DISTRIBUTION AND PRODUCTION OF THREE HYDROBIA SPECIES (GASTROPODA: HYDROBIIDAE) IN A SHALLOW COASTAL LAGOON IN THE BAY OF CADIZ, SPAIN

The abundance, life span, growth and production of the mud snailsHydrobia minoricensis, H. ulvae and H. ventrosa in a semi-naturallagoon system were studied by taking monthly samples at threesites during 1991 and 1992. The most abundant species, H. minoricensisoccurred at mean densities of 12834 to 26264 snails m^–2(10.7 to 25.8g dry weigh m^–2), depending on the site.The least abundant species, H. ulvae, occurred at mean densitiesof 185 to 353 snails m^–2 (3.2 to 2.2g dry weight m^–2).The numerical abundance and biomass of the three Hydrobia specieswere positively related to the biomass of benthic macroalgae(P<0.01). Although H. ulvae egg capsules were recorded throughoutthe year, newly hatched snailsof this species were not observed,in contrast to the other two species. The early spring and summercohorts of H. minoricensis and H. ventrosa seemed to be themost numerous. The average life spans of these two species wereestimated to be about 18 and 13 months respectively. Annualproduction estimates for the whole lagoon system were 29.0 (6.3),5.5 (0.8) and 5.2 (1.0)g dry weight (ash-free dry weight) m^–2yr^–1 for H. minoricensis, H. ulvae and H. ventrosa respectively.The annual P/B ratio was about 2 for H. minoricensis and H.ventrosa. (Received 5 July 1994; accepted 5 October 1994)     

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.     

Character displacement in Hydrobia

Summary 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.     

Character displacement and coexistence in mud snails (Hydrobiidae)

Summary Populations of coexisting and of allopatrically occurring species of hydrobiid snails (Hydrobia ulvae, H. neglecta, H. ventrosa and Potamopyrgus jenkinsi) have been studied in 90 localities within three different areas. When H. ventrosa coexists with H. ulvae they show character displacement, i.e., the average body size of the former is smaller and that of the latter is larger. When these species live alone they are of approximately the same size. It is shown here that the size ratio between the coexisting species usually found (1.3–1.5) allows stable coexistence based on food praticles size selectivity alone. Variation in the degree of character displacement from locality to locality is explained by different degrees of genetical isolation of the populations. Coexisting H. ventrosa and H. ulvae have shorter, more well-defined periods of reproduction than they do when they occur alone. H. neglecta is larger than coexisting H. ventrosa and smaller than coexisting H. ulvae. Due to the patchy distribution and the fluctuating populations of this species, and due to the fact that pure H. neglecta populations are rare, data on this species are difficult to interpret. Potamopyrgus jenkinsi shows a different food particle size selection than the Hydrobia spp. of identical sizes. Competitive interactions between P. jenkinsi and coexisting Hydrobia spp. are therefore probably weak. In accordance with this, P. jenkinsi does not show character displacement when coexisting with, e.g., H. ventrosa.The fact that the major study area, the Limfjord, is only 150 years old as a marine habitat as well as various more recent man-made changes of the coast line allow estimates of the time scale of the microevolutionary changes which lead to character displacement.     

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

The 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.     

Reproductive effort in Danish mudsnails (Hydrobiidae)

Summary The four species of Hydrobiid mudsnails found in Danish brackish water habitats are ranked with respect to reproductive effort. The biology of the species indicate that Hydrobia neglecta and Potamopyrgus jenkinsi, showing the highest reproductive effort are also more r-selected than the other species, H. ventrosa and H. ulvae.     

Resource partitioning on basis of hydrolytic enzymes in deposit-feeding mud snails (Hydrobiidae)

Summary Crude extracts of hydrolytic enzymes from the related mud snailsHydrobia ulvae, H. ventrosa andH. neglecta are compared by use of six different methods and 18 natural carbohydrates. Synthetic substrates for linkage specific carbohydrases and trypsin-like activity were studied in addition to lysozyme-like activity.A significant hydrolysis was only observed with the reserve carbohydrates amylose, glycogen and laminaran. Many algal carbohydrates were, however, digested to some extent.The qualitative spectra are almost identical for the three species but significant quantitative differences were found. The findings are discussed in relation to information on the chemical composition of potential food items and it is concluded that the commonly observed coexistence of the snails can in part be explained by selection for microalgae and detritus particularly meeting their enzymatic potentials.     

