Infectivity of Echinostoma friedi miracidia to different snail species under experimental conditions

The infectivity of Echinostoma friedi (Trematoda: Echinostomatidae) miracidia was studied experimentally in a range of laboratory-reared snails that coexist in the same natural locality, namely Radix peregra, Lymnaea fuscus, L. truncatula (Lymnaeidae), Gyraulus chinensis, Helisoma duryi (Planorbidae) and Physella acuta (Physidae), and snails from different geographical origins acting naturally or experimentally as intermediate hosts of Schistosoma spp., namely Planorbarius metidjensis (from Málaga, Spain), Biomphalaria glabrata (Guadeloupe), B. alexandrina (Egypt) (Planorbidae), Bulinus cernicus (Mauritius), B. globosus (Zambia), B. natalensis (South Africa) and B. truncatus (Niger) (Bulinidae). Six species of snails were found to be susceptible, with the rate of infection ranging from 0 to 36.7%. The highest infection was detected in R. peregra. The low host specificity of E. friedi might have an epidemiological significance as a requisite for a recent establishment in a new geographical area.     

The chemical ecology of some British freshwater gastropod molluses: behavioural responses to short chain carboxylic acids and maltose

SUMMARY. 1. The behavioural responses of six species of British freshwater gastropods to chemical gradients of short chain carboxylic acids (C-2 to C-5, C-8) and maltose were investigated by means of diffusion olfactometers.
2. The species were ranked as follows on the basis of the number of significant behavioural responses to the test chemicals: Lymnaea peregra > Planorbis contortus > Physa fontinalis > Planorbis planorbis > Bithynia tentaculata > Planorbis vortex. These differences were not a result of differences in activity but reflect real differences in ehemoreception.
3. Propanoate (C-3) was a significant attractant or arrestant to five snail species, C-4, C-8 and maltose to four species and C-5 to three species. Acetate (C-2) was a significant attractant to L. peregra and P. fontinalis but it was the only acid to act as a repellent, to P. planorbis and B. tentaculata.
4. Lymnaea peregra continued to respond to repeated butanoate application. Response levels could be enhanced by increasing concentration (minimum thresholds 10^-4M to 10⁻⁵ M), pH (above 6) and food deprivation.
5. The ecological relevance of the results are discussed with particular reference to the distributional patterns of the chemicals and snails and the feeding niches of the latter.     

Effects of irradiation on the reproductive ability of Zonitoides arboreus, a snail pest of orchid roots

The effect of irradiation on the reproductive ability of the orchid snail, Zonitoides arboreus, a serious pest of potted orchids in Hawaii, was investigated. Weekly egg production averaged between 0.8 and 1.9 over a 9–wk period for snails not exposed to irradiation, and egg hatch averaged 61%. In comparison to untreated controls, irradiation of snails at the lowest dose tested (34–37 Gy) reduced egg production and egg hatch by 63% and 94%, respectively over a 9–wk period. None of the snails treated with levels of irradiation ≥ 69 Gy produced viable eggs. This is the first study measuring the effect of ionising irradiation on a terrestrial snail or slug species using sterilising doses. Overall, the results show that the reproductive ability of this snail species is affected by irradiation in a similar manner as for Biomphalaria glabrata, an aquatic snail for which the effects of irradiation have been studied in detail.     

Grazing interactions between pulmonate snails and epiphytic algae and bacteria

SUMMARY. 1. The effects of grazing by Lymnaca peregra and Planorbis planorbis on Ceratophyllum demersum epiphyton were investigated under simulated natural conditions.
2. Grazing resulted in significant reductions in algal cell densities, number of algal taxa and in diversity. Grazing did not influence equi-tabilily values.
3. The use of Ivlev's Electivity index demonstrated positive and negative selection by feeding snails for large, pedunculate, and small, adnate algae, respectively.
4. Inoculation of artificial surfaces revealed that 61.1% and 62.5% of the algal taxa presenl in the diet of L. peregra and P . planorbis , respectively (mainly the smaller species), were able to survive passage through the gut, resettle and grow.
5. The results are discussed with particular reference to the survival strategies available to the epiphytes, and the possible mutual benefits which algae and snails may derive from coexistence.     

