Hydrobia Ventrosa Research Articles

Component Community of Larval Trematodes in the Mudsnail Hydrobia ventrosa: Temporal Variations in Prevalence in Relation to Host Life History

Component Community of Larval Trematodes in the Mudsnail Hydrobia ventrosa: Temporal Variations in Prevalence in Relation to Host Life History

Buried salt-marsh edges and tide-level cycles in the mid-Holocene of the Caldicot Level (Gwent), South Wales, UK

Combining stratigraphical, sedimentological and biofacial (foraminifera) approaches, a high-resolution, time-constrained study has been made of a mid-Holocene ( c. 5800–4900 14C yrs BP) estuarine succession ( c. 2 m) continuously exposed for 1.75 km between two major, late-Holocene palaeochannels. The succession displays great lateral variation in bed thickness and facies; bounded by two laterally extensive peats, it consists of silts split by an impersistent, thin peat. A depositional hiatus gradually rises southwestward through the silts from the top of the lower extensive peat to the base of the higher bed, dividing the succession into two depositional sequences. Sequence I, restricted to the southwestern part of the section, ranges from the lower peat to the sharp top of a thin, laterally impersistent, silty peat (cockle-bed peat). It presents vertically symmetrical patterns of texture and environment, broadly from organic marsh and high salt marsh to middle marsh and back to high marsh. Sequence II, overlying the depositional break, is best developed in the northeastern and central parts of the section, beginning in the latter with an erosively based silt (cockle bed) with a life-assemblage of Cardium edule, Hydrobia ventrosa and ostracods. Environmentally, Sequence II is vertically symmetrical only in the northeastern section, where a high salt marsh was succeeded by a low/middle marsh followed by a high marsh overlain by an organic marsh. Asymmetrical patterns of texture and environment prevail in the central section, where the cockle bed appears. Through annually banded deposits that progressively coarsen, a mudflat or low marsh gave way upward to a middle marsh and finally to high and then organic marshes. Sequence I registers a rise of tide level lasting several hundred radiocarbon years. It appears to represent an estuarine marsh with a depositional edge which overlooked to the northeast a largely exposed shelf formed by the lower extensive peat. Sequence II arose much more rapidly, during a further tide-level rise that lasted only 100–200 years. A new marsh formed in the northeastern part of the section, but a wide, brackish, rapidly infilling embayment lay between this marsh and the previous one to the southwest. Differential autocompaction strongly influenced the rate of accumulation of both sequences, creating bed thickness variations of twofold or more. The mid-Holocene estuary experienced subtle, local geographical changes in addition to the gross changes indicated by the silt-peat alternation.

Life cycles and transmission patterns of seabird digeneans in SW Iceland

Flukes (digeneans) are common parasites of seabirds in Arctic and subarctic regions. This study examined the distribution of such parasites in 1998 and 1999 in the intertidal zones of two study areas in SW Iceland (Skerjafjördur and Grindavik) by investigating seven species of littoral snail, Littorina saxatilis, L. obtusata, L. fabalis, Onoba aculeus, Hydrobia ventrosa, Epheria vincta and Nucella lapillus , which are known to be first intermediate hosts of many species of digenean. A total of 2556 snails was collected and the intramolluscan stages of 23 seabird digenean species were identified. The local occurrence and abundance of final hosts, such as gulls, eiders and waders, largely explain prevalence differences in the study areas. Seabird digeneans with life cycles involving two intermediate hosts and one/two free-living larvae are more frequent in the Icelandic localities studied than in coastal areas of northern Europe (Barents Sea, White Sea, northern Norway). This indicates that the environmental conditions in the coastal ecosystem of SW Iceland are favourable for the transmission of complex life cycles. Also, the abundance and diversity of both potential final hosts (birds) and second intermediate hosts (littoral invertebrates and fishes) are considered to contribute to the richness of the digenean fauna of SW Iceland.