BIOCHEMICAL COMPOSITION OF HELIX SNAILS: INFLUENCE OF GENETIC AND PHYSIOLOGICAL FACTORS

This paper describes the biochemical composition of differentspecies (Helix lucorum, Helix pomatia) and sub-species of snails(Helix aspersa aspersa, Helix aspersa maxima) reared in thesame conditions with a feed (‘Helixal’) speciallydesigned for edible snails. In addition, the composition ofwild H. pomatia and H. lucorum is presented to allow comparisonbetween snails of different origins. Analyses determined thepercentages of proteins, lipids and minerals. They reveal bothsimilarities and differences in composition according to thespecies and the part analysed (whole body, pedal mass, and visceralmass). H. pomatia contains the highest percentage of mineralmatter and the lowest percentage of lipids. Surprisingly, proteincontents are slightly different between artificially rearedH. aspersa maxima of 3 months old and wild H. pomatia. The resultsmake it possible to evaluate nutritional quality of snails withthe composition of the body of four edible snail species. (Received 16 September 1996; accepted 24 May 1997)     

Tolerance and Resistance in Gastropod Mollusks Hydrobia ulvaeand H. ventrosafrom the White Sea to Abiotic Environmental Factors

Tolerance to salinity changes and resistance to desiccation, fresh water, and freezing were studied in two hydrobiid species from the White Sea, Hydrobia ulvae(Pennant) and H. ventrosaMontagu. It was shown that H. ventrosahas a greater tolerance to low salinities in the range of 6–10 but is less tolerant to high salinities (35–45). The interspecies differences in low salinity tolerance persisted after the acclimation of snails to 20, 16, and 12. A comparison of survival in fresh water and under desiccation conditions suggests that H. ulvaeis more resistant to these factors. In fresh water, LT₅₀was 30 and 60 days for H. ventrosaand H. ulvae, respectively. Under desiccation conditions, LT₅₀was 6 and 25 days for H. ventrosaand H. ulvae, respectively. At subzero temperatures, H. ventrosawas shown to be a superior survivor. Within the same species of mud snail, the detrimental effect of freezing depended on the salinity: survival decreased with decreasing salinity. These data suggest significant differences in the mechanisms of resistance and tolerance to abiotic environmental factors between the two species. Despite the partial overlapping of their ecological niches, the interspecies differences may play an important role in the distribution and spatial structure dynamics of coexisting populations of these species in the White Sea region.     

VARIABILITY OF EGG CANNIBALISM IN THE LAND SNAIL HELIX ASPERSA IN RELATION TO THE NUMBER OF EGGS AVAILABLE AND THE PRESENCE OF SOIL

Egg cannibalism by hatchlings has been demonstrated in somepulmonate land snails; this behaviour is promoted by a highhatching asynchrony within the egg-batch. Under laboratory conditions,the percentage of new-born snails Helix aspersa having cannibalisedunhatched eggs was not influenced by the soil factor: about70% of them ingested one egg within their first four days oflife whether soil was present or not. The propensity to eggcannibalism in hatchlings of H. aspersa increased with egg density.However, most of the new-born hatchlings consumed a single eggduring the four days following hatching, and only exceptionallytwo. The consumption of one egg increased the snails wet weightby 38.7% within four days. A weak ingestion of soil componentsalso occurred, but it induced a growth that was three-timesless than that due to the consumption of an egg. In addition,the survival of newly hatched snails maintained under non-dehydratingthermohygrometric conditions was high, even when they were submittedto four days food-deprivation. (Received 22 July 1999; accepted 24 November 1999)     

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)     

BARNACLE-DWELLING EGOTYPES OF THREE BRITISH LITTORINA SPECIES AND THE STATUS OF LITTORINA NEGLECTA BEAN

On the basis of collections from SW Wales and W Ireland it isshown that small (<5 mm) black and/or white Littorina occurringin the barnacle zone on exposed shores, of the L. neglecta shelltype, comprise three species. From their reproductive anatomythese correspond to L. saxatilis, L. arcana and L. nigrolineata,and are interpreted as analogous barnacle-dwelling ecotypesof these species. This adds to the evidence that L. neglectais not a distinct species, but an ecotype of L. saxatilis. (Received 1 June 1992; accepted 23 June 1992)     

PATTERNS OF REPRODUCTION IN FOUR LITTORINA SPECIES

The reproductive periodicities of four Iittorinids, Littorinaneglecta, L. nigrolineata, L. saxatilis and L. arcana were examinedin sympatric populations at Porth Swtan, Anglesey, North Wales.Three species, L. neglecta, L. nigrolineata and L. arcana, werefound to show marked reproductive periodicity. L. saxatilisshowed no periodicity and large numbers of animals were foundin breeding condition throughout the year. The oviparous speciesL. nigrolineata and L. arcana, came into reproductive conditionin the late summer or early autumn, L. nigrolineata in Augustand L. arcana in September, and their populations at Porth Swtanhad few individuals in breeding condition in the late springand early summer. L. neglecta, however, showed a periodicitywith high numbers of breeding animals throughout the springand summer and very low numbers in the autumn. For L. neglectathe basis of reproductive periodicity is shown to be the synchronizedannual life cycle of individuals of this species. It is thereforebelieved to differ from the other three species in which individualsare longer-lived and are supposed to undergo seasonal periodsof reproductive inactivity. (Received 2 March 1982;     

