The influence of infections with trematodes Bunocotyle progenetica (Hemiuridae) and Cryptocotyle cancavum (Heterophiidae) onto mortality of littoral molluscs Hydrobia ulvae (Gastropoda: Prosobranchia) in condition of extremely high temperature

The effect of invasion of two trematode species, Bunocotyle progenetica (Hemiuridae: Bunocotylinae) and Cryptocotyle cancavum (Heterophiidae) on the host vitality is estimated. The mortality of infected and uninfected individuals of Hydrobia ulvae after different exposure time (3, 6, 9 and 12 hours) under extremely high temperature (+42 degrees C) was compared. The total death of the host was observed after 12 hours of experiment. The death rate of molluscs infected with B. progenetica ranged from 60 to 100% and was significantly higher (p < 0.01) than mortality of individuals infected with C. cancavum and uninfected snails in each case of exposure (3, 6 and 9 hours). At the same time the mortality of individuals infected with C. cancavum was significantly higher (p < 0.05) than the mortality of uninfected snails only after 9 hours of exposure (13.3 and 3.1%, respectively). Difference in pathogen effects B. progenetica and C. cancavum is expressed in different temperature resistance of the host that may be explained by different localisation of redia, their motile activity and manner of feeding. Besides, the invasion by C. cancavum causes the sterilization and accelerated growth of hosts. Low decreasing of the resistance to overheating in snails infected with this trematode could be explained by a compensation effect of this infection. This compensation realizes due to the redistribution of some part of the reproductive energy after the sterilization. An absence of parasitic castration in the case of infection with B. progenetica is a key reason of such considerable declining of their vitality. High lethality of the host appears to be adaptive for the parasite with the monoxenous life cycle.     

Comparison of resistance to environmental factors of the molluscs Hydrobia ulvae infected with trematoda parthenitae and free from infection

The influence of trematode infection, mainly with Microphallus claviformis, onto the resistance of mudsnails Hydrobia ulvae to fresh water, desiccation and extremely high temperature has been investigated. It was found out in all variants of experiments that the intensity of mortality in infected individuals is reliably higher than in individuals free of infection. It is suggested, that the negative influence of parasites on the resistance of hosts is related to the disturbance of molluscs' capability to isolate themselves from extremal condition by shutting up the shell with operculum. It is proved by the high rate of salt loss in the infected molluscs in a comparison to non-infected individuals. Our hypothesis based on results obtained and reference data suggests that the rate of trematode parthenites' influence onto the resistance of molluscs depends upon the character of interrelationships in hostparasite systems. Normally, the trematode species having the active cercaria stage in the life cycle show more negative impact onto the resistance of infected molluscs, than those species, larval stages of which develop to metacercaria inside the parthenites.     

Parasites on an intertidal Corophium-bed: factors determining the phenology of microphallid trematodes in the intermediate host populations of the mud-snail Hydrobia ulvae and the amphipod Corophium volutator

The phenology of microphallid trematodes within their intermediate hostpopulations has been studied on an intertidal mud flat. The parasites usethe mud snail Hydrobia ulvae and the infaunal amphipod Corophium volutatoras first and secondary intermediate host, respectively. Migratory shorebirdsact as final hosts. Our results show a general trend of decline in thedensity of infected intermediate hosts during both spring and autumn, whichcould mainly be ascribed to shorebird predation. During summer the densityof both infected snails and infected amphipods increased considerably, witha culmination in June within the snail population (1000 infectedm-² and in August within the amphipod population (40 000infected m-². This time lag in parasite occurrence could berelated to (1) the development time of larval trematodes within the snails,(2) higher ambient temperatures in late summer increasing parasitetransmission between snails and amphipods during this period, and (3) ageneral increase in the Corophium population during late summer. Fromsamples collected between 1990 and 1995 it is shown that microphallidtrematodes occasionally may give rise to mass mortality in the amphipodpopulation. The prerequisites for such an event are a high parasiteprevalence within the first intermediate host population and unusually highambient temperatures, facilitating parasite transmission to the secondaryintermediate host, C. volutator.     