A sieving method for rapid determination of size-frequency distribution of small gastropods. Example of the mud snail Hydrobia ventrosa (Gastropoda: Prosobranchia)

A sieving system using sieves of predefined mesh size has been developed for rapid size–frequency distribution analysis as a basis for the assessment of the secondary production of the hydrobiid species Hydrobia ventrosa Montagu of Lake Ichkeul in northern Tunisia (Mediterranean). The relationship between shell length (SL) and shell width (SW) SL = e0.5005 ċ SW1.4306 calculated by a geometric mean regression is very highly significant allowing the application of sieves in mesh intervals of 0.1 mm for the determination of size–frequency distribution. The mean difference between sieved and measured size distributions, for different distribution patterns, only slightly exceeds 1%. The regression between shell length and ash-free dry mass (AFDM) fits an exponential model AFDM = e(-2.6591 + 0.9194 SL). Mean difference between estimated and observed biomass is in the 10% range. The elementary composition (C, H and N) of the ash-free dry mass is size-dependent. As a result, the proposed sieving method reduces by at least 20 times the data acquisition time (extraction, sorting and size-measuring), avoiding subsampling without significant loss of accuracy for secondary production studies of small gastropods.

Effects of winter severity on life history patterns and population dynamics of Hydrobia ventrosa (Gastropoda: Prosobranchia).

Interannual variations in population structure and life history patterns of the mudsnail Hydrobia ventrosa were investigated in relation to winter seventy in a windflat of the Wismar Bay (southwestern Baltic Sea). The population structure of H. ventrosa was studied in the field from 1995 to 1999 and egg production of different year classes was measured in the laboratory during the reproductive season of 1997. Abundance of H. ventrosa ranged between 7,000 and 150,000 ind./m 2 with density fluctuations according to the abundance of juvenile snails. The abundance of adult mudsnails remained remarkably constant with a mean density of about 25,000 ind./m 2 . Recruitment occurred between June and November with two peaks of reproduction in 1995 and a single one in the following years. Accordingly, the year class born in 1995 consisted of two cohorts, whereas the following year classes consisted of a single cohort only. H. ventrosa reached a maximum life span of about two years and a maximum shell height of about 3.5 mm. Shell growth was similar between year classes. According to laboratory experiments, about 95 % of the total annual egg production was contributed by 1-year-old females and <5 % of the eggs were laid by females which had overwintered twice and by young-of-the-year. Egg production decreased from 1.0 eggs female -1 day -1 in spring to less than 0.5 eggs female -1 day -1 in autumn. The annual production of recruits was higher after the moderate winter of 1996/97 than after the mild winters of 1994/95 and 1997/98. The cold winter of 1995/96 resulted in a delay of the reproductive season in 1996 and it is suggested that this delay restricted recruitment.

Infraspecific mitochondrial sequence diversity in Hydrobia ulvae and Hydrobia ventrosa (Hydrobiidae: Rissooidea: Gastropoda): Do their different life histories affect biogeographic patterns and gene flow?

Infraspecific relationships and population parameters are largely unknown in the ecologically significant mud snail genus Hydrobia s.I. We therefore studied infraspecific variation, population structure and gene flow in two Hydrobia species with different life history strategies: the marine, planktonic H. ulvae and the brackish-water, directly developing H. ventrosa. Based on sequencing data of a 638 bp fragment of the mtDNA gene for cytochrome oxidase 1, we found considerable differences between the two species. H. ulvae shows high average pairwisc nucleotide diversity, low population level differentiation (Fst). AND high average gene flow (Nm) between populations. Dispersal appears to accord with Wright's island model. In contrast, many populations of H. ventrosa have high population level differentiation and low gene flow. The average pairwise nucleotide diversity is relatively low; this species disperses according to Wright's isolation by distance model. Differences in dispersal modes and gene flow could be partly due to differences in type of early ontogeny and quantitative differences in passive dispersal. However, the fact that H. ulvae is a marine species with high tolerance to environmental stress and therefore less sensitive to migration barriers than H. ventrosa may better explain these differences. The extant lineages of H. ulvae and H. ventrosa most likely evolved in the northeastern Atlantic during the Pleistocene.