THE RELATIONSHIP BETWEEN ENVIRONMENTAL VARIABLES AND THE DENSITY OF THE MUD SNAIL HYDROBIA TOTTENI IN A NOVA SCOTIA SALT MARSH

The relationship between environmental variables and the densityof Hydrobia totteni was examined in a restricted area of LawrencetownMarsh, Nova Scotia. Densities averaged 50/m² in Spartina alterniflora,9800/m² in Zostera marina, and 19,000/m² in sand. Of 8 environmentalfactors considered, depth and sediment grain size were the mostimportant in explaining variations in snail density. Since thesefactors together explained only 42.0% of the total variationno single physical factor controls the density of Hydrobia ina restricted area, though salinity does control densities overa wide area. Salinity and temperature tolerances and the abilityof the snails to select appropriate sediment grain sizes weredetermined experimentally and related to the distribution ofthe snails in the marsh. ^*Present address: Western Australian Museum, Francis Street,Perth, Western Australia, 6000 (Received 20 June 1976;     

Laboratory Studies of Parasite Transmission Aspects between Hydrobia spp. (Gastropoda) and Corophium volutator (Amphipoda)

Aspects of parasite transmission between Hydrobia spp. and Corophium volutator, first and second intermediate host of digenetic trematodes, were investigated under laboratory conditions. H. ventrosa is used as an intermediate host by several trematode species. Under laboratory conditions the most frequently observed emergence from H. ventrosa was of cercariae of Maritrema subdolum. The number of cercariae shed per day varied considerably. It was observed that 30 cercariae on average and up to 450 cercariae at maximum can emerge from a single H. ventrosa per day. Cercarial production continued until the death of the snails. The life-span of cercariae of the species M. subdolum decreased as the water temperature increased. It can be concluded that under natural conditions the cercariae, after emerging, have a maximum period of 1 day in which to seek out their second intermediate host C. volutator. Almost all specimens of C. volutator exposed to cercariae of the species M. subdolum died within the test period of 6 days. High average cercarial densities caused short life-spans (<50 h), while at lower densities longer survival times were possible. For cercariae of other Trematoda species, we were unable to find any equally clear evidence of a reduction in the survival rate of C. volutator within the test period. Mortality of C volutator, and other effects of infestation, as observed in our experiment, can be assumed to be a result of the penetration process of the cercariae but they also can be attributed to the mesocercariae.     

GENETIC AND CONCHOLOGICAL COMPARISON OF SNAILS (HELIX ASPERSA) DIFFERING IN SHELL DEPOSITION OF LEAD

Populations of snails inhabiting areas with different historiesof Pb contamination differed in their deposition of Pb in shellrelative to soft tissues. Genetic variation, measured usingisozymes, was not related to Pb history nor geographic distancebetween populations. Shell characteristics were significantlydifferent among sites; shell dry weight was strongly relatedto soil calcium levels. Shells of snails from areas with longhistories of Pb contamination were significantly more robust(greater shell width/shell height ratio) than snails from otherlocations. H. asprsa adaptation to Pb contamination may involvesignificant changes in shell characteristics but these do notcorrelate with genetic traits assessed with allozymes (Received 29 December 1994; accepted 15 October 1995)     

DISTRIBUTION AND ABUNDANCE OF HYDROBIA ULVAE (PENNANT) IN THE LOWER MEDWAY ESTUARY, KENT

Measurements of densities and distribution of populations ofH. ulvae were made in the lower Medway estuaryduring the period1969–1975, together with measurements of salinity, substrateparticle size and macrophyte distribution. Results indicatedthat sheltered siteswere more favourable to the snail as shownby higher population density and greater shell size. ^*Current address: Southern Water Authority, Maidstone, Kent (Received 20 May 1979;     

Neuroendocrine Control Mechanisms in Shell Formation

The brain of Helisoma duryi contains several neurodendocrinecentres. Factors) present in the cerebral ganglia are thoughtto be involved in normal shell growth while neurosecretory substancespresent in the visceral ganglion are involved in the repairof damaged shell. In Lymnaea stagnalis a growth hormone is producedby the cerebral ganglion which stimulates periostracum formationand the calcification of the inner shell layer. The second effectis thought to occur through the action of a mantle edge calciumbinding protein. In Helisoma, mantle collar is able to produce the periostracumin vitro. The presence of brain from a fast growing donor increasesthe amount of periostracum produced by a mantle collar froma slow growing animal. This effect is further enhanced by theremoval of the lateral lobes. The periostracum produced by fastgrowing animals has a higher glycine content than that producedby slow growing snails. The presence of dorsal epithelial tissueenhances the incorporation of calcium into periostracum formedin vitro. These findings suggest that a single factor is present in thebrain of fast growing Helisoma which modulates shell formationrates in vivo and periostracum formation in vitro.