Preliminary observation on the role of conditioning and mechanical Interference with egg masses and Juveniles in the competitive relationships Between Helisoma Durvi (Wetherby) and the Intermediate host of Schistosoma Mansoni Sambon: Biomphalaria Camerunensis (Boettger).

Biological control of the intermediate hosts of schistosomiasis by means of the introduction oa a competitor snail Helisoma duryi has been suggested. The effect of conditioning by Biomphalaria camerunensis and H. duryi respectively on growth and egg laying capacity of B. camerunensis and on the hatchability of B. camerunensis egg masses was compared. No differences were found besides an indication of a growth promoting effect of B. camerunensis conditioning. B. camerunensis egg masses were found to be much more sensitive to mechanical interference from snails than were H. duryi egg masses. Both juvenile and adult H. duryi caused a great reduction in the number of B. camerunensis egg masses and adult B. camerunensis showed a great tendency to destroy its own egg masses. Newly hatched B. camerunensis appeared to be destroyed to a greater extent by adult H. duryi than by adult B. camerunensis.     

Further laboratory studies on the interspecific competition between Helisoma duryi (Wetherby) and the intermediate hosts of schistosoma mansoni sambon: Biomphalaria alexandrina (Ehrenberg) and B. camerunensis (Boettger)

Biological control of schistosomiasis by means of introduction of a competitor, Helisoma duryi, of the intermediate host snails has been proposed. In the present laboratory studies the competitive relationships between H. duryi and two Biomphalaria species were investigated both when direct interference between the two competing species was possible and when not. A reduction of growth and reproduction of Biomphalaria was found when direct interference was involved but the effect of competition was lessened when the two species were separated by a mesh.The data showed that despite continuous presence of food in the aquaria food competition was involved in the direct interference between the two species, while the role of chemical interactions could not be evaluated conclusively from the present experiments.The role of food is discussed.     

Incomplete behavioural isolation and asymmetric female preference in darter sister species (Percidae: Etheostoma)

In this study, female preferences and behavioural isolation were estimated in a pair of allopatric sister species, Etheostoma duryi and Etheostoma flavum. Dichotomous mate preference trials were conducted to determine whether females prefer to associate with conspecific over heterospecific males and free‐spawning assays were conducted to determine whether those preferences translated into behavioural isolation. Dichotomous mate choice trials revealed asymmetric female preference, as female E. flavum preferred conspecific males, whereas female E. duryi showed no preference. Free‐spawning assays indicated that behavioural isolation remains incomplete between E. duryi and E. flavum (I_B = 0·19). In addition to female mating preferences, male behaviour also appeared to influence mating outcomes as male E. flavum consistently courted conspecific females more often in free‐spawning assays whereas male E. duryi did not. The data therefore suggest that despite marked divergence in male nuptial colour, divergence in female preferences between these species may not be sufficient to maintain species boundaries upon secondary contact. These results contrast with similar work in a sympatric darter species pair and may be explained by considering the contributions of reinforcement and differences in colour pattern as well as colour value.     

The effect of temperature,darkness, starvation and various food types on growth,survival and reproduction of Helisoma duryi,Biomphalaria alexandrina and Bulinus truncatus (Gastropoda: Planorbidae)

Helisoma duryi has been proposed as a biological control agent in schistosomiasis due to its superiority in laboratory competition experiments with various species of the intermediate host snails. Therefore it was considered important to evaluate the response of this snail species and the intermediate host species, Biomphalaria alexandrina and Bulinus truncatus, to various physical, chemical and biological factors under laboratory conditions in order to obtain information on the similarities in the ecological niches of these species. The factors considered in the present paper are: temperature, darkness, starvation and food. All three species had optimal growth and egg laying at 26–28 °C. Only H. duryi survived for a longer period at 33°C and it was capable of starting egg laying at this temperature although the onset was delayed. However, low temperature (18°C) caused a relatively larger decrease in egg laying of H. duryi than in the other two species. Growth and egg laying was reduced for H. duryi and B. truncatus kept under darkness and B. alexandrina could not tolerate maintenance under darkness. A few days of starvation of juvenile snails had no effect on later growth and egg laying capacity of the survivors, although mortality in B. truncatus was increased. B. alexandrina had a lower tolerance to starvation than the other two species. Egg laying of snails fed only one of the three laboratory food types decreased for all three species in the order: Vov-vov (dog food in dry pellets), Tetramin (fish food) and lettuce. Combinations of lettuce and one or more proteinaceous food types gave optimal growth and egg laying for all three species.     