Infraspecific mitochondrial sequence diversity in Hydrobia ulvae and Hydrobia ventrosa (Hydrobiidae: Rissooidea: Gastropoda): Do their different life histories affect biogeographic patterns and gene flow?

Infraspecific relationships and population parameters are largely unknown in the ecologically significant mud snail genus Hydrobia s.l. We therefore studied infraspecific variation, population structure and gene flow in two Hydrobia species with different life history strategies: the marine, planktonic H. ulvae and the brackish-water, directly developing H. ventrosa. Based on sequencing data of a 638 bp fragment of the mtDNA gene for cytochrome oxidase I, we found considerable differences between the two species. H. ulvae shows high average pairwise nucleotide diversity, low population level differentiation (FST), and high average gene flow (Nm) between populations. Dispersal appears to accord with Wright's island model. In contrast, many populations of H. ventrosa have high population level differentiation and low gene flow. The average pairwise nucleotide diversity is relatively low; this species disperses according to Wright's isolation by distance model. Differences in dispersal modes and gene flow could be partly due to differences in type of early ontogeny and quantitative differences in passive dispersal. However, the fact that H. ulvae is a marine species with high tolerance to environmental stress and therefore less sensitive to migration barriers than H. ventrosa may better explain these differences. The extant lineages of H. ulvae and H. ventrosa most likely evolved in the northeastern Atlantic during the Pleistocene.

What Determines the Distribution of Coastal Hydrobiid Mudsnails Within North‐Western Europe?

The three north‐west European species of Hydrobia are often partially segregated along salinity gradients and/or sympatric in the nontidal eastern Danish waters and Baltic Sea, but are rarely sympatric and are confined either to nontidal coastal lagoons (Hydrobia ventrosa and Hydrobia neglecta) or to the marine/estuarine intertidal zone (Hydrobia ulvae) along the macrotidal Atlantic and North Sea coasts. Significant interspecific competition has been found in Danish waters but not elsewhere in the Atlantic and North‐Sea Europe. The two nontidal species and H. ulvae also possess contrasting reproductive strategies. The roles of life‐history strategy, interspecific competition and between‐habitat dispersal in restricting these mudsnails to their differing habitats are evaluated in the light of these and other findings. It is concluded that all are likely to be relevant, especially dispersal in maintaining and interspecific competition in opposing, mixed populations in the Kattegat‐Baltic, and life‐history strategy in maintaining the lagoonal versus intertidal dichotomy in the rest of north‐west Europe. This dichotomy prevents segregation of species along common salinity gradients outside the Baltic region and restricts potential interspecific competition to within lagoons, where H. ulvae is at a recruitment disadvantage.

Histopathological effects of larval trematode infections in mudsnails and their impact on host growth: what causes gigantism in Hydrobia ventrosa (Gastropoda: Prosobranchia)?

Abstract The histopathology and the patterns of spatial integration of different types of trematode larvae (sporocysts, rediae) were examined in relation to the stage of gonadal development of the host, the prosobranch semelparous mudsnail Hydrobia ventrosa . Snail growth was analysed in relation to the type of infection (sporocyst or rediae, prepatent or patent). Seven taxa of larval trematodes were found during this investigation: Maritrema subdolum , Microphallus sp., Levinseniella brachysoma , Cryptocotyle concavum , Paramonostomum alveatum , Asymphylodora demeli , and Himasthla sp. H. ventrosa reached maturity at a size of 1.6–2.0 mm, after a lifespan of 9–10 months. Trematode infections were observed 1–2 months after maturation in snails larger than 2 mm. The parthenitae generation of all trematode species was located in the organs of the reproductive system (gonad, albumen gland, prostata) of H. ventrosa . In consequence, the affected organs, in particular the gonads, showed severe histopathological alterations. Effects differed between the prepatent and patent period. If infections appeared before meiosis of germ cells, the germinal epithelium of the gonadal tubules was still present. If infections appeared during meiosis of host germ cells, the germinal epithelium of the gonadal tubules was almost completely absent. The gonadal tissue was completely replaced by daughter sporocysts or rediae during the patent period. Sporocysts invaded also the accessory sex glands. In no instances were histopathological alterations of the digestive gland observed. Growth rates of snails with prepatent infections were similar to growth rates of uninfected snails. Growth of snails with patent infections was significantly enhanced. The increment in shell size per day was higher in snails infected with patent sporocysts than in snails infected with patent rediae. Gigantism in infected H. ventrosa is interpreted as a side-effect of parasitic castration.