Egg size and form as taxonomic criteria in Diphyllobothrium (Cestoda, Pseudophyllidea)

The size and form (length, width, and length: width ratio) of eggs of Diphyllobothrium dendriticum, D. ditremum and D. latum vary considerably among individual worms within each species. The size of eggs varies with host species and a decrease in egg size with increasing intensity of infestation is indicated. The egg size of D. latum increases during the first 10-12 days of egg production. For single worm infections in golden hamsters the mean egg length and width of D. ditremum are significantly smaller than the corresponding means of D. dendriticum and D. latum, while D. latum has significantly wider eggs than D. dendriticum. As taxonomic characteristics, egg size and form may contribute to species delimitation at the population level. For identification at the individual level the best possible accuracy is about 80%. This accuracy is considerably reduced when variation in host species and intensities of infestations are introduced. Scanning electron microscope studies did not reveal any differences among eggs of the three species.     

Cadmium toxicity and snail-digenean interactions in a population of Lymnaea spp

The toxicity of cadmium to a population of Lymnaea peregra and L. stagnalis naturally infected with a range of digeneans and collected from a number of sites in the lower Thames Valley, UK was investigated. Lymnaeid snails were exposed to 100 microg l-1 cadmium and the effects on host survival and emergence of cercariae recorded. Overwintered L. peregra, but not L. stagnalis, showed significantly reduced survival compared to seasonally infected snails, i.e. snails which have acquired an infection during the spring or summer. A significant increase in survival with increasing snail size was demonstrated for L. stagnalis and for seasonally infected L. peregra only. Only L. stagnalis infected with Diplostomum spathaceum and L. peregra infected individually with D. spathaceum, Sanguinicola inermis, Echinoparyphium recurvatum and Notocotylus attenuatus demonstrated a significantly reduced survival compared to laboratory-bred controls. The exposure of L. stagnalis to cadmium resulted in a significant reduction in the emergence of D. spathaceum over a 5-day period but cadmium-exposed L. peregra showed no difference in the emergence of E. recurvatum cercariae over a 3-day exposure period. The mechanisms and importance of metal toxicity to snail-digenean interactions are discussed.     

Variations in the retinal designs of pulmonate snails (Mollusca, Gastropoda): squaring phylogenetic background and ecophysiological needs (I)

Abstract. The eyes of aquatic pulmonates differ from those of terrestrial pulmonates; the latter, in species such as Cepaea nemoralis and Trichia hispida , possess conventional, cup-shaped retinas, but the aquatic species Lymnaea stagnalis, Radix peregra, Physa fontinalis , and Planorbarius corneus have retinas that are partitioned into dorsal and ventral depressions ("pits"). The pits are separated by an internal ridge, called the "crest", and on account of their pigmentation can be seen in vivo . The dominant cellular components of the retinae of terrestrial as well as aquatic snails are pigmented cells and microvillar photoreceptors, the latter occurring in two morphologically distinct types (I and II). Aquatic snails with preferences for shallow water possess eyes with both type I and type II photoreceptive cells, but Pl. corneus , an inhabitant of deeper water, only has type-I receptors, supporting an earlier finding that type I cells represent dim- and type II cells bright-light receptors. On the basis of histological and optical comparisons, we conclude that the eyes of L. stagnalis and R. peregra , species that are known to escape and seek temporary refuge above the water surface, are well adapted to function in water as well as air, but that the eyes of P. fontinalis and Pl. corneus are less modified from those of their terrestrial ancestors.     