Action competitive d'helisoma duryi,wetherby (Planorbidae : Helisomatinae) a l'egard de divers mollusques vecteurs de trematodoses humaines et animales. I. Biomphalaria glabrata,say

Laboratory experiments indicate that Helisoma duryi may be a promising biological control agent of human Schistosomiasis by acting as a predator of very young Biomphalaria glabrata and, when the two planorbid snails are maintained together, in influencing infection rate of Biomphalaria by miracidia of Schistosoma mansoni and their cercarial production.(avec la collaboration technique de R. THOMASSET)     

INTRASPECIFIC AND INTERSPECIFIC VARIATION IN SHELL MORPHOLOGY OF BIOMPHALARIA PFEIFFERI (KRAUSS, 1848) AND HELISOMA DURYI (WETHERBY, 1879) FROM MOSHI, TANZANIA, ANALYSED BY MORPHOMETRIC STUDIES

Helisoma duryi, a planorbid snail species, which is a potentialbiological control agent of the intermediate hosts of schistosomiasis,was introduced in an irrigation scheme (T.P.C.) in northernTanzania in 1972. The species has been present at T.P.C. since,and during a snail survey in January 1981, a great variationin the shell morphology of H. duryi, ranging from typical H.duryi forms to forms resembling Biomphalaria pfeifferi, theintermediate host of Schistosoma mansoni in the area, was observed.The Biomphalaria- Wkc forms of H. duryi could constitute anentire population but also occasionally occurred in populationsof typical H. duryi. This study was undertaken to determine whether simple shellmeasurements, or ratios between some of these could be usefulin discriminating between H. duryi and B. pfeifferi. Followingparameters were considered: shell height, shell diameter, diameterof the umbilicus, ratio between shell height and diameter ofthe umbilicus, and the number of whorls. (Received 29 July 1983;     

Fate of organic compounds in mountain streams of the Marmot basin: A process reactor model

Helisoma duryi is a planorbid snail that has been proposed as biological control agent against intermediate hosts of schistosomiasis. When the population density of this species is increased from 200 to 25 ml per snail in containers of constant volume (i.e. 200 ml) growth decreases. Whereas the single snails (i.e. 200 ml/snail) laid no egg masses, egg-laying in the other groups decreased with increasing density. When four snails were kept in various volumes of water giving the same population densities as above both growth and egg-laying decreased with increasing density. Cross-breeding experiments between an albino and a pigmented strain of H. duryi showed that the albino character is inherited as a recessive autosomal character, cross-fertilization appears to be the only mode of reproduction in H. duryi, stored foreign sperm remains viable for up to five months and stored sperm can survive when snails are aestivating or starving. The implications of the present findings for the use of H. duryi in biological control are discussed.     

ALLOZYME COMPARISONS AMONG SPECIES OF THE BULINUS FORSKALII GROUP (GASTROPODA: PLANORBIDAE) IN CAMEROON

Allozyme comparisons were made for adult Bulinus forskalii,B. camerunensis, and B. senegalensis from populations in Cameroonusing starch gel electrophoresis. Fifteen loci were examinedbut no differences were found between B. forskalii and B. camerunensis,bringing into question the validity of B. camerunensis as aseparate species. Bulinus senegalensis differed from B. forskaliiand B. camerunensis in allozymes for acid phosphatase, alpha-glycero-phosphatedehydxogenase, hydroxybutyrate dehy-drogenase, and phosphoglucomutase.No differences were found, as reported elsewhere, between B.forskalii and B. senegalensis for aspartate amino-transferase,isocitrate debydrogenase, phosphogluco-isomerase, and xanthineoxidise. No polymorphism was seen in B. camerunensis or variousB. senegalensis populations examined. Three alleles for isocitratedehydrogenase were observed for B. forskalii. Mixed populationsof B. forskalii and B. senegalensis were found at four sitesbut no evidence of hybridization between these species was found. (Received 9 May 1988; accepted 30 August 1988)     