Population responses to acute and chronic cadmium exposure in sexual and asexual estuarine gastropods

: The acute lethal and chronic sublethal responses of sexual (Hydrobia ventrosa) and asexual (Potamopyrgus antipodarum) gastropod populations to cadmium exposure were examined. Three questions were addressed: (i) are there differences in responses of sexual and asexual populations; (ii) are the response patterns similar in lethal and sublethal exposure conditions; and (iii) how does preexposure to cadmium influence these responses. No differences between the two species in mean acute tolerance (LC50) could be detected, but a significant difference was found between the slopes of the concentration-response curves. The steeper slope for P. antipodarum indicated a more uniform response for the asexual species than for H. ventrosa. In the sublethal experiment there was a significant difference in mean growth rate where, in general, P. antipodarum grew faster than H. ventrosa. P. antipodarum was more affected by cadmium at the low cadmium exposure, whereas growth rates were reduced equally for the two species at the high cadmium concentration. Pre-exposure to cadmium did not increase the tolerance to chronic exposure in either of the species, but P. antipodarum exhibited a tendency toward increased tolerance to acute cadmium stress after pre-exposure.The results show that there are differences between these closely related sexual and asexual species in response to cadmium, and that these differences are more pronounced under chronic sublethal exposure conditions.

The effect of larval trematodes on the survival rates of two species of mud snails (hydrobiidae) experimentally exposed to desiccation, freezing and anoxia

Digenetic trematodes are widespread among mud snails (Hydrobiidae) living in coastal lagoons and estuaries, but knowledge is generally lacking on their impact on these host organisms. We examined the survival rates of infected and non-infected experimental populations of two mud snail species,Hydrobia ventrosa (Montagu) andHydrobia neglecta Muus, exposed to desiccation, freezing and anoxia in the laboratory. Our experiments indicated that non-infected groups of both species had similar survival rates after being subjected to desiccation and anoxia, whereasH. ventrosa survived freezing better thanH. neglecta. However, infected groups ofH. neglecta specimens subjected to desiccation showed significantly lower survival rates than non-infected groups. Infected and non-infected snails of both species exposed to freezing and anoxia exhibited similar survival rates. The possible mechanisms by which parasites influence their hosts are discussed. It is unlikely that the parasites in the present case mediate the coexistence of the twoHydrobia-species, because the snail with the highest reproductive effort-H. neglecta-showed lower infection rates in situ than its congenerH. ventrosa.

Investment in eggs in lagoonalHydrobia ventrosaand life-history strategies in north-west EuropeanHydrobiaspecies

Expected lifetime investment in eggs and related parameters are described for a lagoonal population ofHydrobia ventrosa(Mollusca: Gastropoda) in East Anglia, UK, and these are compared with those of East Anglian lagoonal populations ofH. neglectaand both lagoonal and intertidal-marine populations ofH. ulvae. In contrast to an earlier study on Danish populations, which suggested thatH. neglectaandH. ventrosaare at opposite ends of the hydrobiid spectrum of investment in reproduction, the order of increasing relative allocation of resources to egg production runsH. neglecta=H. ventrosa→ lagoonalH. ulvae→ intertidal-marineH. ulvae. That is, from the two small, short-lived, specialist lagoonal species producing few, large, directly-developing eggs, to the larger, potentially longer-livedH. ulvaethat produces many small eggs which develop into veliger larvae. The lagoonal populations of all threeHydrobiaspecies, however, are very similar to each other in terms of reproductive investment. When like cohorts are compared, in each, the annual egg weight is equal to the annual adult growth increment, and the weight of eggs produced per expected lifetime is half the adult growth increment at the time of death. These lagoonal populations are markedly dissimilar to intertidal-marine populations ofH. ulvae, which produce an annual weight of eggs some 15 times the annual adult growth increment, and an expected lifetime egg weight some two-and-a-half times the adult growth increment at the time of death.