Re-establishing a life cycle of Schistosoma mansoni from cryopreserved larvae

To study the feasibility of re-establishing a life cycle of Schistosoma mansoni (NMRI strain) from cryopreserved larvae, schistosomules were suspended in the cryoprotectant 1,2-ethanediol and cryopreserved in liquid nitrogen. Mice were injected intramuscularly with samples thawed after 3 days, 3 wk, or 6 mo in liquid nitrogen storage. Two to 5% of the cryopreserved larvae and approximately 18% of corresponding unfrozen control larvae developed into adult worms. Infectivity did not decrease as a function of storage time. The adult worms showed no structural damage or changes in overall size and morphology when examined by light and transmission electron microscopy. Female worms derived from cryopreserved larvae had the same or slightly elevated egg production as controls, but tissue egg distributions were comparable. Subsequent passages through Biomphalaria glabrata snails and mice revealed no difference in snail prepatent death rate, percentage of snails infected, cercarial production per snail, or cercarial infectivity.     

The distribution and variation of Daphnia lumholtzi (Crustacea: Cladocera) in relation to fish predation in Lake Albert, East Africa

In Lake Albert Daphnia lumholtzi is found in two forms. One has a pointed anterior prolongation, or helmet, on the head. The other has a shorter rounded head, and was originally described as a separate species, D. monacha . The latter form dominates the zooplankton in the middle of the lake where planktivorous fish are rare or absent. The helmeted form becomes commoner near the margins of the lake and reaches its greatest abundance in Ndaiga Lagoon, where planktivorous fish are common. The possession of a helmet is associated with a reduction in the size of the carapace compared to the round headed form. The carapace with its contained eggs is the most conspicuous part of a cladoceran, so that the helmeted forms are at an advantage in the presence of planktivorous fish which locate their prey by sight. The mid-lake monacha forms are larger than specimens of the same form in Ndaiga Lagoon, where it is shown that Alestes baremose feeds selectively on the larger specimens of the monacha form.
The helmeted form produces more, but smaller eggs than the monacha form. The total brood volume (= mean egg volume x mean number eggs per female) is greatest in the midlake monacha forms. The selective advantages of variations in egg size and the possession of a helmet are discussed. It is concluded that the data from Lake Albert support the hypothesis of Brooks (1965) concerning the adaptive significance of helmet development in Daphnia .     

The distribution and variation of Daphnia lumholtzi (Crustacea: Cladocera) in relation to fish predation in Lake Albert, East Africa

In Lake Albert Daphnia lumholtzi is found in two forms. One has a pointed anterior prolongation, or helmet, on the head. The other has a shorter rounded head, and was originally described as a separate species, D. monacha . The latter form dominates the zooplankton in the middle of the lake where planktivorous fish are rare or absent. The helmeted form becomes commoner near the margins of the lake and reaches its greatest abundance in Ndaiga Lagoon, where planktivorousfish are common. The possession of a helmet is associated with a reduction in the size of the carapace compared to the round headed form. The carapace with its contained eggs is the most conspicuous part of a cladoceran, so that the helmeted forms are at an advantage in the presence of planktivorous fish which locate their prey by sight. The mid-lake monacha forms are larger than specimens of the same form in Ndaiga Lagoon, where it is shown that Alestes baremose feeds selectively on the larger specimens of the monacha form.
The helmeted form produces more, but smaller eggs than the monacha form. The total brood volume (= mean egg volume x mean number eggs per female) is greatest in the midlake monacha forms. The selective advantages of variations in egg size and the possession of a helmet are discussed. It is concluded that the data from Lake Albert support the hypothesis of Brooks (1965) concerning the adaptive significance of helmet development in Daphnia .     

