the cerebral and buccal ganglia of juvenile snails. At the same time, a continuous bundle of reactive fibers is formed in the neuropil both through and through around the circumesophageal ganglion ring. The localization of NADPH-d activity in the developing nervous system of Lymnaea suggests that nitric oxide participates mainly in sensory processes. However, its role in specific intraganglionic integrative events cannot be excluded following embryonic metamorphosis.

Demography of two landsnail populations ( Placostylus ambagiosus, Pulmonata: Bulimulidae) in relation to predator control in the far north of New Zealand

Two Placostylus ambagiosus populations were studied in New Zealand for 8 years to determine if poisoning rodent predators in the habitat of one population resulted in increased adult snail recruitment. ‘Pulse’ poisoning four times a year commenced a year after the study started in a population of Placostylus ambagiosus paraspiritus. A significant increase in the proportions of all juvenile snails with shells larger than 10 mm long was observed. The only other comparable population of this highly endangered snail species, P. a. michiei, was 33·7 km to the east. Here, predation mostly by birds had a similar effect on reducing adult snail recruitment as rodents did on P. a. paraspiritus. No predator control was done at the site occupied by P. a. michiei, and adult snail recruitment remained low throughout the study. This suggests that long-term pulse poisoning of rodents in remnant ‘islands’ of native habitat on the New Zealand mainland can be beneficial to the recovery of a landsnail population.

Schistosoma mansoni Infection Inhibits Maturation of Ovotestis Allografts in Biomphalaria glabrata (Mollusca: Pulmonata)

The posterior tip of juvenile snails, consisting of immature ovotestis, vesicular connective tissue, and digestive gland acini, all surrounded by body wall, was implanted into the hemocoel of allogeneic adult Biomphalaria glabrata. Recipient snails were either uninfected or had been infected 15 days previously with Schistosoma mansoni. Graft recipients were dissected at 30 or 60 days postimplantation (DPI), and the implants, along with preimplantation controls, were examined histologically. The gonads in preimplantation controls consisted of a partially coiled collecting canal partitioned dorsally by connective tissue septa and contained mainly spermatogonia-like cells and small oocytes. In uninfected recipients, implanted gonads underwent rapid development, i.e., had formed mature acini and were carrying out vigorous gametogenesis, both spermatogenesis and oogenesis, by 30 DPI. At 60 DPI, numbers of spermatozoa had increased substantially in most implants. In infected recipients, relatively little development or gametogenesis occurred at either 30 or 60 DPI, and oogenesis was more strongly inhibited than spermatogenesis. Except for 1 implanted gonad (out of 25) that had undergone necrosis at 60 DPI and a second otherwise healthy 60-day implant partially encapsulated at its cut surface (both in uninfected recipients), no significant hemocytic response occurred against the allografts.

Susceptibility ofBiomphalaria glabratato Infection withEchinostoma paraensei:Correlation with the Effect of Parasite Secretory–Excretory Products on Host Hemocyte Spreading

Susceptibility of the snailBiomphalaria glabratato infection with the digenetic trematodeEchinostoma paraenseiwas correlated with the ability of secretory–excretory products (SEP) derived from sporocysts of this parasite to interfere with the spreading behavior of host hemocytes in anin vitroassay. In 15 separate experiments, the prevalence of infection achieved by miracidia was positively correlated (C.C.=0.779,P<0.001) with the ability ofin vitro-transformed sporocysts derived from the same batches of miracidia to produce SEP that inhibited host hemocyte spreading. Under assay conditions reflecting differences in hemolymph volumes between juvenile snails (susceptible to infection) and adult snails (relatively refractory to infection), SEP had a significantly greater effect on hemocytes from juvenile snails. No significant differences in response to SEP were noted when equivalent numbers of adult and juvenile hemocytes were used in the assay. Snails of the M line strain ofB. glabrataare significantly more susceptible to infection withE. paraenseithan 13-16-R1 strain snails. Exposure to SEP significantly increased the number of unspread hemocytes for both strains. However, significantly more 13-16-R1 than M line hemocytes remained spread following SEP treatment.Echinostoma paraenseisporocyst SEP effects on host hemocyte spreading mirror observed patterns in both age- and strain-related susceptibility ofB. glabratato this parasite. The results suggest that the number of hemocytes available to a particular snail influences its vulnerability to infection withE. paraensei.

