Digenean trematodes-marine mollusc relationships: a stable isotope study.

The stable carbon and nitrogen isotopic composition of digenean trematode parasites and their marine mollusc hosts was investigated to describe the potential influence of parasites on their host and its different tissues, and to obtain further insight into their trophic relationships. Four parasite-host systems were studied: Labratrema minimus-Cerastoderma edule, Monorchis parvus-C. edule, Lepocreadiidae parasites-Nassarius reticulatus and Zoogonidae parasites-N. reticulatus. Among the 4 sampling occasions reported here and corresponding to the 4 parasite-host systems, isotopic shifts from pathologic (i.e. linked to disturbances in host metabolism) and mass-balance (i.e. linked to significant differences between host and parasite isotopic signatures) origins were observed only once. Both corresponded to delta 13C measurements of the L. minimus-C. edule system when the infestation load (percentage parasite dry weight compared to total flesh dry weight) was highest (9 to 25%, mean = 16%) over the sampling period. Overall, measurements indicate that digenean trematode parasitism induced low or no shifts in isotopic signatures of C. edule and N. reticulatus tissues. The 2 endoparasites L. minimus and M. parvus appeared to be slightly depleted in 13C compared to C. edule digestive gland and gonads, which were the most parasitized tissues. In contrast, no fractionation or low 15N trophic enrichments occurred in the parasites. These results highly contrast with the classical trophic enrichment reported in prey-predator systems but are in agreement with the scarce literature regarding other parasite-host systems. Our results indicate that (1) digenean trematodes mainly feed on digestive glands (the cockle tissue with which they are mainly associated) with a possible slight preference for lipids, and (2) fractionation due to parasite metabolism should be low due to abbreviated metabolic pathways and/or slight loss of materials through excretion, tegument diffusion and respiration.

Use of ITS rDNA for discriminating of larval stages of two microphallid (Digenea) species using Hydrobia ulvae (Pennant, 1777) and Corophium volutator (Pallas, 1766) as intermediate hosts.

Digenean trematodes encompass several species with little morphological differentiation in the larval stages and, as a result, uncertainty prevails regarding species identification. The microphallid trematode Maritrema subdolum occurs widespread geographically in mud snail and crustacean hosts in European marine shallow-water ecosystems. Larval stages of this and other congeneric species are, however, difficult to separate morphologically. To verify the species status and to secure identification of two co-occurring microphallids, M. subdolum and microphallid sp. no. 15 (Deblock, 1980), we examined the nucleotide sequences of the internal transcribed spacer regions (ITS1, ITS2). From fragments consisting of both ITS regions and the 5.8S gene (nearly 1,200 bp), a sequence divergence of 2.9% between the two types was recorded. In accordance with the morphological traits of the cercariae (stylet shape, length), the results support the view that the two types actually represent different species. Species-specific primers were prepared for each species. They proved to be efficient diagnostic tools for identifying single larval-stage individuals. Using these primers, infections in host organisms were also verified without performing a dissection of the host individuals.

Carbohydrate production in relation to microphytobenthic biofilm development: an integrated approach in a tidal mesocosm.

Experiments were performed to evaluate short-term changes in sediment extracellular carbohydrates for a multispecific assemblage of benthic diatoms in relation to physiological status, endogenous migratory rhythms, and environmental conditions. For this purpose, a mesocosm was used, which simulated both tidal and dark: light alternating cycles under controlled conditions. Scanning electronic microscopy in combination with picture analyses indicated that natural diatom migration patterns were reproduced in the mesocosm. Two EPS fractions were operationally separated in colloidal carbohydrate measurements: alcohol-soluble EPS (termed "soluble EPS") and alcohol-insoluble EPS (termed "bound EPS"). Microphytobenthic biomass followed a logistic-type curve and converged toward a maximal value termed the "biotic capacity of the local environment." Both EPS fractions showed oscillations with production during photosynthetic periods and sharp decreases during night immersion periods. Productions of both EPS fractions increased with Chl a production during light periods suggesting a light dependence in relation to migratory patterns. The decreases in both EPS fractions, which occurred during night immersion periods suggest that carbohydrate hydrolysis and/or washaway affected both EPS fractions similarly in benthic environments. Our results confirm the theory according to which the two distinct fractions are under different metabolic controls. No change in soluble EPS release was obtained during the transition from logarithmic to stationary phase. On the other hand, a metabolism modification of microalgae, probably related to ammonium depletion, occurred when cells entered the stationary phase, since there was a high enhancement in bound EPS production. Mesocosm results can serve as a system of reference useful to characterize biofilm development in field investigations and to revisit the effective implication of each EPS fraction in sediment stability.

The influence of long emersion on biota, ammonium fluxes and nitrification in intertidal sediments of Marennes-Oléron Bay, France.

A comparative study between waterlogged and reflooded intertidal sediments was undertaken in March and June 1999 through statistical analysis of selected sediment parameters (biota, salinity, O2, Eh), pool sizes and benthic fluxes of nutrients (NH4+, NO2-, NO3-) and nitrification rates. In March samples, absence of polychaetes and oligochaetes from upper sediment horizons were due to erosional events sweeping away surface sediments. Presence of richer annelid assemblages in June samples indicated more stable hydrodynamic conditions that favoured the development of benthic microalgae biofilms. Dewatering of sediments during a 3-day emersion period promoted a salinity rise on top layers, migration of pore water ions towards the sediment surface, and created sediment fissures that accelerated water exchange on reflooding. Reflooded and waterlogged sediment systems were comparable with respect to the release of NH4+ to overlying water but were different with respect to nitrification rates. Sediment-water NH4+ fluxes were higher (P = 0.011) in March (3.3 mmol m(-2) day(-1) compared to June (1.4 mmol m(-2) day(-1) due to higher macrofauna biomasses and lower benthic microalgae concentrations in March samples. Potential nitrification rates (range from 19 to 60 mmol NO3- (-2) day(-1)) were not statistically different between March and June. A thinner oxic layer in reflooded compared with waterlogged systems reflects a decrease of O2 diffusion into sediment at high salinities which resulted in the fall of the actual nitrification rates (P < 0.05). Our data suggest that long term dessication of intertidal sediments may depress the nitrification process at the ecosystem level.

Claire ponds as an experimental model for Marteilia refringens life-cycle studies: new perspectives.

Since its first description, the paramyxean parasite Marteilia refringens (Grizel et al.) has been recognised as a significant pathogen of the European flat oyster Ostrea edulis L. The existence of a complex life-cycle involving several hosts was postulated early on by many authors, although it remains unsolved. Recent developments in the DNA-based diagnosis of M. refringens provides new prospects for the detection of the parasite in potential hosts. However, this screening remains impeded by the number of species living in the vicinity of oyster beds. We report here on the use of semi-closed oyster ponds (so called 'claire' in Marennes-Oléron Bay) as a study model for the life-cycle of M. refringens. Claires are located in an endemic area for M. refringens and transmission of the disease to healthy oysters has been shown to be effective during the course of this study. The environmental characteristics of the claires strongly limit the number of species compared with intertidal areas and oyster beds. Consequently, extensive sampling of a limited number of species cohabiting with oysters was possible. These were preserved for future screening of M. refringens. The experimental model should bring new insights to the life-cycle of M. refringens, as it enables us to propose new conceptual schemes of M. refringens transmission. The role of species as potential hosts is discussed regarding their biology and geographical distribution.