Thermoregulation and activity patterns of two syntopic cicadas,Tibicen chiricahua and T. duryi (Homoptera: Cicadidae), in central New Mexico

Summary Tibicen chiricahua and T. duryi are cicada species that are active as adults early each summer in central New Mexico, and are often syntopic in pinyon-juniper woodlands. Both species regulate thoracic temperature (T_th) within fairly narrow limits by utilizing behavioral mechanisms and evaporative cooling. However, syntopic populations of these two species were found to regulate at different T_th despite having synchronous annual and daily activity periods; overall mean T_th of T. chiricahua was 3.1°C higher than it was for T. duryi. Interspecific differences in evaporative cooling abilities and rates of passive heat exchange could not account for this difference in T_th. Part of the difference in T_th resulted from the fact that individuals of the two species were active in thermally distinct microhabitats. Within each species, mean T_th varied among behavior categories, and differences in how the two species allocated their time between activities also contributed to the interspecific difference in T_th. Though T. duryi is restricted to pinyon-juniper habitats such as the one in this study, T. chiricahua is also found in warmer habitats. The difference in T_th in the syntopic populations probably reflects interspecific differences in thermal preferences and thermal optima that are adaptive over their respective habitat ranges. The degree of dependence of T_th on ambient temperature (T_am) varied between activities within both species, with the least dependence exhibited during singing. Singing involves intense activity of tymbal muscles, which apparently can only function effectively over a relatively narrow range of temperature.     

THE EFFECT OF VARYING RELATIVE DENSITY AND VARYING FOOD SUPPLY ON INTERSPECIFIC COMPETITION BETWEEN HELISOMA DURYI AND BULINUS TRUNCATUS (GASTROPODA, PLANORBIDAE)

Interspecific competition between Helisoma duryi and the intermediatehost of schistosomes, Bulinus truncatus results in reduced growthand reproduction of B. truncatus and the effects are more pronouncedas the density of H. duryi is increased. The drastic reductionin reproduction of B. trunauus is the result of a predationby H. duryi on egg masses. It appears that increasing the foodsupply cannot completely eliminate the effects of competition.Although the growth and reproduction of B. truncatus in competitionwith H. duryi increases significantly with the increased foodsupply, both growth and reproduction are still reduced comparedto control snails given the same amount of food. The effectsof food supply and competition on the allometric size-weightrelationships of the two species are slight. (Received 23 February 1987; accepted 4 May 1988)     

Action competitive d'Helisoma duryi,wetherby (Planorbidae: Helisomatinae) a l'egard de divers mollusques vecteurs de trematodoses humaines et animales. II. Biomphalaria pfeifferi,krauss, Bulinus truncatus sericinus,jickeli et limnea natalensis,krauss

The authors studied biological competition between some african snails (Biomphalaria pfeifferi, Bulinus truncatus sericinus, Limnea natalensis) and Helisoma duryi in the laboratory.The results are irregular and less successful than with the closely related species, Biomphalaria glabrata.

avec la collaboration technique de R. THOMASSET     

Cell contacts between follicle cells and the oocyte of Helisoma (Mollusca,Pulmonata)