Intramolluscan Development of the Digenean Echinostoma paraensei: Rapid Production of a Unique Mother Redia That Adversely Affects Development of Conspecific Parasites

Investigations of the early intramolluscan development of Echinostoma paraensei in juvenile M-line snails, Biomphalaria glabrata, have revealed the presence of a previously uncharacterized stage, a morphologically and behaviorally specialized mother redia, here termed the precocious mother redia (PMR). The PMR, which is produced in a sporocyst attached to the inner surface of the host's ventricle, develops before other mother rediae and is released as early as 6 days post-exposure (DPE). Unlike all other mother rediae later produced by the sporocyst, it attaches to the ventricle wall adjacent to the sporocyst and remains there for at least 31 days. It develops a long, sinuous body and a pharynx approximately twice as large as that of any other mother or daughter redia present. Developing rapidly, the PMR releases daughter rediae at 10 DPE, about 2 days before other mother rediae. When snails containing a single intraventricular sporocyst were challenged by exposure to additional miracidia, fewer challenge sporocysts were observed subsequently in the ventricle than in control snails; this effect was greater if the PMR of the original infection had emerged and was present in the ventricle. The proportion of challenge sporocysts that reached the ventricle but then subsequently disappeared or failed to develop also increased significantly in snails in which a PMR was present. These results suggest that the PMR is specialized to protect the sporocyst and that it is capable of adversely affecting the development of challenge sporocysts. This mode of development was not observed in two other echinostome species (Echinostoma liei and E. trivolvis) examined. Additional key words: Mollusca, Platyhelminthes, Trematoda, parasite development, antagonistic interactions Echinostoma paraensei is a digenetic trematode that reproduces sexually in the intestine of mammalian hosts and asexually in freshwater pulmonate snails of the genus Biomphalaria (Lie & Basch 1967). Miracidia hatch from eggs, penetrate the head-foot region of an appropriate snail (first intermediate host), and transform into sporocysts, which usually, but not always, develop in the ventricle of the heart. Sporocysts produce mother rediae that either remain in the heart or migrate to the digestive gland-gonad complex and in turn produce numerous daughter rediae. Depending on environmental conditions, cercariae begin to emerge from the infected snails around 4 weeks post-infection (Huffman & Fried 1990). E. paraensei is used in our laboratory to study snail and parasite immunological interactions. In the course a Author for correspondence. E-mail: ksapp@unm.edu of other experiments, we observed something peculiar about the intramolluscan development of E. paraensei. A single large redia invariably developed adjacent to the sporocyst in which it was produced, and never left this position. No such pattern of development was explicitly noted in the original life-history description of E. paraensei (Lie & Basch 1967). This preliminary observation led us to re-examine the early stages of larval development and raised some specific questions regarding the development of digenetic trematodes within their molluscan host. Accordingly, we have focused our investigation on two questions. First, does the redial stage described above exhibit specialized developmental, anatomical, or behavioral features that allow it to establish an infection in the snail host? Second, is there evidence for antagonistic interactions between this peculiar redia and challenge sporocysts? This content downloaded from 157.55.39.178 on Sun, 24 Jul 2016 06:12:55 UTC All use subject to http://about.jstor.org/terms Intramolluscan development of a unique mother redia

Feeding behaviour of juvenile snails (Helix pomatia) to four plant species grown at elevated atmospheric CO2

The feeding behaviour of juveniles of the land snail Helix pomatia was examined in model plant communities consisting of Trifolium repens , Hieracium pilosella , Bromus erectus and Prunella vulgaris that are common species in extensively managed calcareous grasslands in the Swiss Jura mountains. The plant communities were grown either at ambient (350 ppm) or elevated (600 ppm) CO 2 concentrations. Leaves of T. repens and P. vulgaris grown in elevated atmospheric CO 2 had a lower specific leaf area, and leaves of T. repens had lower percentage N on a dry weight basis than leaves grown under ambient CO 2 concentration. Snails fed on all four plant species, but showed a overwhelming preference for T. repens (percentages of total biomass consumed were 91.9% at 350 ppm and 97.6% at 600 ppm). The species-specific feeding intensity of juvenile H. pomatia did not differ between the two treatments. The total dry weight of T. repens consumed by the snails was marginally greater ( P = 0.06) at elevated CO 2 , but there were no significant differences in leaf N or leaf area eaten. These findings are similar to numerous other studies showing that invertebrates increase their consumption of plant material to balance reductions in plant N concentrations at elevated CO 2 . The leaf biomass, leaf area and amount of nitrogen consumed of the other three plant species did not differ significantly between the two CO 2 treatments. Helix pomatia that fed on plants grown at elevated CO 2 atmosphere showed a larger increase in relative wet weight than those that fed on plants from ambient CO 2 conditions. However, the weight gain of H. pomatia was poorly correlated with amount of plant tissue consumed, so we suggest that the effect of CO 2 on weight gain in H. pomatia was due to a change in the quality of T. repens leaves.