Single-copy DNA distance between two congeneric sea urchin species exhibiting radically different modes of development

We have investigated the differences between nuclear genomes of two purportedly congeneric species of sea urchin that differ radically in early development. Heliocidaris tuberculata develops by means of a typical pluteus larva, whereas H. erythrogramma develops directly from an egg that is 100-fold the volume of the H. tuberculata egg. Reassociation kinetic analysis shows that the kinetic components of the genomic DNA from the two species are essentially the same. No single repeat component explains the 30% difference between the H. erythrogramma and H. tuberculata genomes. Reciprocal hybridization of tracer-labeled single-copy DNA fractions between these species indicates that approximately 50% of the single-copy DNA is sufficiently similar to form hybrids at standard hybridization criterion. Thermal denaturation profiles of the hybridized single-copy DNA sequence yields median (T50H) values of 13.8 degrees-16.5 degrees C. This result suggests a divergence time of 10-13 Mya, which is comparable to divergence times between congeneric sea urchin species in other genera that do not differ significantly in development. Radical differences in early developmental processes can evolve rapidly between closely related forms.     

内蒙科尔沁草原淡水螺吸虫幼虫期的调查研究

本文报道在内蒙科尔沁草原检查5种淡水螺:耳萝卜螺、卵萝卜螺,豆螺、凸旋螺及半球多脉扁螺共5,616粒。从中查出至少12种吸虫幼虫期,它们是:土耳其斯坦东毕吸虫、有棘血居吸虫、马蹄吸虫、斜睾吸虫、二种背孔吸虫、棘口类吸虫、重盘吸虫、球孔吸虫、异形科吸虫、杯叶科吸虫及鹗形吸虫。其中东毕吸虫、血居吸虫及棘口吸虫的尾蚴分别进行实验动物的感染试验,获得成虫。各吸虫侵袭期幼虫季节动态亦经观察。     

Resource polymorphism and diversity of Arctic charr Salvelinus alpinus in a series of isolated lakes

Morphological, dietary and life‐history variation in Arctic charr Salvelinus alpinus were characterized from three geographically proximate, but isolated lakes and one large lake into which they drain in south‐western Alaska. Polymorphism was predicted to occur in the first three lakes because S. alpinus tend to become polymorphic in deep, isolated lakes with few co‐occurring species. Only one morph was evident in the large lake and two of the three isolated lakes. In the third isolated lake, Lower Tazimina Lake, small and large morphs were found, the latter including two forms differing in growth rate. The small morph additionally differed from the two large forms by having more gill rakers and a deeper body than same‐sized individuals of the large morph, consuming more limnetic and fewer benthic resources, having a greater gonado‐somatic index and maturing at a smaller size. The two large forms consumed only slightly different foods (more terrestrial insects were consumed by the medium‐growth form; more snails by the high‐growth form). Trends in consumption of resources with body shape also differed between lakes. Variability in life history of S. alpinus in these Alaskan lakes was as broad as that found elsewhere. This variability is important for understanding lake ecosystems of remote regions where this species is commonly dominant.     

Comparative life histories of two diplostomid trematodes, Ornithodiplostomum ptychocheilus and Posthodiplostomum minimum

A lack of comparative studies limits our understanding of interspecific variation in parasite life histories, especially for species that incorporate asexual and resting stages into their life cycles. Ornithodiplostomum ptychocheilus and Posthodiplostomum minimum are sympatric diplostomid trematodes that share the same first (pond snail, Physa gyrina) and second (fathead minnow, Pimephales promelas) intermediate hosts within lakes in Alberta, Canada. Interspecific differences in the body sizes of adults, cercariae, and metacercariae were evaluated from parasites recovered from laboratory-raised chickens, snails, and minnows, respectively. Differences in adult worm fecundity and cercariae production were estimated from chickens and snails exposed to known numbers of parasite larvae. Posthodiplostomum minimum adults, metacercariae, and eggs were 97, 86, and 5% larger than those of 0. ptychocheilus, respectively. The average numbers of eggs produced per day by P. minimum was 3 times higher than for O. ptychocheilus, and P. minimum produced approximately twice as many cercariae per day. Although the larger of the 2 species had higher egg and cercariae production, conclusions regarding covariation between adult worm size and the reproductive rates of other life cycle stages require further studies on related species.     