The cell contacts between follicle cells, and follicle cells and oocytes of egg-laying populations of Helisoma duryi and non-egg-laying populations of H. trivcolvis have been studied. Scanning electron microscopy reveals that four to six follicle cells envelop a single developing oocyte. Thin sections and lanthanum impregnations demonstrate apical zonulae adherentes followed by winding pleated-type septate junctions between follicle cells. Gap junctions and septate junctions have been found between follicle cells and vitellogenic oocytes. Freeze-fracture replicas show relatively wide sinuous rows of septate junctional particles, and nemerous large gap junctional particle aggregates on the P-face between vitellogenic oocytes and follicle cells. Septate and gap junctions between immature or nonvitellogenic oocytes and follicle cells are fewer compared to those in vitellogenic oocytes. Similarly, the junctional complexes are less developed in non-egg-laying H. trivolvis compared to those in egg-laying H. duryi. It is possible that intimate interaction between follicle cells and a developing oocyte is necessary for the maturation of the oocyte. The junctional complexes could be involved in the interaction of the follicle cells and the oocyte, and they must disassemble at the onset of ovulation. Rhombic particle arrays and nonjunctional ridges of particles have been found in the basal part of the oolemma.     

Interference competition among native and invader amphipods

Aquarium experiments were used to study indications of interference competition, such as substratum choice shifts, swimming activities and mortality of invasive and indigenous gammarids in each other's presence. The more recent invaders Gammarus tigrinus and Dikerogammarus villosus were more likely to prefer stone substratum, whereas the native Gammarus pulex and an earlier invader Gammarus roeseli were found more frequently in the water layer. Sand was the least likely substratum to be chosen by any of the species. G. pulex and G. roeseli did not alter their substratum preference in each other's presence. In the presence of D. villosus, G. pulex shifted towards smaller stones and increased its swimming activities, whereas D. villosus did not change its behaviour in the presence of G. pulex. These shifts may indicate interference competition, with D. villosus being the stronger competitor. The greatest shifts in substratum preference arose when one species had occupied a substratum before the other one was introduced, especially when D. villosus was already present before G. pulex was introduced, possibly indicating pre-emptive competition. Swimming activities of G. pulex increased in the presence of D. villosus, whereas D. villosus spent little time swimming. Mortality was comparable between the different experiments without any indication of predation. The effect of Intra Guild Predation (IGP) may not be reflected adequately by short-time experiments as moults occurred seldom during the experiments. Although no IGP was observed during our experiments, habitat shifts occurred, which may indicate that competitive interactions are apparent before IGP starts. Such shifts may serve to avoid intraguild competition.     

Food selection by freshwater snails in the Gezira irrigation canals,Sudan

Stomach content analysis was carried out on samples of the freshwater snail species Biomphalaria pfeifferi, Bulinus truncatus, Bulinus forskalii (Pulmonata, Planorbidae), Lymnaea natalensis (Pulmonata, Lymnaeidae), Melanoides tuberculata, Cleopatra bulimoides (Prosobranchia, Thiaridae) and Lanistes carinatus (Prosobranchia, Ampullariidae) from different irrigation canals in Sudan. In order to evaluate overlap in diet selection among these species, sites with two or more of the above-mentioned species present were selected. For some species food choice was examined in relation to size groupings. In addition, samples of Marisa cornuarietis (Prosobranchia, Ampullariidae) from small ponds in Sudan, samples of Biomphalaria pfeifferi and Helisoma duryi (Pulmonata, Planorbidae) from drainage canals in an irrigation scheme in northern Tanzania, and samples of H. duryi from fish ponds in the coastal area of Kenya were also analysed. The results indicate a great similarity in the food choice of these species, especially among the pulmonate species. All species feed on fine detritus, epiphytic algae and decaying macrophytes. No fresh fragments of aquatic macrophytes were found and animal remains were found only on a few occasions. However, the stomach contents of the ampullarid species were characterized by large fragments of dead macrophyte tissue, while the composition of the finer particles showed a great resemblance to that of the pulmonate species. The diet of the thiarid species is essentially the same as that of the pulmonate species, although in one site Cleopatra bulimoides showed a greater preference for green algae. Apart from the avoidance of blue-green algae, there was little evidence of selection of certain algal components of the Aufwuchs for the pulmonate species. Detritus constitutes the major component of the stomach content of all these snail species.     