Age/size- and time-specific effects of Schistosoma mansoni on energy allocation patterns of its snail host Biomphalaria glabrata.

Snail hosts of different ages constitute different resource environments for larval trematodes because each individual has a particular energy budget and energy allocation pattern at the time of infection. Effects of monomiracidial trematode infections on shell growth, body dry weight and reproductive effort of the snail host were compared between controls and infected juvenile and adult snails during the prepatent and patent periods. The results demonstrate phenotypic plasticity in the host response. There is an age/size-specific effect characterized by limitation of growth rate when snails are infected as juveniles and reduction of reproductive effort when snails are infected as adults, and a time-specific effect with early enhancement of growth rate and reproductive effort for infected juvenile and adult snails respectively during prepatency, before reduction and cessation during patency. The parasite is considered as a generalist in its energy exploitation strategy of the snail host, but taking into account the differential host responses observed relative to the host age, the possibility of a host preference for juvenile or adult snails is discussed.

Ilyanassa obsoleta (Gastropoda) as a Host for Trematodes in Delaware Estuaries

Extensive data characterizing larval trematode communities in marine gastropods do not exist for many systems. The purpose here is to report on the infections encountered in Delaware Ilyanassa obsoleta and to compare this trematode community with certain others that have been well studied. Over 15 yr, 11,774 I. obsoleta from 9 different estuarine habitats were examined. Trematode parasitism, sex, and size were determined for each snail. In the total collection, 9 trematode species were encountered; 51.04% of snails were infected with 1 or more species. The sexes were equally susceptible to being parasitized. Smaller snails were less likely to be infected than larger ones. Juvenile snails can be infected, but few have been colonized because of the short time they have been exposed to infective stages. Other studies of trematodes in this snail have revealed few multispecies infections. In contrast, they were strikingly common in this study (12.57% of all snails). Himasthla quissetensis seldom infects the same snail with Lepocreadium setiferoides or Austrobilharzia variglandis, but other species combinations can coexist. The important element in this trematode community seems to be time. The observed frequencies of different infracommunities (single and multispecies combinations) in samples of snails are considered to result from a variety of causes. These include commonness of the trematode species in the system, where and how the sample was collected, definitive host behavior, the low probability of a snail becoming infected, the long life of the snail and its infections, the vagility of the snail, and the incompatibility of certain species in terms of infecting the same snail.

Dose-dependent effects of cadmium on the growth of snails in toxicity bioassays.

The effects on survival and growth of exposure to cadmium (Cd) in the food were analyzed in juvenile snails (age one month, mean weight 1 g) of the two subspecies Helix aspersa aspersa (H.a.a.) and H. aspersa maxima (H.a.m.). The experiments lasted for four weeks and the animals were fed with special snail food containing 0-, 50-, 100-, 200-, 400-, and 800 microg Cd/g of dry food. No significant adverse effect (NOEC) was noted at 50 microg/g for three weeks. A negative effect of Cd on growth was noted from 100 to 800 microg/g and plotting the growth coefficient variation against the Cd concentration led to an estimate of the EC75 at day 14 of 370 microg/g in H.a.a. and 470 microg/g in H.a.m. and at day 28 of 290 microg/g in H.a.a. and 330 microg/g in H.a.m. Juvenile snails are thus a suitable material for use as bioindicators in the assay of contamination of food.

Simulated Field Trials to Evaluate the Effect of Sargochromis codringtoni and Tilapia rendalli on Snails in the Presence and Absence of Aquatic Plants

Two cichlid fish were studied under semi-natural conditions to evaluate their potential as agents for the biological control of schistosome intermediate host snails. Sargochromis codringtoni is known to eat snails, whereas Tilapia rendalli survives primarily on aquatic plants but may ingest snail egg masses and juvenile snails. Sargochromis codringtoni reduced snail numbers significantly, but this effect was less pronounced when plants were present. Tilapia rendalli did not significantly reduce the biomass of aquatic plant species used in the experiment, but did significantly reduce snail numbers, albeit to a much lesser extent than observed for S. codringtoni. It was concluded that the two fish species may complement each other in the control of schistosome intermediate host snails, but further investigations are needed to determine: whether they can co-exist in nature, and the extent to which T. rendalli can control other species of aquatic plants under natural conditions.