Lake resident and migratory smelt, Retropinna retropinna (Richardson), of the lower Waikato River system, New Zealand

Retropinna retropinna occurred in two forms in Lake Waahi, North Island, New Zealand. One form was characterized by high mean gill raker counts ( c . 28) and low mean vertebral counts ( c . 52)—similar to those of non-migratory, lake resident, native smelt in some other lakes of the lower Waikato River system. The other form had low mean gill raker counts ( c . 21) and high vertebral counts ( c . 60)—similar to migratory smelt at five sites along the lower Waikato River down to its estuary. The lake resident form was shorter but deeper bodied than the migratory form, as indicated by significant differences in mean length and in weight-length regression analyses.
The two forms overlapped in time of sexual maturity. As in other native lacustrine smelt populations of the lower Waikato system, the resident form had more but smaller eggs than the migratory form. The egg number to body length regression and the mean egg diameter of migratory Lake Waahi smelt were similar to those of lower Waikato River smelt. Evidence for reproductive separation of the two forms is presented and mechanisms controlling their meristic and other differences are considered.     

蚌科钩介幼虫比较形态学研究──Ⅰ.四个种幼虫的形态

本文报道圆顶珠蚌、鱼尾楔蚌、中国尖嵴蚌、卵形尖嵴蚌育儿囊的特点和钩介幼虫的形态。应用光镜和扫描电镜对四种蚌的钩介幼虫形态进行了观察和比较。结果表明,四种蚌的育儿囊均为外鳃类的同生型,钩介幼虫为有钩型,幼虫的大小、形状、壳表面、壳钩、棘刺、幼虫丝、感觉毛等在不同种之间存在着差异。文中对这些特征在分类上的意义进行了讨论。     

Evaluation of phylogenetic relationships between five polyploid Aegilops L. species of the U-genome cluster by means of chromosomal analysis

Four tetraploid (Aegilops ovata, Ae. biuncialis, Ae. columnaris, and Ae. triaristata) and one hexaploid (Ae. recta) species of the U-genome cluster were studied using C-banding technique. All species displayed broad C-banding polymorphism and high frequency of chromosomal rearrangements. Chromosomal rearrangements were represented by paracentric inversions and intragenomic and intergenomic translocations. We found that the processes of intraspecific divergence of Ae. ovata, Ae. biuncialis, and Ae. columnaris were probably associated with introgression of genetic material from other species. The results obtained confirmed that tetraploid species Ae. ovata and Ae. biuncialis occurred as a result of hybridization of a diploid Ae. umbellulata with Ae. comosa and Ae. heldreichii, respectively. The dissimilarity of the C-banding patterns of several chromosomes of these tetraploid species and their ancestral diploid forms indicated that chromosomal aberrations might have taken place during their speciation. Significant differences of karyotype structure, total amount and distribution of C-heterochromatin found between Ae. columnaris and Ae. triaristata, on the one hand, and Ae. ovata and Ae. biuncialis, on the other, evidenced in favor of different origin of these groups of species. In turn, similarity of the C-banding patterns of Ae. columnaris and Ae. triaristata chromosomes suggested that they were derived from a common ancestor. A diploid species Ae. umbellulata was the U-genome donor of Ae. columnaris and Ae. triaristata; however, the donor of the second genome of these species was not determined. We assumed that these tetraploid species occurred as a result of introgressive hybridization. Similarity of the C-banding patterns of chromosomes of Ae. recta and its parental species Ae. triaristata and Ae. uniaristata indicated that the formation of the hexaploid form was not associated with large modifications of the parental genomes.     

Polymorphic segregation in Arctic charr Salvelinus alpinus (L.) from Vatnshlidarvatn, a shallow Icelandic lake

We studied the salmonid fish Arctic charr, Salvelinus alpinus , in a small and shallow landlocked lake in NW Iceland. The lake is productive but die only fish present is Arctic charr. Despite the apparent absence of discrete benthic and limnetic habitats for fish, two forms of Arctic charr are found in the lake. They show subde differences in morphology related to swimming performance and manoeuvrability, but differences in life history such as growth, and age and size at sexual maturation are more pronounced. Both forms have benthic feeding habits with one form consuming greater number of species than the other. We suggest that the segregation of these forms is based on the evolution of a specialist from a local generalist and that this has been made possible by the absence of a common fish competitor in similar lakes, the threespined stickleback Gasterosteous aculeatus.     