Correlation of the zonal schemes in the Quaternary sediments of the Bulgarian Black Sea shelf according to molluscs and ostracods

The Candona Ostracod Zone corresponds the Upper Chaudinian where the Dreissena rostriformis tschaudae Molluscan Zone is indicated. The Didacna crassa pontocaspia – Dreissena euxinica and Didacna crassa pontocaspia – Alba ovata Molluscan Concurrent-range-zones corresponding to the Cryptocyprideis bogatschovi Ostracod Zone are interpreted respectively in the Middle Pleistocene, Old Euxinian and Uzunlarian Stages. The section of the Upper Pleistocenian (Karangatian) molluscan zones has not been presented for analysis of the Ostracod sequences. The sequence of the molluscan and ostracod zones in the inner shelf has been broken by a barren zone which corresponds to the continental sediments, deposited in the Early New Euxinian. The Loxoconcha lepida Ostracod Zone corresponds to the following in superposition Upper Euxinian Dreissena polymorpha – Dreissena rostriformis distincta Molluscan Concurrent-rage-zone. The Hydrobia ventrosa – Monodacna caspia Molluscan Zone together with the following in superposition Mytilus galloprovincialis Zone comprise the sediments of the Old Chernomorian. The intervals occupied by the Cyprideis torosa Ostracod Zone correspond to these molluscan zones. The Falunia rubra – Loxoncha granulata Osatracod Zone comprises the area occupied by the Spisula subtruncata triangula Molluscan zone including also the upper parts of the Mytilus galloprovincialis Molluscan Zone.

The Distribution of Hydrobiids and the Effects of Sediment Characteristics on the Population Dynamics of Hydrobia ventrosa in a Coastal Region of the Southern Baltic

AbstractThe composition of the hydrobiid fauna in a coastal water of the southern Baltic was studied in 1990/91 at three stations with different sediment characteristics (grain size, organic content). Four species were found (Hydrobia ventrosa, H. ulvae, H. neglecta and Potamopyrgus jenkinsi). H. ventrosa was dominant at all stations. The mean hydrobiid abundances were 4,000 (stn. C), 18,000 (stn. B) and 40,000 (stn. A) ind./m2 respectively. Abundance was found to correlate with the organic content of the sediment. The reproduction and growth of H. ventrosa was studied. Two reproduction phases were observed, one in summer and the other in late autumn. The autumn reproduction peaks were staggered owing to the different growth rates at the various stations. Possible correlations between the differences in shell size of H. ventrosa at the various stations and both sediment characteristics and abundance are discussed.

Local mollusc standard zones of Quaternary sediments from the Bulgarian Black Sea shelf

The only boundary which can be used to differentiate the Bulgarian Black Sea shelf into stratigraphic zones is the spatial distribution of mollusc fauna. This boundary is bathymetrically determined and divides the central morphological shelf zone into two uneven parts given inner and outer shelf. Another boundary is drawn which follows roughly the parallel of Cape Kaliakra and divides it into two uneven parts. The sediments of Low Pleistocene series were not deposited in the inner shelf zone and here a stratigraphic gap is established there which coincides with Chaudian Age. A similar gap at the time of Early Chaudinian Subage is established in the outer zone of the shelf where the Dreissena rostriformis tschaudae local mollusc Range-zone is indicated. In both shelf zones the sedimentation continues by formation of the Middle Pleistocene Old Euxinian and Uzunlarian Regional Stage in whose sediments the corresponding are Didacna crassa pontocaspia – Dreissena euxinica and Didacna crassa pontocaspia – Abra ovatа local mollusc Parallel Range-zones. The Upper Pleistocene (Karangatian) mollusc zones are established only at the inner parts of the Bourgas Bay and there three: Corbula gibba – Rissoa parva, Paphia senescens – Cardium tuberculatum and Cardium edule – Bittium reticulatum local mollusc Parallel Range-zone are found out which correspond to the three Substages of the Karangatian Regional Stage. The succession of the mollusc zones from the inner shelf is interrupted by a barren zone which corresponds to the continental deposits sedimented at the time of the Early New Euxinian Substage. During the greater part of the Late Pleistocene Subepoch in the zone of the outer shelf a stratigraphic gap is established. In the composition of the zone community of the next in superposition Dreissena polymorpha – Dreissena rostriformis distincta local mollusc Parallel Range-zone two associations are established – one in the inner and another in the outer shelf. The climatic changes which took place after the Würm glaciation restored the connection of the Black Sea with the World Ocean. In the Black Sea basin this Holocene transgression causes uplift of the sea level and increasing of the salinity to its recent values. The latter determines the successional changes of the mollusc communities which from the beginning of the Holocene became more halophilic. The community of Hydrobia ventrosa – Monodacna caspia caspia local mollusc Parallel Range-zone and Mytilus galloprovincialis local mollusc Range-zone comprise the sediments of Old Chernomorian Substage. The two New Chernomorian Spisula subtruncata triangula and Modiolus phaseolinus local mollusc Range-zone are fingering spatially in the zone of the outer shelf together with increase of the bottom at practically one and same composition of the zonal community.

