Freshwater mussels (Anodonta anatina) reduce transmission of a common fish trematode (eye fluke, Diplostomum pseudospathaceum).

Recent results suggest that bivalves can play an important role in restraining the spread of various aquatic infections. However, the ability of mussels to remove free-living stages of macroparasites and reduce their transmission is still understudied, especially for freshwater ecosystems. We investigated the influence of the common freshwater mussel (Anodonta anatina) on the transmission of a trematode (eye fluke, Diplostomum pseudospathaceum), which frequently infects fish in farms and natural habitats. In our experiments, mussels caused a significant decrease (P < 0·001) in the abundance of trematode free-living stages, from 6520 to 1770 cercariae L-1 on average (about 4-fold in 2 h). Individual clearance rates of mussels were 0·6‒3·7 L per hour (mean 1·9). These tests were followed by experimental infections of rainbow trout (Oncorhynchus mykiss) with different doses of D. pseudospathaceum cercariae in the presence or absence of mussels. Exposure of fish to cercariae in the presence of mussels significantly (P < 0·05) reduced the infection intensities in fish (by 30-40%) at all exposure doses. Our results indicate that freshwater bivalves can markedly reduce local cercariae densities and could be useful in mitigation of trematodoses harmful to fish farming.

Rhipidocotyle fennica (Digenea: Bucephalidae) from Anodonta anatina and pike Esox lucius in Lithuania.

Ribosomal DNA sequences of Rhipidocotyle sp. adults from Esox lucius were shown to be identical to sequences of larval Rhipidocotyle fennica, occurring in Anodonta anatina in Lake Vilkoksnis, Lithuania. Morphological features and host specificity of this adult worm correspond with that, determinate in the first description of R. fennica in Finland. These data give the first evidence that a viable population of R. fennica exists in east central Europe. Bucephalus polymorphus which was reported in unionids in all previous publications is probably R. fennica.

An invasive species reverses the roles in a host-parasite relationship between bitterling fish and unionid mussels.

The impact of multiple invading species can be magnified owing to mutual facilitation--termed 'invasional meltdown'--but invasive species can also be adversely affected by their interactions with other invaders. Using a unique reciprocal host-parasite relationship between a bitterling fish (Rhodeus amarus) and unionid mussels, we show that an invasive mussel reverses the roles in the relationship. Bitterling lay their eggs into mussel gills, and mussel larvae parasitize fish. Bitterling recently colonized Europe and parasitize all sympatric European mussels, but are unable to use a recently invasive mussel, Anodonta woodiana. The parasitic larvae of A. woodiana successfully develop on R. amarus, whereas larvae of European mussels are rejected by bitterling. This demonstrates that invading species may temporarily benefit from a coevolutionary lag by exploiting evolutionarily naive hosts, but the resulting relaxed selection may facilitate its exploitation by subsequent invading species, leading to unexpected consequences for established interspecific relationships.

Trichodinid ectoparasites (Ciliophora: Peritricha) from Misgurnus anguillicaudatus (Cantor) and Anodonta woodiana (Lea) in China, with descriptions of two new species of Trichodina Ehrenberg, 1838.

During parasitological surveys in Chongqing, China, three trichodinids infecting a freshwater fish, Misgurnus anguillicaudatus (Cantor), and a mollusc, Anodonta woodiana (Lea), were investigated. Two of these species were found to be new. Trichodina lechriodentata n. sp., from the fish, was identified by its small-sized body and irregularly rectangular blade, while T. rara n. sp., from the mollusc, is characterised by its overall dimensions, denticle morphology and the presence of a white speckle in some specimens. T. modesta Lom, 1979 is also described from the fish. Photomicrographs and morphometric data are presented for all of the species.

Burrowing behaviour affects Paraergasilus rylovi abundance in Anodonta piscinalis.

Burrowing depth may affect predation rate, feeding ability and reproduction in bivalve clams. We studied the effect of burrowing depth on the abundance of the ergasilid Paraergasilus rylovi in the freshwater bivalve clam Anodonta piscinalis. We transplanted uninfected clams to a lake where they were allowed to choose their preferred burrowing depth, and were exposed naturally to copepodids of the parasite. There was a significant positive correlation between proportionate burrowing depth (PBD) and the abundance of P. rylovi at the end of the 17-day experiment, the deeper-burrowed clams harbouring more P. rylovi. Original PBD (0%, 50%, 100%) did not influence the final PBD or parasite abundance. Clam length affected PBD, smaller clams burrowing deeper, but it did not affect parasite abundance. Infected experimental clams and naturally-burrowed uninfected clams, both originating from the same lake, did not differ in their mean PBD. This indicated that burrowing of the experimental clams affected parasitism rather than the parasites altering burrowing of the clams. In line with the experimental result, we observed a significant positive correlation between PBD and the abundance of P. rylovi also among clams collected from 2 natural A. piscinalis populations.