Prey selection by inexperienced predators: do early juvenile snails maximize net energy gains on their first attack?

Through encounters with prey, a predator may learn to become more selective and thus forage more efficiently. This is the case for several muricid gastropods. When searching for their first prey, however, early juveniles have no such foraging experience. Prey species and prey size preferences of inexperienced Nucella emarginata (Deshayes) hatchlings from Barkley Sound, British Columbia, Canada, were therefore examined to determine if selective feeding occurs in the absence of foraging experience and, if so, whether the preferences are consistent with the energy maximization hypothesis of optimal foraging theory. When given a choice between 5 prey species (small Mytilus spp., Balanus glandula (Darwin), Chthamalus dalli (Pillsbury), Pollicipes polymerus (Sowerby) and Lasaea spp.), 80% of the hatchlings attacked Mytilus spp. on their first attack. When offered 5 size classes (1, 2, 3, 4 and 5 mm shell length) of Mytilus spp., ≈ 73% of the hatchlings attacked 1-mm or 2-mm size classes. Hatchlings were, in fact, more selective than late juveniles and adults. Hence, strong prey species and size preferences can exist without prior foraging experience even in species which use such experience later in life to make foraging decisions. But hatchlings feeding on Mytilus spp. for 25 days did not grow faster than hatchlings feeding on Balanus glandula or Chthamalus dalli, and there was no difference in energy content between Mytilus spp. and Chthamalus dalli. Thus, the strong preference for small Mytilus spp. can not be explained by considerations of energy gain. In the field, small Mytilus spp. are mostly located in microhabitats which provide hatchlings with substantial protection from mortality factors. The preference for small Mytilus spp. should keep the young snails within protective microhabitats once these sites have been reached, and might be an adaptation to help the hatchlings locate these havens from a distance, in an otherwise highly dangerous environment.

Energy constraint exerted by the parasite Schistosoma mansoni on the locomotion of its snail host, Biomphalaria glabrata

The consequences of the constraint caused by the parasite Schistosoma mansoni on the locomotory activity of its snail host, Biomphalaria glabrata, were studied during the patent period. Rates of locomotion were determined 6 times per 24-h period for juvenile and adult snails with single-miracidium infections, then compared with those of healthy snails of the same age. The locomotory activity of infected snails was the same during the day and at night, whereas control snails moved less at night than during the day. The locomotion of snails infected when immature was similar to that of the controls during the day and superior at night. The locomotion of snails infected when mature decreased regularly during patency and clearly decreased in comparison with that of healthy snails. The results are interpreted in terms of energy constraint on the mode of resource allocation of the host due to the parasite.

Evolution of digestion of carbohydrates in the separate parts of the digestive tract of the edible snail Helix lucorum (Gastropoda: Pulmonata: Stylommatophora) during a complete 24-hour cycle and the first days of starvation

In the present study we examined carbohydrase activities during a complete 24-h cycle and during the first days of starvation in both adult and juvenile snails. The results indicated the predominant role of the digestive gland in the secretions of the enzymes responsible for degradation of most of the carbohydrates tested. Salivary glands secreted some digestive enzymes but in amounts lower than secreted by digestive gland. Enzymatic activities fluctuated during the first hours of digestion and also after the digestive tract was empty. The relatively high enzymatic activities recorded 24 h after the intake of food and during starvation could be due to the circadian rhythm of this species and/or to the participation of an existing microflora in the digestive tract of Helix lucorum. The double origin (exogenous and endogenous) of some digestive enzymes such as cellulases is discussed.

Parasite-Induced Gigantism in a Snail: A Host Adaptation?