Nutrient regeneration processes in bottom sediments in a Po delta lagoon (Italy) and the role of bioturbation in determining the fluxes at the sediment-water interface

A study on nutrient regeneration processes and a measure of their fluxes at the sediment-water interface was carried out in two different stations of a shallow lagoon of the Po delta river (Italy). A few parameters on the solid fraction (grain-size, porosity, C, N) and pore water profiles of o-P, NH3, NO inf3 sup− , SiO2, Tot-CO2, SO inf4 sup2− , Fe, Mn, Ca, Mg, pH, Eh were determined. At both stations the results were typical for fine sediments rich in organic matter. The ratio of variations of sulphate (ΔSO inf4 sup2− ) to total carbonate demonstrates the main role sulphate reduction plays on the organic matter decay. The use of the ratios of variations of sulphate (ΔSO inf4 sup2− ) to ammonia (ΔNH3) and of sulphate (ΔSO inf4 sup2− ) to phosphate (Δo-P) in pore waters enabled us to calculate the C/N/P of the decomposing organic matter. Obtained C/N/P indicated an enrichment of N and P with regard to C/N/P ratios of the solid fraction, due to the selective stripping of N and P during organic matter mineralization. This phenomenon decreases with depth, where organic matter becomes more refractory. Calculations on saturation degrees have shown the possibility of authigenic calcite, apatite and rhodochrosite precipitation in sediments. Nutrient fluxes were estimated for SiO2, NH3 and o-P by means of benthic chambers and modelling the pore water profiles. The model used for the calculation of fluxes allowed us to account for the bioturbation-irrigation influence near the interface, by means of a cumulative diffusion coefficient. Directly measured fluxes proved to be always significantly greater than the theoretical ones. These differences seem to be due to surface processes which do not affect pore water concentrations (degradation of fresh materials at the interface; micro-bioturbation by small gasteropoda such as Hydrobia ventrosa) and/or to the different concept of the two methods in time and space. Number, size and biomass of macrobenthic species living in the sediment underneath the benthic chambers were determined. The comparison between data on macrobenthic populations and flux values showed a good relationship between the number of organisms and benthic fluxes within each station. However, flux variations between stations are to be attributed mainly to the different arrangement of the tubes of the polychaetes Polydora ciliata in the sediment.