Measurement of ingestion rate of Hydrobia ulvae (Pennant) on intertidal epipelic microalgae: the effect of mud snail density

The individual mean ingestion rate of Hydrobia ulvae was measured experimentally in controlled microcosms, in the dark to avoid primary production during measurement and at constant temperature. The experimental design was based on the addition of prelabelled epipelic microalgae to microcosms in a constant proportion with unlabelled diatoms, and in such a way that algal food availability was not a limiting factor within the range of tested densities (0.3 to 4.1 snails cm(-2)). Results show that the individual mean ingestion rate decreased significantly from 26.6+/-1.1 ng Chl a snail(-1) h(-1) to 22.4+/-1.0 ng Chl a snail(-1) h(-1) between 0.7 and 3 snails cm(-2). We hypothesize that this sharp decrease (the threshold density was between 1.4 and 2.5 snails cm(-2)) may account for a density-dependent effect. We have tested this hypothesis by using a simple random walk model including basic behavioural processes such as a break in feeding activity when two individuals contact each other. The model represents quantitatively well the threshold effect, suggesting that behavioural processes have to be taken into account for estimating a global feeding activity of H. ulvae populations.     

The Hydrobia ulvae-Maritrema subdolum association: cercarial emergence controlled by host activity

The release of Maritrema subdolum cercariae (Digenea: Microphallidae) from the marine mud snail Hydrobia ulvae is significantly affected by temperature, salinity, light and exudates from the second intermediate amphipod host. Based on (i) previously published data on temperature-salinity dependent H. ulvae activity, (ii) new experimental data on H. ulvae activity in light and darkness as well as in the presence and absence of host exudates, and (iii) the cercarial emergence rate from free moving snails and snails prevented from crawling, the present analysis indicates that emergence of M. subdolum larvae is regulated mainly by host activity as the ultimate factor for release. The adaptive significance of such an emergence strategy is emphasized.     

Mechanisms of adaption to salinity stress in marine gastropods <Emphasis Type="Italic">Littorina saxatilis</Emphasis>: a proteomic analysis

Salinity is one of the most important abiotic environmental factors for marine animals. If salinity deviates from optimum, adaptive mechanisms switch on to maintain organism’s physiological activity. In this study the proteome of the marine snails Littorina saxatilis from natural habitats (12, 23 and 32‰ and in response to experimental salinity decreasing (from 20‰ to 10‰) was analyzed. The isolation of all snails inside their shells and gradually declining mortality was observed under an acute experimental salinity decrease. Proteomic changes were evaluated in the survived experimental mollusks compared to control individuals using differential 2D gel-electrophoresis (DIGE) and subsequent LC-MS/MS-identification of proteins. Approximately 10% of analyzed proteins underwent upor down regulation during the experiment. Proteins of folding, antioxidant response, intercellular matrix, and metabolic enzymes were identified among them. Proteomic changes observed in experimental hypoosmotic stress partially reproduced in the proteomes of molluscs that live in conditions of natural freshening (estuaries). Possible mechanisms involved in the adaptation process of L. saxatilis individuals to hypoosmotic stress are discussed.     