I experimentally tested the hypothesis that parasite-induced gigantism in molluscs may be a host adaptation by increasing a host's probability of outliving an infection. I exposed juvenile pulmonate snails from two populations of the hermaphrodite Lymnaea peregra to infections with the trematode Diplostomum phoxini. Only one of the two snail populations studied is naturally in contact with this parasite (P + ) and the other is not (P - ). Diplostomum phoxini induced gigantism only in P + snails. Furthermore, P + snails showed a lower parasite-induced mortality compared to P - snails but infected P + snails did not lay eggs before they died. These results are in agreement with three out of four predictions derived from the hypothesis of gigantism as a host adaptation. The fourth prediction, that some P + snails should survive long enough to outlive the infection and reproduce, was not met. My results do not directly support the idea that parasite-induced gigantism is of adaptive value for L. peregra but suggest that gigantism is unlikely to be purely a parasite adaptation. This study cannot exclude the alternative hypothesis that gigantism is a non-adaptive side-effect of infection.

Brain grafts of cerebral ganglia have effectiveness in growth restoration of damaged Helix aspersa mesocerebrum

The microsurgical extirpation of the mesocerebrum from the brain of fast-growing juvenile snails ( Helix aspersa aspersa: H.a.a.) stops their growth. This suggests that neurosecretory cells of the mesocerebrum secrete a growth hormone. Neural grafting has been used as a tool to restore the impaired growth function after mesocerebrum removal in juvenile H.a.a. snails. The transplantation of desheathed cerebral ganglia (CG) (i.e. CG with their glioconjunctive outer covering removed), into the place where the mesocerebrum had been re-established growth which depended on the age of the donors. For the grafts of H.a.a. CG into H.a.a, it was CG from the youngest donors that restored growth best. However, the CG of adult snails still conserved a slight growth-stimulating activity. Transplantation of the CG from the large, fast-growing sub-species H. aspersa maxima (H.a.m.) into the brain of H.a.a. with mesocerebrum removed induced faster growth than the H.a.a CG probably because of a more abundant secretion of growth hormone. Our results show that intracerebral CG grafts are well tolerated in snails and that labeling of the neurones of the transplanted CG with a vital fluorescent stain (Fast blue), allowed the observation, over several months, of their integration into the lesion zone of the host brain.

Characterizing temperate rocky shores from the perspective of an early juvenile snail: the main threats to survival of newly hatched Nucella emarginata

Mortality factors most likely to constitute substantial selective pressures for early juvenile gas- tropods on temperate rocky shores were identified by examining the vulnerability of hatchlings of an inter- tidal snail, Nucella emarginata, to heat stress, desicca- tion, and predation in 1992 and 1993. The highest temperature of substrata measured at tidal heights col- onized by N. emarginata in Barkley Sound, British Columbia, Canada, was 28.5 ~ This temperature was not lethal to hatchlings in laboratory tests. In labora- tory and field desiccation experiments, all hatchlings died within 6 h of emersion. Early juveniles could not survive direct exposure to even moderate drying condi- tions for the duration of a low tide. Hence, intertidal microhabitats which dry up even for short periods during low tides would prove lethal. Of 45 intertidal animal species to which hatchlings were exposed in the laboratory, small decapod crustaceans were the only organisms to cause substantial hatchling mortality. Of these, Pagurus hirsutiusculus and Hemigrapsus nudus were by far the most abundant in the field, and are probably the only important predators of early juvenile N. emarginata at most sites. Total predator densities in the field were as high as 438 individuals m-z, sugges- ting that predation pressure may be intense. Desicca- tion and predation by decapod crustaceans appear to be the most significant threats to early juvenile N. emarginata. These factors commonly occur on most temperate rocky shores and undoubtedly constitute major selective agents influencing population param- eters and shaping life-history strategies and early juvenile traits of intertidal invertebrates.

Experimental Infection of Juvenile Biomphalaria glabrata with Cercariae of Echinostoma trivolvis

Infectivity studies were done on laboratory-reared Biomphalaria glabrata juvenile (neonatal) snails exposed to cercariae of Echinostoma trivolvis. Of the 40 snails, 0.7-1.0 mm in shell diameter, which were exposed to a total of 1,000 cercariae for 24 hr, all were infected with 1-29 viable encysted metacercariae per snail. Biomphalaria glabrata, 1-2 mm in shell diameter, were used to determine the effects of an infectious dose of 25 cercariae/juvenile on subsequent snail survival. The median survival time of 24 exposed snails occurred between day 3 and 4 postinfection, at which time more than 90% of 24 control (unexposed) snails were alive. The high mortality in the E. trivolvis-infected B. glabrata juveniles was probably related to cercarial penetration and encystment in the snail tissue, although histological examination of snails after exposure would be required to determine the actual cause of the observed mortality.