Unusual snail species involved in the transmission of Fasciola hepatica in watercress beds in central France

Four freshwater pulmonate species (Lymnaea ovata, L. stagnalis, Physa acuta, Planorbis leucostoma) were living in several watercress beds known for their relationships with human cases of fasciolosis, whereas L. truncatula was never found. The aims of these studies were to determine the prevalence of natural infections with Fasciola hepatica in snails and to verify if these species might ensure the full larval development of this trematode (with cercarial shedding) when they were experimentally subjected to F. hepatica only, or to co-infections with an other trematode species. Investigations were so carried out in six snail populations living in watercress beds (including three for P. acuta) and in four others originating from three brooks or a pond (as controls). Snails naturally infected with F. hepatica were found in two watercress beds inhabited by L. ovata (prevalence of infection: 1.4%) and P. leucostoma (0.1%), respectively. The L. ovata from the watercress bed could be infected at a higher size than those from the control population and the prevalence of this infection was greater in the bed population. Similar findings were noted for L. stagnalis. Despite single or dual infections, the results obtained with the four populations of P. acuta were unsuccessful. In contrast, the co-infections of young P. leucostoma with Paramphistomum daubneyi and F. hepatica resulted in the shedding of some F. hepatica cercariae. According to the authors, the occurrence of fasciolosis in these watercress beds would be the consequence of frequent natural encounters between parasite and snails (L. ovata, L. stagnalis), or of co-infections with P. daubneyi and F. hepatica (P. leucostoma). In watercress beds only colonized by P. acuta, a lymnaeid species would have ensured the larval development of F. hepatica but it would have been eliminated by P. acuta, as this last species was known to be invasive and could colonize open drainage ditches on siliceous soil.     

Interactions between freshwater snails and tadpoles: competition and facilitation

Summary Freshwater snails and anuran tadpoles have been suggested to have their highest population densities in ponds of intermediate size where abiotic disturbance (e.g. desiccation) is low and large predators absent. Both snails and tadpoles feed on periphytic algae and, thus, there should be a large potential for competitive interactions to occur between these two distantly related taxa. In a field experiment we examined the relative strength of competition between two closely related snail species, Lymnaea stagnalis and L. peregra, and between L. stagnalis and tadpoles of the common frog, Rana temporaria. Snail growth and egg production and tadpole size at and time to metamorphosis were determined. Effects on the common food source, periphyton, were monitored with the aid of artificial substrates. Periphyton dry weight was dramatically reduced in the presence of snails and/or tadpoles. There were no competitive effects on growth or egg production of the two snail species when they were coexisting. Mortality of L. peregra was high (95%) after reproduction, but independent of treatment. Growth of L. stagnalis was reduced only at the highest tadpole densities, whereas egg production was reduced both by intraspecific competition and by competition with tadpoles. Differences in egg production were retained after tadpole metamorphosis. Tadpole larval period increased, weight of metamorphosing frogs decreased and growth rate was reduced as a function of increasing tadpole density. However, contrary to expectation, snails had a positive effect on tadpole larval period, weight and growth rate. Further, in experimental containers without snails there was a dense growth of the filamentous green alga Cladophora sp. We suggest that the facilitative effects of snails on tadpoles are due to an indirect mutualistic mechanism, involving competition between food sources of different quality (microalgae and Cladophora sp.) and tadpoles being competitively dominant over snails for the preferred food source (microalgae). In the presence of tadpoles snails will be forced to feed on low-quality Cladophora, increasing nutrient turnover rates, which results in enhanced productivity of microalgae, increasing tadpole food resources. Thus, tadpoles have a negative effect on snails through resource depression, while snails facilitate tadpole growth through an indirect enhancement of food availability.