Musculature of the penial complex: A new criterion in unravelling the phylogeny of Hygrophila (Gastropoda: Pulmonata)

The taxonomy of freshwater pulmonates (Hygrophila) has been in a fluid state warranting the search for new morphological criteria that may show congruence with molecular phylogenetic data. We examined the muscle arrangement in the penial complex (penis and penis sheath) of most major groups of freshwater pulmonates to explore to which extent the copulatory musculature can serve as a source of phylogenetic information for Hygrophila. The penises of Acroloxus lacustris (Acroloxidae), Radix auricularia (Lymnaeidae), and Physella acuta (Physidae) posses inner and outer layers of circular muscles and an intermediate layer of longitudinal muscles. The inner and outer muscle layers in the penis of Biomphalaria glabrata consist of circular muscles, but this species has two intermediate longitudinal layers separated by a lacunar space, which is crossed by radial and transverse fibers. The muscular wall of the penis of Planorbella duryi is composed of transverse and longitudinal fibers, with circular muscles as the outer layer. In Planorbidae, the penial musculature consists of inner and outer layers of longitudinal muscles and an intermediate layer of radial muscles. The penis sheath shows more variation in muscle patterns: its muscular wall has two layers in A. lacustris, P. acuta, and P. duryi, three layers in R. auricularia and Planorbinae and four layers in B. glabrata. To trace the evolution of the penial musculature, we mapped the muscle characters on a molecular phylogeny constructed from the concatenated 18S and mtCOI data set. The most convincing synapomorphies were found for Planorbinae (inner and outer penis layers of longitudinal muscles, three-layered wall of the penis sheath). A larger clade coinciding with Planorbidae is defined by the presence of radial muscles and two longitudinal layers in the penis. The comparative analysis of the penial musculature appears to be a promising tool in unraveling the phylogeny of Hygrophila.     

THE EFFECT OF SODIUM CHLORIDE CONCENTRATION ON GROWTH AND EGG LAYING OF HELISOMA DURYI, BIOMPHALARIA ALEXANDRINA AND BULINUS TRUNCATUS (GASTROPODA: PLANORBIDAE)

Helisoma duryi, Biomphalaria alexandrine and Bulinus truncatuswere maintained in media with seven different sodium chlorideconcentrations ranging from 0.6 to 32 mM. Growth of H. duryiwas significantly reduced only in the 32 mM NaCl treatment,while no significant effects were observed in the net reproductiverate or in egg-laying rates. Bulinus truncatus showed optimalgrowth within the range of 1 to 8 mM NaCl and the net reproductiveand egg-laying rates were reduced in the 32 mM NaCl treatment.Growth of B. alexandrina was reduced in the 16 and 32 mM NaCltreatments, while the net reproductive rate was reduced in the16 and 32 mM NaCl treatments, while the net reproductive ratewas reduced in media with more than 2.0 mM NaCl. Hatching ofB. truncatus eggs was not affected by the high NaCl concentrations,while the hatchability of H. duryi eggs was reduced in the 32mM treatment. Survival of newly-hatched H. duryi was best inthe 1 mM NaCl treatment while for B. truncatus survival wasreduced only in the 32 mM treatment. (Received 28 March 1988; accepted 7 April 1989)     