Distributional ecology of snails (Gastropoda: Mollusca) in seasonal ponds of Tamil Nadu,South India

The present study was focused to examine diversity and distributional pattern of freshwater snails and to identify the schistosome infected snails in ten different ponds of Madurai district, Tamil Nadu, South India. Snails were collected from native substrates in ponds using dip net from Madurai North, Tamil Nadu province of south India and microscopic examination for detecting cercariae larvae in snails were assessed. In total, 1250 individuals belonging to six species under five families of freshwater snails were collected. The alpha and beta diversity analyses were calculated for estimating the diversity of snails. The cercariae infection was observed in four collected freshwater snail species. Of four infected snails, Lymnaea luteola had the highest percentage of cercariae infection. Statistical tests showed that the environmental factors of total dissolved solids, pH and salinity were significantly related with snail distribution and cercariae infection rather than spatial factor. The study demonstrated that the salinity, conductivity and total dissolved solids of pond water play a key factor for the distribution of snails and cercariae infection. The findings of our study will improve the understanding of the dynamics of schistosomiasis transmission and would ease the implementation of integrated disease control measures based on ecological knowledge of schistosomiasis vector in Madurai and Tamil Nadu zones.     

The effect of wintering sites on the survival and reproduction of Gyraulus acronicus (Gastropoda) in a partly frozen river

Summary In a boreal river about 95% of the individuals of Gyraulus acronicus overwinter in the littoral zone which freezes solid each year. These snails were compared with those overwintering in the unfrozen sublittoral area: The littoral snails had a higher survival rate, a higher tissue dry mass/CaCO₃ ratio, and they deposited a higher number of eggs. Littoral snails had a more pronounced tissue degrowth during winter. High winter survival in the frozen littoral zone, a refuge totaly free from predation, indicates that overwintering here is advantageous. However, during frozen periods of short duration (<1 month) the high initial mortality to which the snails were exposed when freezing into the ice was not compensated for by higher survival after the initial phase. Under such conditions when the frozen period is very short the snails would have higher survival in unfrozen parts and are thus expected to avoid the ice.     

Salinity adaptation of the invasive New Zealand mud snail (Potamopyrgus antipodarum) in the Columbia River estuary (Pacific Northwest, USA): physiological and molecular studies

In this study, we examine salinity stress tolerances of two populations of the invasive species New Zealand mud snail Potamopyrgus antipodarum, one population from a high salinity environment in the Columbia River estuary and the other from a fresh water lake. In 1996, New Zealand mud snails were discovered in the tidal reaches of the Columbia River estuary that is routinely exposed to salinity at near full seawater concentrations. In contrast, in their native habitat and throughout its spread in the western US, New Zealand mud snails are found only in fresh water ecosystems. Our aim was to determine whether the Columbia River snails have become salt water adapted. Using a modification of the standard amphipod sediment toxicity test, salinity tolerance was tested using a range of concentrations up to undiluted seawater, and the snails were sampled for mortality at daily time points. Our results show that the Columbia River snails were more tolerant of acute salinity stress with the LC₅₀ values averaging 38 and 22 Practical Salinity Units for the Columbia River and freshwater snails, respectively. DNA sequence analysis and morphological comparisons of individuals representing each population indicate that they were all P. antipodarum. These results suggest that this species is salt water adaptable and in addition, this investigation helps elucidate the potential of this aquatic invasive organism to adapt to adverse environmental conditions.     

The influence of salinity on certain aspects of the biology of Bulinus (Physopsis) africanus

The hatchability of eggs and the fecundity and survival of adult Bulinus (Physopsis) africanus was investigated in different salinities. Experimental results revealed egg masses and hatchlings to be considerably more sensitive to salinity than the adult snails. Egg-laying was recorded in salinities 4·5 ‰ and further increases in salinity resulted in a progressive reduction in the hatching success up to a lethal concentration of 5·25 ‰ Survival of these hatchings was adversely affected by salinities as low as 1·0 ‰ and a salinity of 4·5 ‰ was lethal within 6 days. In contrast, adult survival was unaffected in salinities < 3·5 ‰ while further increases in salinity resulted in significant reductions in survival up to a lethal salinity of 8·7 ‰, which caused 100% mortality within 24 h. The survival of B. africanus infected with Schistosoma haematobium and Schistosoma mattheei was lower in the different salinities and control than that of their uninfected counterparts.     