Response of Hydrobia ventrosa (Montagu) to Environmental Stress: Effects of Salinity Fluctuations and Cadmium Exposure on Growth

As estuarine environments are often rich in both natural and man-induced stresses, estuarine species may provide important insights into the study of how organisms respond to stress. Exposure to cadmium concentrations of 100200 [Lg 1-1 significantly reduced growth rate of the estuarine gastropod, Hydrobia ventrosa (Montagu). Snails acclimated to 23%o salinity sea-water (S) showed reduced growth rates at 13 and 33%o S relative to snails grown at 23%o S. At 13%o S snails did not grow, and any additional effects of cadmium exposure on growth were masked. The results are not consistent with expectations based on estimated cadmium bioavailability as the relative reduction in growth in response to cadmium increased with increasing salinity. The growth trajectory of H. ventrosa at 23%o S (no cadmium) fitted a logistic growth model. Changing salinity and exposure to cadmium altered daily specific growth rates (% day-1) in a size(or time-) dependent manner such that a logistic model no longer described the growth trajectory of this species. Maximum growth rates occurred at a higher salinity than expected, based on reports of the natural distribution of H. ventrosa, and the results suggest substantial intraspecific variability in salinity tolerance in this species. Key-words: Cadmium, growth, Hydrobia, salinity

The Life Cycle of Pygidiopsis ardeae Koie, 1990 (Digenea, Heterophyidae)

Ocellate cercariae, consistent with the description of Cercaria angularis Honer, 1961, developing in rediae in Hydrobia ulvae (Pennant) and Hydrobia ventrosa (Montagu) (Prosobranchia, Hydrobiidae) from Denmark, were shown to be larvae of Pygidiopsis ardeae Køie, 1990 (Heterophyidae, Pygidiopsinae). Euryhaline fishes such as the sticklebacks Gasterosteus aculeatus Linnaeus and Pungitius pungitius (Linnaeus), and the common goby Pomatoschistus microps (Krøyer) are second intermediate hosts in Danish inlets. These species and the flatfishes Platichthys flesus (Linnaeus) and Limanda limanda (Linnaeus) were laboratory infected. Cercariae penetrated the gills where some encysted, but most scattered throughout the body via efferent branchial arteries and the dorsal aorta to encyst mostly in the mesenteries, spleen, and on the surface of other viscera. Twelve-week-old (15 C) metacercariae fed to domestic chickens and pigeons developed to adults of P. ardeae in less than 4 days. Grey herons, Ardea cinerea Linnaeus, were naturally infected with P. ardeae, which was not found in gulls or cormorants from the same area and feeding on similar fishes.

Dispersal-mediated coexistence of mud snails (Hydrobiidae) in an estuary

The distribution of the mud snails Hydrobia ventrosa (Montagu), H. neglecta Muus, and Potamopyrgus jenkinsi (Smith) was studied in the Kysing Fjord estuary, Denmark, from November 1977 to February 1979. The hydrobiids show habitat selection with regard to salinity but coexist in large areas of the estuary: P. jenkinsi at the innermost parts of the estuary together with H. ventrosa, with the latter species extending its distribution further to the mouth of the estuary, where H. neglecta dominates. H. ventrosa and H. neglecta are the only species that have permanent populations in the estuary. P. jenkinsi dies out in the estuary in the winter, but recolonizes there from a small river in the spring and builds up a population in the innermost parts of the estuary during the summer. H. ventrosa and H. neglecta coexist in the middle part of the estuary due to continued dispersal into this area.

Character displacement in Hydrobia.

Fenchel's study of size variation in Hydrobiid snails in the Limfjord, Denmark, provides one of the most convincing cases of ecological character displacement available. In order to assess the generality of the phenomena within the Hydrobia genus, allopatric and sympatric Hydrobia ventrosa, H. neglecta and H. ulvae were collected from 24 coastal sites around Eastern England in July and October, 1982. Shell heights of 5,850 snails from 55 samples were recorded. These data were analysed for intraspecific allopatric-sympatric, and interspecific height differences. Such differences were not significant for H. ventrosa/H. neglecta, but the species exhibited parallel variation at sympatric sites. H. ventrosa/H. ulvae showed only significant interspecific differences, but in both allopatry and sympatry. The pattern of mean size variation for these species resembles that in the Limfjord. However the statistical analyses fail to provide support for character displacement. Differences in character states attributed to the process of character displacement may result from a number of other causes. Environmental conditions at sympatric and allopatric sites may act differentially on the heights of H. ulvae and H. ventrosa. The conditions that prevail at sympatric H. ulvae sites appear to lead to increased size in this species irrespective of the presence of H. ventrosa.