Interference competition between alien invasive gammaridean species

The relative abundances of gammaridean species in the river Rhine have profoundly changed since the invasion of Dikerogammarus villosus in 1994/1995. This study tested whether these changes in gammaridean dominance could have been determined by interspecific competition and unequal mortality, for example by intraguild predation (IGP). Single and two species tests have been carried out in aquariums provided with all substrata present in the main channel of the Rhine. Changes in substratum choice, increased swimming activity and increased mortality of a species were used as indicators of interspecific competition during interaction between gammaridean species. Interspecific competition and mortality between the most abundant invasive gammaridean species in the Rhine, viz. Gammarus tigrinus, Echinogammarus ischnus and Dikerogammarus villosus were tested. In single-species experiments, G. tigrinus and D. villosus showed similar preferences for a stony substratum, whereas E. ischnus mostly occupied the water column. The two-species aquarium experiments indicated direct interference competition for substratum and unequal mortality between G. tigrinus and D. villosus, with D. villosus being the stronger competitor. Competitive stress was influenced by population density, was size-dependent and varied between the different types of substratum due to substratum choice. G. tigrinus did not show any behaviour indicative of interference competition in the presence of E. ischnus, and neither did E. ischnus or D. villosus in the presence of any of the other gammarideans. Swimming in the water layer may already enable E. ischnus to minimise its encounters with the stone-dwelling D. villosus and G. tigrinus. To maximise the encounters between E. ischnus and D. villosus, a fish (Lepomis gibbosus) was added to occupy the water layer during the aquarium experiments. E. ischnus showed a higher mortality in the presence of both D. villosus and fish, probably due to increased stress, as shelter opportunities to escape the predators had been minimised. The study shows that interference competition between gammaridean species can explain the replacement of the North American invader G. tigrinus by D. villosus in the river Rhine. E. ischnus and D. villosus both Ponto-Caspian invaders did not show interference competition in our experiments and co-exist in the Rhine.     

Interspecific competition between Sitophilus zeamais and Sitotroga cerealella in a patchy environment

The maize weevil, Sitophilus zeamais (Motchulsky) (Coleoptera: Curculionidae), and the Angoumois grain moth, Sitotroga cerealella (Olivier) (Lepidoptera: Gelechiidae), are important insect pests of stored products. The coexistence in nature of the two species on maize has been difficult to explain from laboratory set‐ups, as the moth is inevitably eliminated by S. zeamais. However, early laboratory experiments ignored the spatial dimension, while several studies have lately revealed that two competing species may coexist in a spatially divided environment even though one of the species is competitively superior in a simpler universe. Two hypotheses for coexistence were examined by studying the interactions in a system where the resources were spatially divided. It was found that spatial subdivision in the current experiment was insufficient to ensure coexistence between the two species, so neither of the hypotheses could be confirmed. A simple model describing the population dynamics of S. zeamais was fitted to the data. Its parameters provide important information about how resource depletion influences the per capita growth rate. Finally, the study indicates that the mechanism responsible for S. zeamais's rapid elimination of S. cerealella is the superiority of the former species to colonize and monopolize new patches. The design of this study may serve as a template for further laboratory experiments revealing the effect of a spatially divided environment on competitive interactions. However, the present study also indicates that choosing the appropriate spatial scale may be critical for relating the results to natural environments.     

Direct innervation of the mantle edge gland by neurosecretory axons in Helisoma duryi (Mollusca: Pulmonata)

Summary The mantle edge gland of Helisoma duryi is innervated by neurosecretory axons from the pallial nerves. Synaptoid contacts occur between axons and gland cells, and there is ultrastructural evidence for the release of neurosecretory material. The mantle edge gland contributes to the deposition of periostracum during shell formation, and direct neurosecretory innervation may control shell growth and regeneration.Supported by a National Research Council of Canada Grant (A-4673) and Negotiated Grant D-61     

Song discrimination suggests premating isolation among sympatric indigobird species and host races

Indigobirds (Vidua spp.) are host-specific brood parasites thathave diversified in a recent radiation apparently driven byhost colonization. Behavioral imprinting of both male and femaleindigobirds on host song is thought to promote rapid speciationbecause it results in assortative mating between indigobirdsassociated with a particular host. We conducted a song playbackexperiment to test whether male indigobirds discriminate amongpotential competitors based on song. Of particular interestwas the behavior of two sympatric host races of the Cameroonindigobird Vidua camerunensis that differ only in host songmimicry and other components of their vocal repertoires. Territorialmales of the two V. camerunensis host races and Vidua raricola,a morphologically distinct indigobird species, were tested withplaybacks of each other's songs. Males of all three groups respondedmost aggressively to songs of their own species and/or hostrace, as evidenced by strong and highly significant differencesin a variety of response variables. This differential territorialdefense suggests that an intruding male with different songsdoes not represent a competitive threat and is expected if femalesmate assortatively with respect to song. Thus, our results provideevidence of premating reproductive isolation among recentlyevolved indigobird species and host races.