Effect of starvation on parasite-induced mortality in a freshwater snail (Potamopyrgus antipodarum)

The level of host exploitation is expected, under theory, to be selected to maximise (subject to constraints) the lifetime reproductive success of the parasite. Here we studied the effect of two castrating trematode species on their intermediate snail host, Potamopyrgus antipodarum. One of the trematode species, Microphallus sp., encysts in the snail host and the encysted larvae “hatch” following ingestion of infected snails by birds. The other species, Notocotylus gippyensis, by contrast, releases swimming larvae; ingestion of the snail host is not required for, and does not aid, transmission to the final host. We isolated field-collected snails for 3 months in the laboratory, and followed the survival of infected and uninfected snails under two conditions: not fed and fed ad libitum. Mortality of the infected hosts was higher than mortality of the uninfected ones, but the response to starvation treatment was parasite species specific. N. gippyensis induced significantly higher mortality in starved snails than did Microphallus. Based on these results, we suggest that host exploitation by different species of trematodes may depend on the type of transmission. Encysting in the snail host may select for a reduced rate of host exploitation so as to increase the probability of transmission to the final host. Received: 29 July 1998 / Accepted: 1 February 1999     

Effects of trematode infection on metabolism and activity in a freshwater snail, Semisulcospira libertina.

Changes in the metabolism and activity of the freshwater snail Semisulcospira libertina infected with larval trematodes were studied experimentally. In snails up to 11 mm in shell width, crawling distance, feeding frequency, and the proportion of individuals located on vertical walls did not differ among snails infected with mature or immature cercariae, or uninfected snails (p > 0.05). In snails larger than 11 mm, individuals infected with mature cercariae tended to feed more frequently during the light period (p = 0.0081), but the distance they crawled and the proportion of individuals located on vertical walls did not differ, regardless of infection (p > 0.05). Infection with mature cercariae significantly increased the oxygen consumption rate (p = 0.016), which was measured only in the large size.     

Physiological and behavioral responses of the mud snails Hydrobia glyca and Hydrobia ulvae to extreme water temperatures and salinities: implications for their spatial distribution within a system of temperate lagoons

Physiological responses (oxygen consumption) and behavioral responses (feeding and activity) of the mud snails Hydrobia ulvae and Hydrobia glyca at different salinities (20 per thousand-80 per thousand) and temperatures (20 degrees and 30 degrees C) were studied. After 24 h under experimental conditions, both Hydrobia species already showed maximal activities (>90%) for a wide salinity range (30 per thousand-70 per thousand), with significant differences in activity between species only outside the usual salinity range of the studied lagoon. In contrast, egestion rates of H. glyca were significantly higher at the lowest salinities tested (30 per thousand and 40 per thousand) irrespective of water temperature, whereas egestion rates of H. ulvae were always significantly higher (57% on average) at 20 degrees C than at 30 degrees C and at the usual salinities found in the field (40 per thousand and 50 per thousand). Both species showed an oxyregulatory response to dissolved oxygen concentrations ranging from saturation to 1.5 mg O(2) L(-1), although specific oxygen consumption rates were significantly higher at 30 degrees C than at 20 degrees C (Q(10)=1.47+/-0.08 for H. ulvae and Q(10)=12.1+/-0.06 for H. glyca) and at the lowest salinities (30 per thousand-50 per thousand for H. ulvae and 30 per thousand-40 per thousand for H. glyca). On average, specific rates were higher for the smaller-sized H. glyca (1.64+/-0.03 microg O(2) mg(-1) ash-free dry weight [AFDW]) than for H. ulvae (1.35+/-0.03 microg O(2) mg(-1) AFDW). Despite the overlapping of their tolerances to high temperatures and salinities, the observed interspecies differences could play a certain role in the distribution of H. ulvae and H. glyca in the studied habitat. In particular, the decreasing feeding activity but increasing respiration of H. ulvae at 30 degrees C for salinities that usually occur in the studied lagoon could represent disadvantages to H. glyca during the warm period.     

