X. de Montaudouin

Needle in a haystack: involvement of the copepod Paracartia grani in the life-cycle of the oyster pathogen Marteilia refringens

Marteilia refringens is a major pathogen of the European flat oyster, Ostrea edulis Linnaeus. Since its description, the life-cycle of this protozoan parasite has eluded discovery. Attempts to infect oysters experimentally have been unsuccessful and led to the hypothesis of a complex life-cycle involving several hosts. Knowledge of this life-cycle is of central importance in order to manage oyster disease. However, the exploration of M. refringens life-cycle has been previously limited by the detection tools available and the tremendous number of species to be screened in enzootic areas. In this study, these two restrictions were circumvented by the use of both molecular detection tools and a mesocosm with low biodiversity. Screening of the entire fauna of the pond for M. refringens DNA was systematically undertaken using PCR. Here, we show that the copepod Paracartia (Acartia) grani is a host of M. refringens. Not only was DNA of M. refringens consistently detected in P. grani but also the presence of the parasite in the ovarian tissues was demonstrated using in situ hybridization. Finally, successful experimental transmissions provided evidence that P. grani can be infected from infected flat oysters.

The proliferating Gastropoda Crepidula fornicata may stimulate macrozoobenthic diversity

Since the late 19th century, the accidentally introduced Gastropoda Crepidula fornicata has been invading sheltered coastal waters of western Europe. The consequences of this proliferation on macrozoobenthic communities were studied in the Bay of Marennes-Oleron, France. Crepidula fornicata was found in a wide range of sediment grain sizes and depths, with however, a predilection for shallow muddy areas where abundance and biomass reached 4770 ind m -2 and 354g DW m -2 , respectively. Soft-sediment macrofaunal assemblages were compared in similar habitats, in the presence and absence of C. fornicata. In muddy and medium sand areas, abundance, biomass and species richness of macrofauna were generally greater in presence of C. fornicata, with Annelida dominating, although the differences were significant (P<0.05) only in one location out of five. The species composition was moderately different in the presence of C. fornicata. In coarse sand, the abundance of C. fornicata was low and did not affect abundance, biomass and species richness of the macrofauna. However, species composition differed where C. fornicata was absent, with a higher proportion of mobile Crustaceana. The effect of C. fornicata on benthic communities differs in relation to the habitat they colonize: in muddy sediments, the presence of C. fornicata apparently stimulates zoobenthic community diversity and abundance (mostly deposit-feeders), whereas in coarser sediments, macrofauna community is different (more suspension-feeders) from the community associated with C. fornicata.

Potential of bivalves' secondary settlement differs with species: a comparison between cockle (Cerastoderma edule) and clam (Ruditapes philippinarum) juvenile resuspension

Abstract Juvenile bivalves may be dispersed by entering a bysso-pelagic phase where they drift through the water mass aided by a long thread. The ability to resuspend and control the specific weight in two bivalve species, the cockle Cerastoderma edule (L.) and the Japanese clam Ruditapes philippinarum (Adams and Reeves), was documented with juveniles through flume and still-water experiments. Cockle juveniles initially placed on an unsuitable substratum were exposed to two shear velocities (u*). At the end of the experiment, 42 (±15)% (for u*=0.51 cm s−1) and 79 (±9)% (for u*=0.99 cm s−1) of individuals were retrieved from the sand area which represents only 8% of the total flume surface. Most juveniles (70.5%) with shell lengths <2.5 mm migrated from the unsuitable Plexiglas substratum to the sand array by resuspension in the water column. The percentage was lower (21.5%) for larger individuals. The same experimental design was applied to clams, which immediately adhered to the Plexiglas substratum and remained attach to it. Sinking rates of live and dead specimens of both species were measured in a 1 m long transparent PVC tube. Cockle fall velocities showed severe deceleration, probably due to byssus secretion (up to 15-fold slower than dead cockles), sometimes interrupted by brutal acceleration probably due to byssal rupture. Cockles were able to reduce their sinking rate for shell lengths up to 4.25 mm. By contrast, clam sinking rates were constant, and similar to dead clam sinking rates. Specific weights of all experimental juveniles were calculated in relation to their lengths, and their passive motion into the boundary layer was theoretically assessed with Shields curve. In short, C. edule and R. philippinarum can both exhibit dense populations in the field with a good capacity to colonize, although juveniles display different abilities to resuspend in the water column.

