Different Host Exploitation Strategies in Two Zebra Mussel-Trematode Systems: Adjustments of Host Life History Traits

The zebra mussel is the intermediate host for two digenean trematodes, Phyllodistomum folium and Bucephalus polymorphus, infecting gills and the gonad respectively. Many gray areas exist relating to the host physiological disturbances associated with these infections, and the strategies used by these parasites to exploit their host without killing it. The aim of this study was to examine the host exploitation strategies of these trematodes and the associated host physiological disturbances. We hypothesized that these two parasite species, by infecting two different organs (gills or gonads), do not induce the same physiological changes. Four cellular responses (lysosomal and peroxisomal defence systems, lipidic peroxidation and lipidic reserves) in the host digestive gland were studied by histochemistry and stereology, as well as the energetic reserves available in gonads. Moreover, two indices were calculated related to the reproductive status and the physiological condition of the organisms. Both parasites induced adjustments of zebra mussel life history traits. The host-exploitation strategy adopted by P. folium would occur during a short-term period due to gill deformation, and could be defined as “virulent.” Moreover, this parasite had significant host gender-dependent effects: infected males displayed a slowed-down metabolism and energetic reserves more allocated to growth, whereas females displayed better defences and would allocate more energy to reproduction and maintenance. In contrast, B. polymorphus would be a more “prudent” parasite, exploiting its host during a long-term period through the consumption of reserves allocated to reproduction

Macroparasite community in molluscs of a tidal basin in the Wadden Sea

We provide a quantitative inventory of macroparasites in intertidal molluscs from a tidal basin in the Wadden Sea (eastern North Sea). Gastropods and bivalves contained a species rich macroparasite community consisting of trematodes (26 species), turbellarians (1), nematodes (1), copepods (2) and polychaetes (1) in 3,800 host individuals from 10 host species. Highest parasite burdens were observed in the gastropods Hydrobia ulvae and Littorina littorea and in the bivalves Cerastoderma edule and Mytilus edulis. In contrast, only one parasite species and no trematodes were found in Crepidula fornicata. The parasite community in the molluscs was similar to other Western European localities but some parasite species showed obvious differences, related to the large-scale distribution of intermediate and final hosts. Parasitism seems to be a common phenomenon in molluscs of the Wadden Sea and hence the detrimental effects observed in experiments can be expected to frequently happen in the field.Keywords: Trematodes - Nematodes - Copepods - Polydora ciliata - Parasitism - Intertidal - Wadden Sea - Gastropods - Bivalve

Testing the enemy release hypothesis: trematode parasites in the non-indigenous Manila clam Ruditapes philippinarum

International audienceThe present study tested the ‘Enemy Release Hypothesis’ (ERH) which states that the success of an introduced species is related to the scarcity of natural enemies in the introduced range compared with the native range. Digeneans are dominant macroparasites of molluscs; therefore, the interaction between R. philippinarum and these parasites was ideal for investigation. A two-year monitoring in Arcachon Bay (SW France) was performed to estimate digenean loads in R. philippinarum and in three infaunal native bivalves (R. decussatus, Paphia aurea, Cerastoderma edule). A laboratory experiment allowed comparison of infection success among these bivalves (except P. aurea) by generalist digenean larvae (Himasthla elongata cercariae). R. philippinarum digenean abundance in Arcachon Bay was much lower than in native bivalves, with values depending on species, sites and time. Similarly, mean digenean species richness per host individual was always lower in R. philippinarum than in sympatric bivalves. A comparison of digenean metacercariae abundance between R. decussatus and C. edule in Mundaka Estuary (Spain) showed that both species had similar digenean loads but that R. decussatus was depleted in digenean species encysting in host tissues (the non-gymnophallid species). Experimental infection confirmed that the two species of the genus Ruditapes (and not R. philippinarum only) were resistant to encysting digeneans, with an infection success 3–5 times lower than that of C. edule. The lack of infection that was observed in the field would therefore be the consequence of a tissue barrier, R. philippinarum epithelium being too tough for cercariae penetration. Concordantly, according to the literature, digenean infection in the native range of R. philippinarum is also low. Consequently, the ERH, as an explanation for R. philippinarum success in Europe, is not totally consistent in the case of digenean trematodes as enemies, R. philippinarum hosting low load of digeneans in its native as well as colonized range

Review: Bucephalus minimus, a deleterious trematode parasite of cockles Cerastoderma spp.

Trematodes are the most prevalent and abundant macroparasites in coastal waters. They display a complex life cycle with alternation of free-living and parasitic stages generally involving three host species. The most deleterious stage is in the first intermediate host (a mollusc) where the parasite penetrates as miracidium larvae and asexually multiplicates in sporocysts/rediae to provide cercariae larvae. However, due to basic low prevalence in ecosystems, this system remains difficult to study. Taking the example of the cockle (Cerastoderma edule), an exploited bivalve along North-Eastern Atlantic coasts, and Bucephalus minimus, its most prevalent parasite as first intermediate host, we summarised the 51 most relevant papers (1887-2015). Besides, a 16-year monthly monitoring was performed at Banc d'Arguin (Atlantic coast of France), and allowed to obtain a sufficient number of infected cockles (276 out of 5,420 individuals) in order to provide new information concerning this parasite/host system. Sporocysts (diameter 80-500 μm) and developing cercariae (length 300-500 μm) are not visible before cockle reaches 16-mm shell length and then prevalence increases with host size. Seasonality of infection was not observed but variation of prevalence was significant among years and negatively correlated to the temperature of the former year, which could correspond to the period of infection by miracidium. Seven other species of trematode were identified in cockles as second intermediate host. For six of them, metacercariae abundance per individual was 2 to 12 folds higher in B. minimus-infected cockles, exacerbating the potential negative impact on host. From the parasite point of view, metacercariae can be considered as hitchhikers, taking advantage of the abnormal migration of B. minimus-infected cockles to the sediment surface where they become more vulnerable to predators that are also the final hosts of many of these parasites