中国自然湿地螺类生态学研究进展

螺类是软体动物腹足纲的通称,是湿地生态系统大型无脊椎动物的重要组成部分。湿地螺类在维持湿地生物多样性和复杂食物网结构,保障湿地物质循环和能量流动等方面具有重要的生态功能。从基本组成、生活型、功能群方面归纳了螺类群落结构特征;分析了螺类的时空分布格局;重点讨论了影响螺类群落结构的温度、盐度、底质等非生物因子和植被、物种间影响等生物因子以及人类对螺类的影响;概述了湿地演替过程中螺类群落的变化和螺类的环境指示功能。依据目前中国自然湿地螺类的研究特点和国际研究动态,展望了未来我国螺类群落的生态学研究的重点。     

Does parasitic infection compromise host survival under extreme environmental conditions? The case for Cerithidea californica (Gastropoda: Prosobranchia)

Summary This laboratory study examined the influence of parasitic infection by larval trematodes on the survival of extreme environmental conditions by the salt marsh snail, Cerithidea californica. Experimental treatments simulated the durations, combinations, and levels of potentially lethal environmental extremes to which the snail is exposed in its natural habitat, as determined from long-term field measurements. No significant difference was found in the rates of mortality suffered by infected and uninfected snails when exposed to simulated natural extremes of water temperature, water salinity, or exposure in air. Exposure to low levels of dissolved oxygen was the only treatment that caused differential mortality: infected snails died at higher rates than uninfected. This differential mortality was accentuated by high water temperature, and varied with the species of infecting parasite. The potential impact of this interaction between parasitism and anoxia on snail survival and population dynamics is discussed.     

Influence of Trichobilharzia regenti (Digenea: Schistosomatidae) on the Defence Activity of Radix lagotis (Lymnaeidae) Haemocytes

Radix lagotis is an intermediate snail host of the nasal bird schistosome Trichobilharzia regenti. Changes in defence responses in infected snails that might be related to host-parasite compatibility are not known. This study therefore aimed to characterize R. lagotis haemocyte defence mechanisms and determine the extent to which they are modulated by T. regenti. Histological observations of R. lagotis infected with T. regenti revealed that early phases of infection were accompanied by haemocyte accumulation around the developing larvae 2–36 h post exposure (p.e.) to the parasite. At later time points, 44–92 h p.e., no haemocytes were observed around T. regenti. Additionally, microtubular aggregates likely corresponding to phagocytosed ciliary plates of T. regenti miracidia were observed within haemocytes by use of transmission electron microscopy. When the infection was in the patent phase, haemocyte phagocytic activity and hydrogen peroxide production were significantly reduced in infected R. lagotis when compared to uninfected counterparts, whereas haemocyte abundance increased in infected snails. At a molecular level, protein kinase C (PKC) and extracellular-signal regulated kinase (ERK) were found to play an important role in regulating these defence reactions in R. lagotis. Moreover, haemocytes from snails with patent infection displayed lower PKC and ERK activity in cell adhesion assays when compared to those from uninfected snails, which may therefore be related to the reduced defence activities of these cells. These data provide the first integrated insight into the immunobiology of R. lagotis and demonstrate modulation of haemocyte-mediated responses in patent T. regenti infected snails. Given that immunomodulation occurs during patency, interference of snail-host defence by T. regenti might be important for the sustained production and/or release of infective cercariae.     

Heart Rate in the Blue Mussel <Emphasis Type="Italic">Mytilus edulis</Emphasis> (Bivalvia) under Salinity Change

The heart rate (HR) was studied applying noninvasive monitoring in the blue mussel Mytilus edulis L. from the White Sea. It was shown that the sublittoral mussels had a higher cardiac activity than the littoral mussels. Mollusks responded to a drop in salinity from 25 to 15‰ with a decrease in HR, which was more pronounced in sublittoral individuals. Acclimated mollusks sharply increased their HR upon being returned to the initial salinity (deacclimation). The cardiac activity normalized completely within two days after the beginning of the deacclimation.     