Evidence of an altered protective effect of metallothioneins after cadmium exposure in the digenean parasite-infected cockle (Cerastoderma edule).

The aim of the present study was to analyse the relation between parasitism and subsequent metallothioneins (MT) in the case of metal contamination. Experimental exposure of parasitized and unparasitized cockles (Cerastoderma edule) to cadmium (Cd) was performed, with the cockle as first or second intermediate host of 2 digenean species. After 7 days of Cd exposure in microcosms, cockles infected as first intermediate host by Labratrema minimus exhibited metal concentrations in tissues double that in uninfected cockles. Jointly, MT concentrations of parasitized cockles were not modified in comparison with uninfected individuals in which concentrations were increased 4.3-fold compared with controls. In cockles experimentally infected as the second intermediate host by Himasthla elongata, cadmium concentrations significantly increased again in parasitized cockles compared with uninfected individuals in contaminated conditions. Simultaneously, MT concentrations in healthy cockles increased, whereas they significantly decreased in parasitized individuals. Therefore, the presence of digenean parasites in Cd-exposed cockles leads to a maintenance or a decrease in MT concentrations compared with healthy individuals, whereas Cd accumulation in tissues is significantly increased. These experiments indicate a significant alteration of the protective effect of metallothioneins towards metals which could consequently enhance cockle vulnerability. Moreover, these results highlight the limit of the use of MT as a biomarker of metal pollution in field monitoring if parasitism is not taken into account.

Why does the introduced gastropod Crepidula fornicata fail to invade Arcachon Bay (France)

Arcachon Bay was the last important shell culture area of the French Atlantic coast without a stock assessment of the accidentally introduced Crepidula fornicata (Gastropoda). Following a stratified sampling strategy, 205 stations were sampled with a dredge. The total biomass of C. fornicata was estimated at 155 tn (confidence limits: 72 tn) scattered on 2·4 km 2 , i.e. about 5% of the infralittoral area. This biomass is very low compared to other exploited shellfish sites. Amongst mean environmental parameters (such as bathymetry, temperature, salinity, sediment grain-size, anthropic activity), the major keyfactors differentiating Arcachon Bay from other French sites were searched to explain the differences in C. fornicata colonization pattern. It seems that the low biomass of C. fornicata , 30 y after its occurrence was first recorded in the bay, could be correlated to: (1) the scarcity of subtidal muddy sediments which are suitable for C. fornicata settlement; (2) the presence of large Zostera spp. beds on both intertidal and subtidal areas; and (3) the absence of bottom trawl fishing.  Growth, production and productivity (P/B) ratios were compared between five stations characterized by different hydrological and sediment parameters. Growth and productivity (0·3 y −1 ) were not affected by mean biomass suggesting a lack of population regulation by intraspecific competition.

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.

Virus-like particles associated with brown muscle disease in Manila clam, Ruditapes philippinarum, in Arcachon Bay (France)

Recently, Manila clam, Ruditapes philippinarum, populations have suffered mortalities in Arcachon Bay (SW France). Mortality was associated with extensive lesions of the posterior adductor muscle, which become progressively brown and calcified. Ultrastructural observations by transmission electron microscopy revealed tissue degradation with necrotized muscle fibres and granulocytomas. Unenveloped virus-like particles (VLPs) were detected in muscle, granulocytic, epithelial and rectal cells. VLPs were abundant in the extracellular space, in the cytoplasm (free or enclosed in vesicles) and in the nucleoplasm of granulocytes. Nuclei and mitochondria of granulocytes displayed changes which suggested reactive oxygen species production and apoptosis induction. VLPs exhibited an icosahedral structure with a diameter of 25 to 35 nm. These observations suggest that the VLPs could belong to the family Picornaviridae or the Parvoviridae.