Effects of sediment type and intra- and interspecific competition on growth rate of the marine snails Hydrobia ulvae and Hydrobia ventrosa

The combined effects of sediment grain size and intra- and interspecific competition on growth rate of Hydrobia ulvae and H. ventrosa were investigated in a 2-month laboratory experiment. Individuals of H. ulvae grew faster than H. ventrosa at all competition intensities. Growth rate did not differ between fine- and coarse-grained sediments in any of the species. This suggests that other factors than the sediment quality cause the often observed size differences between Hydrobia snails from these sediment types in nature. Competition from 1.6 to 1.9 times larger competitors affected growth of the smaller target snails. Competition between the species was asymmetrical and, in both species, per capita effect of competition was stronger from H. ulvae than from H. ventrosa, regardless of sediment type. From growth, Lotka-Volterra competition coefficients were estimated to 0.2 from H. ventrosa on H. ulvae and 1.8 from H. ulvae on H. ventrosa. Since competition between the species seems to be independent of sediment grain size, the shift in numerical dominance in nature, from H. ulvae on coarse-grained sediment to H. ventrosa on fine-grained, could not be explained with different competitive ability in different environments.     

Effects of temperature,salinity and aerial exposure on predation and lysosomal stability of the dogwhelk Thais (Nucella) lapillus (L.)

The ingestion and absorption rate of standard length Thais lapillus (L.) stepwise-acclimated to constant temperature-salinity conditions and preying on Mytilus edulis (L.) varied directly with environmental salinity at 10, 15 and 20°C. Dogwhelk ingestion and absorption rates indicate that cold torpor existed at 5°C and heat stress was evident at 20°C. The feeding cycle duration was significantly longer for dogwhelks acclimated to 20%. S than in those acclimated to 30%. S at 10°C even though no significant difference existed between the two groups of snails in the drilling and ingestion or postfeeding phases of the cycle. Ingestive conditioning of dogwhelks to mussels occurred; the duration of the drilling and ingestion and total feeding cycle declined as a function of the number of mussels consumed by a snail. Dogwhelks of all sizes prey on a wide length range of mussels and there is also a high degree of variability in the ingestion rate of snails as a function of their size. A prominent feature of the lack of a relationship between dogwhelk ingestion rate and snail size was that the percentage of nonfeeding snails increased at low salinity and temperature extremes. Digestive-tubule cell lysosomal stability was tested as an index of digestive capability and animal condition; in stepwise-acclimated dogwhelks, it correlated well with their ingestion and absorption rates. The ingestion rate of dogwhelks acclimated to 30%. S and subjected to a 30?17.5?30%. S semidiurnal pattern of fluctuating salinity for 21 days was significantly lower than for snails maintained at 30%. S; however, snails acclimated to 17.5%. S and exposed to the same pattern of fluctuating salinity fed at a higher rate than snails maintained at 17.5%. S. Aerial exposure of snails maintained at 30%. S and 10°C water temperature resulted in an ingestion rate 2.1 times faster than for snails constantly submerged suggesting that tidal emersion is not always stressful to intertidal carnivores. The postfeeding phase of the feeding cycle was shortened in dogwhelks subjected to aerial exposure. Although significant variation occurred in digestive-tubule cell lysosomal stability during the first cycle of fluctuating salinity, the variability had declined significantly by Day 21. This observation suggests that digestive tubule lysosomal stability becomes adapted to a fluctuating osmotic environment, although the initial changes in lysosomal stability are probably related to intralysosomal protein catabolism and production of amino acids for intracellular osmoregulation. Variations in the osmotic environment of T. lapillus have resulted in unexpected outcomes with respect to their ingestion rate under conditions of fluctuating salinity and aerial exposure.